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<div class="title">HDF5 File Format Specification Version 3.0 </div> </div>
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HDF5 File Format Specification Version 3.0
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<center>
<table border="0" width="90%">
<tr>
<td valign="top">
<ol type="I">
<li><a href="#Intro">Introduction</a></li>
<font size="-1">
<ol type="A">
<li><a href="#ThisDocument">This Document</a></li>
<li><a href="#ChangesForHdf5_1_12">Changes for HDF5 1.12</a></li>
<li><a href="#ChangesForHdf5_1_10">Changes for HDF5 1.10</a></li>
</ol>
</font>
<li><a href="#FileMetaData">Disk Format: Level 0 - File Metadata</a></li>
<font size="-1">
<ol type="A">
<li><a href="#Superblock">Disk Format: Level 0A - Format Signature
and Superblock</a></li>
<li><a href="#DriverInfo">Disk Format: Level 0B - File Driver
Info</a></li>
<li><a href="#SuperblockExt">Disk Format: Level 0C - Superblock
Extension</a></li>
</ol>
</font>
<li><a href="#FileInfra">Disk Format: Level 1 - File Infrastructure</a></li>
<font size="-1">
<ol type="A">
<li><a href="#Btrees">Disk Format: Level 1A - B-trees and B-tree
Nodes</a>
<ol type="1">
<li><a href="#V1Btrees">Disk Format: Level 1A1 - Version 1
B-trees</a></li>
<li><a href="#V2Btrees">Disk Format: Level 1A2 - Version 2
B-trees</a></li>
</ol>
</li>
<li><a href="#SymbolTable">Disk Format: Level 1B - Group Symbol
Table Nodes</a></li>
<li><a href="#SymbolTableEntry">Disk Format: Level 1C - Symbol
Table Entry</a></li>
<li><a href="#LocalHeap">Disk Format: Level 1D - Local Heaps</a></li>
<li><a href="#GlobalHeap">Disk Format: Level 1E - Global Heap</a></li>
<li><a href="#GlobalHeapVDS">Disk Format: Level 1F - Global Heap
Block for Virtual Datasets</a></li>
<li><a href="#FractalHeap">Disk Format: Level 1G - Fractal Heap</a></li>
<li><a href="#FreeSpaceManager">Disk Format: Level 1H - Free-space
Manager</a></li>
<li><a href="#SOHMTable">Disk Format: Level 1I - Shared Object
Header Message Table</a></li>
</ol>
</font>
<li><a href="#DataObject">Disk Format: Level 2 - Data Objects</a></li>
<font size="-1">
<ol type="A">
<li><a href="#ObjectHeader">Disk Format: Level 2A - Data Object Headers</a></li>
<ol type="1">
<li><a href="#ObjectHeaderPrefix">Disk Format: Level 2A1 -
Data Object Header Prefix</a>
<ol type="a">
<li><a href="#V1ObjectHeaderPrefix">Version 1 Data
Object Header Prefix</a></li>
<li><a href="#V2ObjectHeaderPrefix">Version 2 Data
Object Header Prefix</a></li>
</ol>
</li>
<li><a href="#ObjectHeaderMessages">Disk Format: Level 2A2 -
Data Object Header Messages</a></li>
<ol type="a">
<li><a href="#NILMessage">The NIL Message</a></li> <!-- 0x0000 -->
<li><a href="#DataspaceMessage">The Dataspace Message</a></li> <!-- 0x0001 -->
<li><a href="#LinkInfoMessage">The Link Info Message</a></li> <!-- 0x0002 -->
<li><a href="#DatatypeMessage">The Datatype Message</a></li> <!-- 0x0003 -->
<li><a href="#OldFillValueMessage">The Data Storage -
Fill Value (Old) Message</a></li> <!-- 0x0004 -->
</ol>
</ol>
</ol>
</font>
</ol>
</td>
<td> </td>
<td valign="top">
<ol type="I" start="4">
<li><a href="#DataObject">Disk Format: Level 2 - Data
Objects</a><font size="-1"><i> (Continued)</i></li>
<ol type="A">
<li><a href="#ObjectHeader">Disk Format: Level 2A - Data Object
Headers</a><i> (Continued)</i>
<ol type="1" start="2">
<li><a href="#ObjectHeaderMessages">Disk Format: Level 2A2 -
Data Object Header Messages</a><i> (Continued)</i></li>
<ol type="a" start="6">
<li><a href="#FillValueMessage">The Data Storage -
Fill Value Message</a></li> <!-- 0x0005 -->
<li><a href="#LinkMessage">The Link Message</a></li> <!-- 0x0006 -->
<li><a href="#ExternalFileListMessage">The Data Storage -
External Data Files Message</a></li> <!-- 0x0007 -->
<li><a href="#LayoutMessage">The Data Layout Message</a></li> <!-- 0x0008 -->
<li><a href="#BogusMessage">The Bogus Message</a></li> <!-- 0x0009 -->
<li><a href="#GroupInfoMessage">The Group Info
Message</a></li> <!-- 0x000a -->
<li><a href="#FilterMessage">The Data Storage -
Filter Pipeline Message</a></li> <!-- 0x000b -->
<li><a href="#AttributeMessage">The Attribute
Message</a></li> <!-- 0x000c -->
<li><a href="#CommentMessage">The Object Comment
Message</a></li> <!-- 0x000d -->
<li><a href="#OldModificationTimeMessage">The Object
Modification Time (Old) Message</a></li> <!-- 0x000e -->
<li><a href="#SOHMTableMessage">The Shared Message
Table Message</a></li> <!-- 0x000f -->
<li><a href="#ContinuationMessage">The Object Header
Continuation Message</a></li> <!-- 0x0010 -->
<li><a href="#SymbolTableMessage">The Symbol
Table Message</a></li> <!-- 0x0011 -->
<li><a href="#ModificationTimeMessage">The Object
Modification Time Message</a></li> <!-- 0x0012 -->
<li><a href="#BtreeKValuesMessage">The B-tree
‘K’ Values Message</a></li> <!-- 0x0013 -->
<li><a href="#DrvInfoMessage">The Driver Info
Message</a></li> <!-- 0x0014 -->
<li><a href="#AinfoMessage">The Attribute Info
Message</a></li> <!-- 0x0015 -->
<li><a href="#RefCountMessage">The Object Reference
Count Message</a></li> <!-- 0x0016 -->
<li><a href="#FsinfoMessage">The File Space Info
Message</a></li> <!-- 0x0017 -->
</ol>
</ol>
</li>
<li><a href="#DataStorage">Disk Format: Level 2B - Data Object Data Storage</a></li>
</ol>
</font>
<li><a href="#AppendixA">Appendix A: Definitions</a></li>
<li><a href="#AppendixB">Appendix B: File Space Allocation
Types</a></li>
<li><a href="#AppendixC">
Appendix C: Types of Indexes for Dataset Chunks</a></li>
<font size="-1">
<ol type="A">
<li><a href="#SingleChunk">The Single Chunk Index</a></li>
<li><a href="#Implicit">The Implicit Index</a></li>
<li><a href="#FixedArray">The Fixed Array Index</a></li>
<li><a href="#ExtensibleArray">The Extensible Array Index</a></li>
<li><a href="#AppendV2Btrees">The Version 2 B-trees Index</a></li>
</ol>
</font>
<li><a href="#AppendixD">
Appendix D: Encoding for Dataspace and Reference</a></li>
<font size="-1">
<ol type="A">
<li><a href="#DataspaceEncode">Dataspace Encoding</a></li>
<li><a href="#ReferenceEncodeRV">Reference Encoding (Revised)</a></li>
<li><a href="#ReferenceEncodeDP">Reference Encoding (Backward Compatibility)</a></li>
</ol>
</font>
</ol>
</td></tr>
</table>
</center>
<a name="Intro"><h2>I. Introduction</h2></a>
<table align="right" width="100">
<tr><td> </td><td align="center">
<hr />
<img src="FF-IH_FileGroup.gif" alt="HDF5 Groups" hspace="15" vspace="15">
</td><td> </td></tr>
<tr><td> </td><td align="center">
<strong>Figure 1:</strong> Relationships among the HDF5 root group, other groups, and objects
<hr />
</td><td> </td></tr>
<tr><td> </td><td align="center">
<img src="FF-IH_FileObject.gif" alt="HDF5 Objects" hspace="15" vspace="15">
</td><td> </td></tr>
<tr><td> </td><td align="center">
<strong>Figure 2:</strong> HDF5 objects -- datasets, datatypes, or dataspaces
<hr />
</td><td> </td></tr>
</table>
<p>The format of an HDF5 file on disk encompasses several
key ideas of the HDF4 and AIO file formats as well as
addressing some shortcomings therein. The new format is
more self-describing than the HDF4 format and is more
uniformly applied to data objects in the file.</p>
<p>An HDF5 file appears to the user as a directed graph.
The nodes of this graph are the higher-level HDF5 objects
that are exposed by the HDF5 APIs:</p>
<ul>
<li>Groups</li>
<li>Datasets</li>
<li>Committed (formerly Named) datatypes</li>
</ul>
<p>At the lowest level, as information is actually written to the disk,
an HDF5 file is made up of the following objects:</p>
<ul>
<li>A superblock</li>
<li>B-tree nodes</li>
<li>Heap blocks</li>
<li>Object headers</li>
<li>Object data</li>
<li>Free space</li>
</ul>
<p>The HDF5 Library uses these low-level objects to represent the
higher-level objects that are then presented to the user or
to applications through the APIs. For instance, a group is an
object header that contains a message that points to a local
heap (for storing the links to objects in the group) and to a
B-tree (which indexes the links). A dataset is an object header
that contains messages that describe the datatype, dataspace,
layout, filters, external files, fill value, and other elements
with the layout message pointing to either a raw data chunk or
to a B-tree that points to raw data chunks.</p>
<a name="ThisDocument"><h3>I.A. This Document</h3></a>
<p>This document describes the lower-level data objects;
the higher-level objects and their properties are described
in the <a href="UG/HDF5_User_Guide-Responsive HTML5/index.html"><cite>HDF5 User Guide</cite></a>.</p>
<p>Three levels of information comprise the file format.
Level 0 contains basic information for identifying and
defining information about the file. Level 1 information contains
the information about the pieces of a file shared by many objects
in the file (such as B-trees and heaps). Level 2 is the rest
of the file and contains all of the data objects with each object
partitioned into header information, also known as
<em>metadata</em>, and data.</p>
<p>The various components of the lower-level data objects are
described in pairs of tables. The first table shows the format
layout, and the second table describes the fields. The titles
of format layout tables begin with “Layout”. The
titles of the tables where the fields are described begin with
“Fields”. For example, the table that describes the
format of the <a href="#V2Btrees">version 2 B-tree header</a> has
a title of “Layout: Version 2 B-tree Header”, and the
fields in the version 2 B-tree header are described in the table
titled “Fields: Version 2 B-tree Header”.
<p>The sizes of various fields in the following layout tables are
determined by looking at the number of columns the field spans
in the table. There are exceptions: </p>
<ul>
<li> The size may be overridden by specifying a size in
parentheses</li>
<li> The size of addresses is determined by the
<em><a href="#SizeOfOffsetsV0">Size of Offsets</a></em> field
in the superblock and is indicated in this document with a
superscripted ‘O’</li>
<li> The size of length fields is determined by the
<em><a href="#SizeOfLengthsV0">Size of Lengths</a></em> field in
the superblock and is indicated in this document with a
superscripted ‘L’</li>
</ul>
<p>Values for all fields in this document should be treated as unsigned
integers, unless otherwise noted in the description of a field.
Additionally, all metadata fields are stored in little-endian byte
order.
</p>
<p>All checksums used in the format are computed with the
<a href="http://www.burtleburtle.net/bob/hash/doobs.html">Jenkins’
lookup3</a> algorithm.
</p>
<p>Whenever a bit flag or field is mentioned for an entry, bits are
numbered from the lowest bit position in the entry.
</p>
<p>Various format tables in this document have cells with
“This space inserted only to align table nicely”. These
entries in the table are just to make the table presentation nicer
and do not represent any values or padding in the file.
</p>
<a name="ChangesForHdf5_1_12">
<h3>I.B. Changes for HDF5 1.12</h3></a>
<p>The following sections have been
changed or added for the 1.12 release:</p>
<ul>
<li>Under <a href="#DatatypeMessage">“The Datatype Message”</a>,
in the Description for “Fields:Datatype Message”,
version 4 was added and Reference class (7) of the datatype was updated to describe version 4.</li>
<li><a href="#AppendixD">
“Appendix D: Encoding for Dataspace and Reference”</a>
was added. </li>
</ul>
<a name="ChangesForHdf5_1_10">
<h3>I.C. Changes for HDF5 1.10</h3></a>
<p>The following sections have been
changed or added for the 1.10 release:</p>
<ul>
<li>In the <a href="#Superblock">
“Disk Format: Level 0A - Format Signature and
Superblock”</a> section, version 3 of the superblock was
added. </li>
<li>In the <a href="#SuperblockExt">
“Disk Format: Level 0C - Superblock Extension”</a>
section, a link to the Data Storage message was added. </li>
<li>In the <a href="#V2Btrees">
“Disk Format: Level 1A2 - Version 2 B-trees”</a>
section, additional B-tree types were added. Tables that
describe the <a href="#V2BtreesType10">type 10</a> and
<a href="#V2BtreesType11">11</a> record layouts were added at
the end of the section.</li>
<li>The <a href="#GlobalHeapVDS">“Disk Format: Level 1F -
Global Heap Block for Virtual Datasets”</a> was added.
</li>
<li><a href="#LayoutMessage">
“The Data Layout Message”</a> section was changed.
The name was changed, and <a href="#DataLayoutV4">version 4</a>
of the data layout message was added for the virtual type.</li>
<li>The <a href="#FsinfoMessage">
“The File Space Info Message”</a> header message
type was added.</li>
<li><a href="#AppendixC">
“Appendix C: Types of Indexes for Dataset Chunks”</a>
was added. Five indexing types were added.</li>
</ul>
<h2><a name="FileMetaData">
II. Disk Format: Level 0 - File Metadata</a></h2>
<h3><a name="Superblock">
II.A. Disk Format: Level 0A - Format Signature and Superblock</a></h3>
<p>The superblock may begin at certain predefined offsets within
the HDF5 file, allowing a block of unspecified content for
users to place additional information at the beginning (and
end) of the HDF5 file without limiting the HDF5 Library’s
ability to manage the objects within the file itself. This
feature was designed to accommodate wrapping an HDF5 file in
another file format or adding descriptive information to an HDF5
file without requiring the modification of the actual file’s
information. The superblock is located by searching for the
HDF5 format signature at byte offset 0, byte offset 512, and at
successive locations in the file, each a multiple of two of
the previous location; in other words, at these byte offsets:
0, 512, 1024, 2048, and so on.</p>
<p>The superblock is composed of the format signature, followed by a
superblock version number and information that is specific to each
version of the superblock.
<p>Currently, there are four versions of the superblock format:
<ul>
<li>Version 0 is the default format.</li>
<li>Version 1 is the same as version 0 but with the
“<em>Indexed Storage Internal Node K</em>” field
for storing non-default B-tree ‘K’ value.</li>
<li>Version 2 has some fields eliminated and compressed from
superblock format versions 0 and 1. It has added checksum support
and superblock extension to store additional superblock
metadata.</li>
<li>Version 3 is the same as version 2 except that the field
“<em>File Consistency Flags</em>” is used for file
locking. This format version will enable support for the latest
version.</li>
</ul>
<p>Versions 0 and 1 of the superblock are described below:</p>
<div align="center">
<table class="format">
<caption>
Layout: Superblock (Versions 0 and 1)
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan="4"><br />Format Signature
<em>(8 bytes)</em><br /><br /></td>
</tr>
<tr>
<td>Version # of Superblock</td>
<td>Version # of File’s Free Space Storage</td>
<td>Version # of Root Group Symbol Table Entry</td>
<td>Reserved <em>(zero)</em></td>
</tr>
<tr>
<td>Version Number of Shared Header Message Format</td>
<td>Size of Offsets</td>
<td>Size of Lengths</td>
<td>Reserved <em>(zero)</em></td>
</tr>
<tr>
<td colspan="2">Group Leaf Node K</td>
<td colspan="2">Group Internal Node K</td>
</tr>
<tr>
<td colspan="4">File Consistency Flags</td>
</tr>
<tr>
<td colspan="2" style="border:dotted;">Indexed Storage Internal Node K<sup>1</sup></td>
<td colspan="2" style="border:dotted;">Reserved
<em>(zero)</em><sup>1</sup></td>
</tr>
<tr>
<td colspan="4"><br />Base Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Address of File Free space Info<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />End of File Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Driver Information Block Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4">Root Group Symbol Table Entry</td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with a ‘1’ in the above table are
new in version 1 of the superblock.)
</td></tr>
<tr>
<td> </td>
<td>
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Superblock (Versions 0 and 1)
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Format Signature</p></td>
<td><p>This field contains a constant value and can be used to
quickly identify a file as being an HDF5 file. The
constant value is designed to allow easy identification of
an HDF5 file and to allow certain types of data corruption
to be detected. The file signature of an HDF5 file always
contains the following values:</p>
<center>
<table border align="center" cellpadding="4">
<tr align="center">
<td align="right">Decimal:</td>
<td width="8%">137</td>
<td width="8%">72</td>
<td width="8%">68</td>
<td width="8%">70</td>
<td width="8%">13</td>
<td width="8%">10</td>
<td width="8%">26</td>
<td width="8%">10</td>
</tr>
<tr align="center">
<td align="right">Hexadecimal:</td>
<td>89</td>
<td>48</td>
<td>44</td>
<td>46</td>
<td>0d</td>
<td>0a</td>
<td>1a</td>
<td>0a</td>
</tr>
<tr align="center">
<td align="right">ASCII C Notation:</td>
<td>\211</td>
<td>H</td>
<td>D</td>
<td>F</td>
<td>\r</td>
<td>\n</td>
<td>\032</td>
<td>\n</td>
</tr>
</table>
</center>
<p>This signature both identifies the file as an HDF5 file
and provides for immediate detection of common
file-transfer problems. The first two bytes distinguish
HDF5 files on systems that expect the first two bytes to
identify the file type uniquely. The first byte is
chosen as a non-ASCII value to reduce the probability
that a text file may be misrecognized as an HDF5 file;
also, it catches bad file transfers that clear bit
7. Bytes two through four name the format. The CR-LF
sequence catches bad file transfers that alter newline
sequences. The control-Z character stops file display
under MS-DOS. The final line feed checks for the inverse
of the CR-LF translation problem. (This is a direct
descendent of the
<a href="http://www.libpng.org/pub/png/spec/iso/index-object.html#5PNG-file-signature">PNG</a> file
signature.)</p>
<p><em>This field is present in version 0+ of the superblock.</em>
</p></td>
</tr>
<tr>
<td><p>Version Number of the Superblock</p></td>
<td><p>This value is used to determine the format of the
information in the superblock. When the format of the
information in the superblock is changed, the version number
is incremented to the next integer and can be used to
determine how the information in the superblock is
formatted.</p>
<p>Values of 0, 1 and 2 are defined for this field (the
format of version 2 is described below, not here).
</p>
<p><em>This field is present in version 0+ of the superblock.</em>
</p>
</td>
</tr>
<tr>
<td><p>Version Number of the File’s Free Space
Information</p></td>
<td>
<p>This value is used to determine the format of the
file’s free space information.
</p>
<p>The only value currently valid in this field is ‘0’, which
indicates that the file’s free space is as described
<a href="#FreeSpaceManager">below</a>.
</p>
<p><em>This field is present in versions 0 and 1 of the
superblock.</em>
</p>
</td>
</tr>
<tr>
<td><p>Version Number of the Root Group Symbol Table
Entry</p></td>
<td><p>This value is used to determine the format of the
information in the Root Group Symbol Table Entry. When the
format of the information in that field is changed, the
version number is incremented to the next integer and can be
used to determine how the information in the field
is formatted.</p>
<p>The only value currently valid in this field is ‘0’,
which indicates that the root group symbol table entry is
formatted as described <a href="#SymbolTableEntry">below</a>.</p>
<p><em>This field is present in version 0 and 1 of the
superblock.</em></p>
</td>
</tr>
<tr>
<td><p>Version Number of the Shared Header Message Format</p></td>
<td><p>This value is used to determine the format of the
information in a shared object header message. Since the format
of the shared header messages differs from the other private
header messages, a version number is used to identify changes
in the format.
</p>
<p>The only value currently valid in this field is ‘0’, which
indicates that shared header messages are formatted as
described <a href="#ObjectHeaderMessages">below</a>.
</p>
<p><em>This field is present in version 0 and 1 of the superblock.</em>
</p>
</td>
</tr>
<tr>
<td><p><a name="SizeOfOffsetsV0">Size of Offsets</a></p></td>
<td><p>This value contains the number of bytes used to store
addresses in the file. The values for the addresses of
objects in the file are offsets relative to a base address,
usually the address of the superblock signature. This
allows a wrapper to be added after the file is created
without invalidating the internal offset locations.
</p>
<p><em>This field is present in version 0+ of the superblock.</em>
</p>
</td>
</tr>
<tr>
<td><p><a name="SizeOfLengthsV0">Size of Lengths</a></p></td>
<td><p>This value contains the number of bytes used to store
the size of an object.
</p>
<p><em>This field is present in version 0+ of the superblock.</em>
</p>
</td>
</tr>
<tr>
<td><p>Group Leaf Node K</p></td>
<td>
<p>Each leaf node of a group B-tree will have at
least this many entries but not more than twice this
many. If a group has a single leaf node then it
may have fewer entries.
</p>
<p>This value must be greater than zero.
</p>
<p>See the <a href="#Btrees">description</a> of B-trees below.
</p>
<p><em>This field is present in version 0 and 1 of the superblock.</em>
</p>
</td>
</tr>
<tr>
<td><p>Group Internal Node K</p></td>
<td>
<p>Each internal node of a group B-tree will have at
least this many entries but not more than twice this
many. If the group has only one internal
node then it might have fewer entries.
</p>
<p>This value must be greater than zero.
</p>
<p>See the <a href="#Btrees">description</a> of B-trees below.
</p>
<p><em>This field is present in version 0 and 1 of the superblock.</em>
</p>
</td>
</tr>
<tr>
<td><p>File Consistency Flags</p></td>
<td>
<p>This field is unused and should be ignored.
</p>
<p><em>This field is present in version 0+ of the superblock.</em>
</p>
</td>
</tr>
<tr>
<td><p>Indexed Storage Internal Node K</p></td>
<td>
<p>Each internal node of an indexed storage B-tree will have at
least this many entries but not more than twice this
many. If the index storage B-tree has only one internal
node then it might have fewer entries.
</p>
<p>This value must be greater than zero.
</p>
<p>See the <a href="#Btrees">description</a> of B-trees below.
</p>
<p><em>This field is present in version 1 of the superblock.</em>
</p>
</td>
</tr>
<tr>
<td><p>Base Address</p></td>
<td>
<p>This is the absolute file address of the first byte of
the HDF5 data within the file. The library currently
constrains this value to be the absolute file address
of the superblock itself when creating new files;
future versions of the library may provide greater
flexibility. When opening an existing file and this address does
not match the offset of the superblock, the library assumes
that the entire contents of the HDF5 file have been adjusted in
the file and adjusts the base address and end of file address to
reflect their new positions in the file. Unless otherwise noted,
all other file addresses are relative to this base
address.
</p>
<p><em>This field is present in version 0+ of the superblock.</em>
</p>
</td>
</tr>
<tr>
<td><p>Address of Global Free-space Index</p></td>
<td>
<p>The file’s free space is not persistent for version 0 and 1 of
the superblock.
Currently this field always contains the
<a href="#UndefinedAddress">undefined address</a>.
</p>
<p><em>This field is present in version 0 and 1 of the superblock.</em>
</p>
</td>
</tr>
<tr>
<td><p>End of File Address</p></td>
<td>
<p>This is the absolute file address of the first byte past
the end of all HDF5 data. It is used to determine whether a
file has been accidentally truncated and as an address where
file data allocation can occur if space from the free list is
not used.
</p>
<p><em>This field is present in version 0+ of the superblock.</em>
</p>
</td>
</tr>
<tr>
<td><p>Driver Information Block Address</p></td>
<td>
<p>This is the relative file address of the file driver
information block which contains driver-specific
information needed to reopen the file. If there is no
driver information block then this entry should be the
<a href="#UndefinedAddress">undefined address</a>.
</p>
<p><em>This field is present in version 0 and 1 of the superblock.</em>
</p>
</td>
</tr>
<tr>
<td><p>Root Group Symbol Table Entry</p></td>
<td>
<p>This is the <a href="#SymbolTableEntry">symbol table entry</a>
of the root group, which serves as the entry point into
the group graph for the file.
</p>
<p><em>This field is present in version 0 and 1 of the superblock.</em>
</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<p>Versions 2 and 3 of the superblock are described below:</p>
<div align="center">
<table class="format">
<caption>
Layout: Superblock (Versions 2 and 3)
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan="4"><br />Format Signature
<em>(8 bytes)</em><br /><br /></td>
</tr>
<tr>
<td>Version # of Superblock</td>
<td>Size of Offsets</td>
<td>Size of Lengths</td>
<td>File Consistency Flags</td>
</tr>
<tr>
<td colspan="4"><br />Base Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Superblock Extension Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />End of File Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Root Group Object Header Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4">Superblock Checksum</td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Superblock (Versions 2 and 3)
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Format Signature</p></td>
<td>
<p>This field is the same as described for versions 0 and 1 of the
superblock.
</p></td>
</tr>
<tr>
<td><p>Version Number of the Superblock</p></td>
<td>
<p>This field has a value of 2 and has the same meaning as for
versions 0 and 1.
</p>
</td>
</tr>
<tr>
<td><p>Size of Offsets</p></td>
<td>
<p>This field is the same as described for
<a href="#SizeOfOffsetsV0">versions 0 and 1</a> of the
superblock.
</p>
</td>
</tr>
<tr>
<td><p>Size of Lengths</p></td>
<td>
<p>This field is the same as described for
<a href="#SizeOfLengthsV0">versions 0 and 1</a> of the
superblock.
</p>
</td>
</tr>
<tr>
<td><p>File Consistency Flags</p></td>
<td>
<p>For superblock version
2: This field is unused and should be ignored.</p>
<p>For superblock version
3: This value contains flags to ensure file consistency for
file locking. Currently, the following bit flags are defined:
<ul>
<li>Bit 0 if set indicates that the file has been opened for
write access.</li>
<li>Bit 1 is reserved for future use.</li>
<li>Bit 2 if set indicates that the file has been opened for
single-writer/multiple-reader (SWMR) write access.</li>
<li>Bits 3-7 are reserved for future use.</li>
</ul>
<p>
Bit 0 should be set as the first action when a file has been
opened for write access. Bit 2 should be set when a file
has been opened for SWMR write access. These two bits should
be cleared only as the final action when closing a file.
</p>
<p><em>This field is present in version 0+ of the superblock.</em>
</p>
<p><em>The size of this
field has been reduced from 4 bytes in superblock format
versions 0 and 1 to 1 byte.</em>
</p>
</td>
</tr>
<tr>
<td><p>Base Address</p></td>
<td>
<p>This field is the same as described for versions 0 and
1 of the superblock.
</p>
</td>
</tr>
<tr>
<td><p>Superblock Extension Address</p></td>
<td>
<p>The field is the address of the object header for the
<a href="#SuperblockExt">superblock extension</a>.
If there is no extension then this entry should be the
<a href="#UndefinedAddress">undefined address</a>.
</p>
</td>
</tr>
<tr>
<td><p>End of File Address</p></td>
<td>
<p>This field is the same as described for versions 0 and 1 of the
superblock.
</p>
</td>
</tr>
<tr>
<td><p>Root Group Object Header Address</p></td>
<td>
<p>This is the address of
the <a href="#DataObject">root group object header</a>,
which serves as the entry point into the group graph for the file.
</p>
</td>
</tr>
<tr>
<td><p>Superblock Checksum</p></td>
<td>
<p>The checksum for the superblock.
</p>
</td>
</tr>
</table>
</div>
<br />
<h3><a name="DriverInfo">
II.B. Disk Format: Level 0B - File Driver Info</a></h3>
<p>The <b>driver information block</b> is an optional region of the
file which contains information needed by the file driver
to reopen a file. The format is described below:</p>
<div align="center">
<table class="format">
<caption>
Layout: Driver Information Block
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td>Version</td>
<td colspan="3">Reserved</td>
</tr>
<tr>
<td colspan="4">Driver Information Size</td>
</tr>
<tr>
<td colspan="4"><br />Driver Identification
<em>(8 bytes)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br /><br />Driver Information
<em>(variable size)</em><br /><br /><br /></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Driver Information Block
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Version</p></td>
<td>
<p>The version number of the Driver Information Block.
This document describes version 0.
</p>
</td>
</tr>
<tr>
<td><p>Driver Information Size</p></td>
<td>
<p>The size in bytes of the <em>Driver Information</em> field.
</p>
</td>
</tr>
<tr>
<td><p>Driver Identification</p></td>
<td>
<p>This is an eight-byte ASCII string without null
termination which identifies the driver and/or version number
of the Driver Information Block. The predefined driver encoded
in this field by the HDF5 Library is identified by the
letters <code>NCSA</code> followed by the first four characters of
the driver name. If the Driver Information block is not
the original version then the last letter(s) of the
identification will be replaced by a version number in
ASCII, starting with 0.
</p>
<p>
Identification for user-defined drivers is also eight-byte long.
It can be arbitrary but should be unique to avoid
the four character prefix “NCSA”.
</p>
</td>
</tr>
<tr valign="top">
<td><p>Driver Information</p></td>
<td>Driver information is stored in a format defined by the
file driver (see description below).</td>
</tr>
</table>
</div>
<br />
<p>The two drivers encoded in the <em>Driver Identification</em>
field are as follows:</p>
<ul>
<li>
Multi driver:
<p>
The identifier for this driver is “NCSAmulti”.
This driver provides a mechanism for segregating raw data and different types of metadata
into multiple files.
These files are viewed by the library as a single virtual HDF5 file with a single file address.
A maximum of 6 files will be created for the following data:
superblock, B-tree, raw data, global heap, local heap, and object header.
More than one type of data can be written to the same file.
</p></li>
<li>
Family driver
<p>
The identifier for this driver is “NCSAfami” and is encoded in this field for library version 1.8 and after.
This driver is designed for systems that do not support files larger than 2 gigabytes
by splitting the HDF5 file address space across several smaller files.
It does nothing to segregate metadata and raw data;
they are mixed in the address space just as they would be in a single contiguous file.
</p></li>
</ul>
<p>The format of the <em>Driver Information</em> field for the
above two drivers are described below:</p>
<div align="center">
<table class="format">
<caption>
Layout: Multi Driver Information
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td>Member Mapping</td>
<td>Member Mapping</td>
<td>Member Mapping</td>
<td>Member Mapping</td>
</tr>
<tr>
<td>Member Mapping</td>
<td>Member Mapping</td>
<td>Reserved</td>
<td>Reserved</td>
</tr>
<tr>
<td colspan="4"><br />Address of Member File 1<br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />End of Address for Member File 1<br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Address of Member File 2<br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />End of Address for Member File 2<br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />... ...<br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Address of Member File N<br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />End of Address for Member File N<br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Name of Member File 1
<em>(variable size)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Name of Member File 2
<em>(variable size)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />... ...<br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Name of Member File N
<em>(variable size)</em><br /><br /></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Multi Driver Information
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Member Mapping</p></td>
<td><p>These fields are integer values from 1 to 6
indicating how the data can be mapped to or merged with another type of
data.
<table class="list">
<tr>
<th width="20%" align="center">Member Mapping</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center">1</td>
<td>The superblock data.</td>
</tr>
<tr>
<td align="center">2</td>
<td>The B-tree data.</td>
</tr>
<tr>
<td align="center">3</td>
<td>The raw data.</td>
</tr>
<tr>
<td align="center">4</td>
<td>The global heap data.</td>
</tr>
<tr>
<td align="center">5</td>
<td>The local heap data.</td>
</tr>
<tr>
<td align="center">6</td>
<td>The object header data.</td>
</tr>
</table></p>
<p>For example, if the third field has the value 3 and all the rest have the
value 1, it means there are two files: one for raw data, and one for superblock,
B-tree, global heap, local heap, and object header.</p>
</td>
</tr>
<tr>
<td><p>Reserved</p></td>
<td><p>These fields are reserved and should always be zero.</p></td>
</tr>
<tr>
<td><p>Address of Member File N</p></td>
<td><p>This field Specifies the virtual address at which the member file starts.</p>
<p>N is the number of member files.</p>
</td>
</tr>
<tr>
<td><p>End of Address for Member File N</p></td>
<td><p>This field is the end of the allocated address for the member file.
</p></td>
</tr>
<tr>
<td><p>Name of Member File N</p></td>
<td><p>This field is the null-terminated name of the member file and
its length should be multiples of 8 bytes.
Additional bytes will be padded with <em>NULL</em>s. The default naming
convention is <em>%s-X.h5</em>, where <em>X</em> is one of the letters
<em>s</em> (for superblock), <em>b</em> (for B-tree), <em>r</em> (for raw data),
<em>g</em> (for global heap), <em>l</em> (for local heap), and <em>o</em> (for
object header). The name of the whole HDF5 file will substitute the <em>%s</em>
in the string.
</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Family Driver Information
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td colspan="8"><br />Size of Member File<br /><br /></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Family Driver Information
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Size of Member File</p></td>
<td><p>This field is the size of the member file in the family of files.</p></td>
</tr>
</table>
</div>
<h3><a name="SuperblockExt">
II.C. Disk Format: Level 0C - Superblock Extension</a></h3>
<p>The <em>superblock extension</em> is used to store superblock metadata
which is either optional, or added after the version of the superblock
was defined. Superblock extensions may only exist when version 2
or later of the superblock is used. A superblock extension is an object
header which may hold the following messages:</p>
<ul>
<li>
<a href="#SOHMTableMessage">Shared Message Table message</a> containing
information to locate the master table of shared object header message
indices.</li>
<li>
<a href="#BtreeKValuesMessage">B-tree ‘K’ Values message</a> containing
non-default B-tree ‘K’ values.</li>
<li>
<a href="#DrvInfoMessage">Driver Info message</a> containing information
needed by the file driver in order to reopen a file.
See also the
<a href="#DriverInfo">“Disk Format: Level 0B - File Driver
Info”</a> section above.</li>
<li>
<a href="#FsinfoMessage">File Space Info message</a> containing
information about file space handling in the file.</li>
</ul>
<h2><a name="FileInfra">
III. Disk Format: Level 1 - File Infrastructure</a></h2>
<h3><a name="Btrees">
III.A. Disk Format: Level 1A - B-trees and B-tree Nodes</a></h3>
<p>B-trees allow flexible storage for objects which tend to grow
in ways that cause the object to be stored discontiguously. B-trees
are described in various algorithms books including “Introduction to
Algorithms” by Thomas H. Cormen, Charles E. Leiserson, and Ronald
L. Rivest. B-trees are used in several places in the HDF5 file format,
when an index is needed for another data structure.</p>
<p>The version 1 B-tree structure described below is the original
index structure. The version 1 B-trees are being phased out in
favor of the version 2 B-trees described below. Note that both
types of structures may be found in the same file depending on
the application settings when creating the file.</p>
<h4><a name="V1Btrees">
III.A.1. Disk Format: Level 1A1 - Version 1 B-trees</a></h4>
<p>Version 1 B-trees in HDF5 files are an implementation of the
B-link tree. The sibling nodes at a particular level in
the tree are stored in a doubly-linked list. See the
“Efficient Locking for Concurrent Operations on B-trees”
paper by Phillip Lehman and S. Bing Yao as published in the
<cite>ACM Transactions on Database Systems</cite>, Vol. 6, No. 4,
December 1981.</p>
<p>The B-trees implemented by the file format contain one more
key than the number of children. In other words, each child
pointer out of a B-tree node has a left key and a right key.
The pointers out of internal nodes point to sub-trees while
the pointers out of leaf nodes point to symbol nodes and
raw data chunks.
Aside from that difference, internal nodes and leaf nodes
are identical.</p>
<div align="center">
<table class="format">
<caption>
Layout: B-tree Nodes
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan="4">Signature</td>
</tr>
<tr>
<td>Node Type</td>
<td>Node Level</td>
<td colspan="2">Entries Used</td>
</tr>
<tr>
<td colspan="4"><br />Address of Left Sibling<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Address of Right Sibling<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4">Key 1 <em>(variable size)</em></td>
</tr>
<tr>
<td colspan="4"><br />Address of Child 1<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4">Key 2 <em>(variable size)</em></td>
</tr>
<tr>
<td colspan="4"><br />Address of Child 2<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4">...</td>
</tr>
<tr>
<td colspan="4">Key 2<em>K</em> <em>(variable size)</em></td>
</tr>
<tr>
<td colspan="4"><br />Address of Child 2<em>K</em><sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4">Key 2<em>K</em>+1
<em>(variable size)</em></td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: B-tree Nodes
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Signature</p></td>
<td>
<p>The ASCII character string “<code>TREE</code>”
is used to indicate the beginning of a B-tree node. This
gives file consistency checking utilities a better chance
of reconstructing a damaged file.
</p>
</td>
</tr>
<tr>
<td><p>Node Type</p></td>
<td>
<p>Each B-tree points to a particular type of data.
This field indicates the type of data as well as
implying the maximum degree <em>K</em> of the tree and
the size of each Key field.
<table class="list">
<tr>
<th width="20%" align="center">Node Type</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center">0</td>
<td>This tree points to group nodes.</td>
</tr>
<tr>
<td align="center">1</td>
<td>This tree points to raw data chunk nodes.</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Node Level</p></td>
<td>
<p>The node level indicates the level at which this node
appears in the tree (leaf nodes are at level zero). Not
only does the level indicate whether child pointers
point to sub-trees or to data, but it can also be used
to help file consistency checking utilities reconstruct
damaged trees.
</p>
</td>
</tr>
<tr valign="top">
<td><p>Entries Used</p></td>
<td>
<p>This determines the number of children to which this
node points. All nodes of a particular type of tree
have the same maximum degree, but most nodes will point
to less than that number of children. The valid child
pointers and keys appear at the beginning of the node
and the unused pointers and keys appear at the end of
the node. The unused pointers and keys have undefined
values.
</p>
</td>
</tr>
<tr valign="top">
<td><p>Address of Left Sibling</p></td>
<td>
<p>This is the relative file address of the left sibling of
the current node. If the current
node is the left-most node at this level then this field
is the <a href="#UndefinedAddress">undefined address</a>.
</p>
</td>
</tr>
<tr valign="top">
<td><p>Address of Right Sibling</p></td>
<td>
<p>This is the relative file address of the right sibling of
the current node. If the current
node is the right-most node at this level then this
field is the <a href="#UndefinedAddress">undefined address</a>.
</p>
</td>
</tr>
<tr valign="top">
<td><p>Keys and Child Pointers</p></td>
<td>
<p>Each tree has 2<em>K</em>+1 keys with 2<em>K</em>
child pointers interleaved between the keys. The number
of keys and child pointers actually containing valid
values is determined by the node’s <em>Entries
Used</em> field. If that field is <em>N</em>, then the
B-tree contains <em>N</em> child pointers and
<em>N</em>+1 keys.
</p>
</td>
</tr>
<tr valign="top">
<td><p>Key</p></td>
<td>
<p>The format and size of the key values is determined by
the type of data to which this tree points. The keys are
ordered and are boundaries for the contents of the child
pointer; that is, the key values represented by child
<em>N</em> fall between Key <em>N</em> and Key
<em>N</em>+1. Whether the interval is open or closed on
each end is determined by the type of data to which the
tree points.
</p>
<p>
The format of the key depends on the node type.
For nodes of node type 0 (group nodes), the key is formatted as
follows:
<table class="list">
<tr>
<td width="20%">A single field of
<i><a href="#SizeOfLengthsV0">Size of Lengths</a></i>
bytes:</td>
<td width="80%">Indicates the byte offset into the local heap
for the first object name in the subtree which
that key describes.
</td>
</tr>
</table>
</p>
<p>
For nodes of node type 1 (chunked raw data nodes), the key is
formatted as follows:
<table class="list">
<tr>
<td width="20%">Bytes 1-4:</td>
<td width="80%">Size of chunk in bytes.</td>
</tr>
<tr>
<td>Bytes 4-8:</td>
<td>Filter mask, a 32-bit bit field indicating which
filters have been skipped for this chunk. Each filter
has an index number in the pipeline (starting at 0, with
the first filter to apply) and if that filter is skipped,
the bit corresponding to its index is set.</td>
</tr>
<tr>
<td>(<em>D + 1</em>) 64-bit fields:</td>
<td>The offset of the
chunk within the dataset where <i>D</i> is the number
of dimensions of the dataset, and the last value is the
offset within the dataset’s datatype and should
always be zero. For example, if
a chunk in a 3-dimensional dataset begins at the
position <code>[5,5,5]</code>, there will be three
such 64-bit values, each with the value of
<code>5</code>, followed by a <code>0</code> value.</td>
</tr>
</table>
</p>
</td>
</tr>
<tr valign="top">
<td><p>Child Pointer</p></td>
<td>
<p>The tree node contains file addresses of subtrees or
data depending on the node level. Nodes at Level 0 point
to data addresses, either raw data chunks or group nodes.
Nodes at non-zero levels point to other nodes of the
same B-tree.
</p>
<p>For raw data chunk nodes, the child pointer is the address
of a single raw data chunk. For group nodes, the child pointer
points to a <a href="#SymbolTable">symbol table</a>, which contains
information for multiple symbol table entries.
</p>
</td>
</tr>
</table>
</div>
<p>
Conceptually, each B-tree node looks like this:</p>
<center>
<table>
<tr valign="top" align="center">
<td>key[0]</td><td> </td>
<td>child[0]</td><td> </td>
<td>key[1]</td><td> </td>
<td>child[1]</td><td> </td>
<td>key[2]</td><td> </td>
<td>...</td><td> </td>
<td>...</td><td> </td>
<td>key[<i>N</i>-1]</td><td> </td>
<td>child[<i>N</i>-1]</td><td> </td>
<td>key[<i>N</i>]</td>
</tr>
</table>
</center>
<br />
where child[<i>i</i>] is a pointer to a sub-tree (at a level
above Level 0) or to data (at Level 0).
Each key[<i>i</i>] describes an <i>item</i> stored by the B-tree
(a chunk or an object of a group node). The range of values
represented by child[<i>i</i>] is indicated by key[<i>i</i>]
and key[<i>i</i>+1].
<p>The following question must next be answered:
“Is the value described by key[<i>i</i>] contained in
child[<i>i</i>-1] or in child[<i>i</i>]?”
The answer depends on the type of tree.
In trees for groups (node type 0), the object described by
key[<i>i</i>] is the greatest object contained in
child[<i>i</i>-1] while in chunk trees (node type 1) the
chunk described by key[<i>i</i>] is the least chunk in
child[<i>i</i>].</p>
<p>That means that key[0] for group trees is sometimes unused;
it points to offset zero in the heap, which is always the
empty string and compares as “less-than” any valid
object name.</p>
<p>And key[<i>N</i>] for chunk trees is sometimes unused;
it contains a chunk offset which compares as “greater-than”
any other chunk offset and has a chunk byte size of zero
to indicate that it is not actually allocated.</p>
<h4><a name="V2Btrees">
III.A.2. Disk Format: Level 1A2 - Version 2 B-trees</a></h4>
<p>Version 2 (v2) B-trees are “traditional” B-trees
with one major difference. Instead of just using a simple pointer
(or address in the file) to a child of an internal node, the pointer
to the child node contains two additional pieces of information:
the number of records in the child node itself, and the total number
of records in the child node and all its descendants. Storing this
additional information allows fast array-like indexing to locate
the n<sup>th</sup> record in the B-tree.</p>
<p>The entry into a version 2 B-tree is a header which contains global
information about the structure of the B-tree. The <em>root node
address</em>
field in the header points to the B-tree root node, which is either an
internal or leaf node, depending on the value in the header’s
<em>depth</em> field. An internal node consists of records plus
pointers to further leaf or internal nodes in the tree. A leaf node
consists of solely of records. The format of the records depends on
the B-tree type (stored in the header).</p>
<div align="center">
<table class="format">
<caption>
Layout: Version 2 B-tree Header
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<tr>
<td colspan="4">Signature</td>
</tr>
<tr>
<td>Version</td>
<td>Type</td>
<td colspan="2" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4">Node Size</td>
</tr>
<tr>
<td colspan="2">Record Size</td>
<td colspan="2">Depth</td>
</tr>
<tr>
<td>Split Percent</td>
<td>Merge Percent</td>
<td colspan="2" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4"><br />Root Node Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="2">Number of Records in Root Node</td>
<td colspan="2" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4"><br />Total Number of Records in B-tree<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4">Checksum</td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
<tr>
<td> </td>
<td>
(Items marked with an ‘L’ in the above table are
of the size specified in the <a href="#SizeOfLengthsV0">Size
of Lengths</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Version 2 B-tree Header
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Signature</p></td>
<td>
<p>The ASCII character string “<code>BTHD</code>”
is used to indicate the header of a version 2 (v2) B-tree
node.
</p>
</td>
</tr>
<tr>
<td><p>Version</p></td>
<td>
<p>The version number for this B-tree header. This document
describes version 0.
</p>
</td>
</tr>
<tr>
<td><p>Type</p></td>
<td>
<p>This field indicates the type of B-tree:
<table class="list">
<tr>
<th width="20%" align="center">Value</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center">0</td>
<td>This B-tree is used for testing only. This
value should <em>not</em> be used for storing
records in actual HDF5 files.
</td>
</tr>
<tr>
<td align="center">1</td>
<td>This B-tree is used for indexing indirectly accessed,
non-filtered ‘huge’ fractal heap objects.
</td>
</tr>
<tr>
<td align="center">2</td>
<td>This B-tree is used for indexing indirectly accessed,
filtered ‘huge’ fractal heap objects.
</td>
</tr>
<tr>
<td align="center">3</td>
<td>This B-tree is used for indexing directly accessed,
non-filtered ‘huge’ fractal heap objects.
</td>
</tr>
<tr>
<td align="center">4</td>
<td>This B-tree is used for indexing directly accessed,
filtered ‘huge’ fractal heap objects.
</td>
</tr>
<tr>
<td align="center">5</td>
<td>This B-tree is used for indexing the ‘name’ field for
links in indexed groups.
</td>
</tr>
<tr>
<td align="center">6</td>
<td>This B-tree is used for indexing the ‘creation order’
field for links in indexed groups.
</td>
</tr>
<tr>
<td align="center">7</td>
<td>This B-tree is used for indexing shared object header
messages.
</td>
</tr>
<tr>
<td align="center">8</td>
<td>This B-tree is used for indexing the ‘name’ field for
indexed attributes.
</td>
</tr>
<tr>
<td align="center">9</td>
<td>This B-tree is used for indexing the ‘creation order’
field for indexed attributes.
</td>
</tr>
<tr>
<td align="center">10</td>
<td>This B-tree is used for indexing chunks of
datasets with no filters and with more than one
dimension of unlimited extent.
</td>
</tr>
<tr>
<td align="center">11</td>
<td>This B-tree is used for indexing chunks of
datasets with filters and more than one dimension
of unlimited extent.
</td>
</tr>
</table></p>
<p>The format of records for each type is described below.</p>
</td>
</tr>
<tr valign="top">
<td><p>Node Size</p></td>
<td>
<p>This is the size in bytes of all B-tree nodes.
</p>
</td>
</tr>
<tr valign="top">
<td><p>Record Size</p></td>
<td>
<p>This field is the size in bytes of the B-tree record.
</p>
</td>
</tr>
<tr valign="top">
<td><p>Depth</p></td>
<td>
<p>This is the depth of the B-tree.
</p>
</td>
</tr>
<tr valign="top">
<td><p>Split Percent</p></td>
<td>
<p>The percent full that a node needs to increase above before it
is split.
</p>
</td>
</tr>
<tr valign="top">
<td><p>Merge Percent</p></td>
<td>
<p>The percent full that a node needs to be decrease below before it
is split.
</p>
</td>
</tr>
<tr valign="top">
<td><p>Root Node Address</p></td>
<td>
<p>This is the address of the root B-tree node. A B-tree with
no records will have the <a href="#UndefinedAddress">undefined
address</a> in this field.
</p>
</td>
</tr>
<tr valign="top">
<td><p>Number of Records in Root Node</p></td>
<td>
<p>This is the number of records in the root node.
</p>
</td>
</tr>
<tr valign="top">
<td><p>Total Number of Records in B-tree</p></td>
<td>
<p>This is the total number of records in the entire B-tree.
</p>
</td>
</tr>
<tr valign="top">
<td><p>Checksum</p></td>
<td>
<p>This is the checksum for the B-tree header.
</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Version 2 B-tree Internal Node
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan="4">Signature</td>
</tr>
<tr>
<td>Version</td>
<td>Type</td>
<td colspan="2">Records 0, 1, 2...N-1 <em>(variable size)</em></td>
</tr>
<tr>
<td colspan="4"><br />Child Node Pointer 0<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Number of Records N<sub>0</sub> for Child
Node 0 <em>(variable size)</em></td>
</tr>
<tr>
<td colspan="4"><br />Total Number of Records for Child Node 0
<em>(optional, variable size)</em></td>
</tr>
<tr>
<td colspan="4"><br />Child Node Pointer 1<sup>O</sup><br /><br /></td>
</tr>
<td colspan="4"><br />Number of Records N<sub>1</sub> for
Child Node 1 <em>(variable size)</em></td>
</tr>
<tr>
<td colspan="4"><br />Total Number of Records for Child Node 1
<em>(optional, variable size)</em></td>
</tr>
<tr>
<td colspan="4">...</td>
</tr>
<tr>
<td colspan="4"><br />Child Node Pointer N<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Number of Records N<sub>n</sub> for
Child Node N <em>(variable size)</em></td>
</tr>
<tr>
<td colspan="4"><br />Total Number of Records for Child Node N
<em>(optional, variable size)</em></td>
</tr>
<tr>
<td colspan="4">Checksum</td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Version 2 B-tree Internal Node
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Signature</p></td>
<td>
<p>The ASCII character string “<code>BTIN</code>” is
used to indicate the internal node of a B-tree.
</p>
</td>
</tr>
<tr>
<td><p>Version</p></td>
<td>
<p>The version number for this B-tree internal node.
This document describes version 0.
</p>
</td>
</tr>
<tr>
<td><p>Type</p></td>
<td>
<p>This field is the type of the B-tree node. It should always
be the same as the B-tree type in the header.
</p>
</td>
</tr>
<tr>
<td><p>Records</p></td>
<td>
<p>The size of this field is determined by the number of records
for this node and the record size (from the header). The format
of records depends on the type of B-tree.
</p>
</td>
</tr>
<tr>
<td><p>Child Node Pointer</p></td>
<td>
<p>This field is the address of the child node pointed to by the
internal node.
</p>
</td>
</tr>
<tr>
<td><p>Number of Records in Child Node</p></td>
<td>
<p>This is the number of records in the child node pointed to by
the corresponding <em>Node Pointer</em>.
</p>
<p>The number of bytes used to store this field is determined by
the maximum possible number of records able to be stored in the
child node.
</p>
<p>
The maximum number of records in a child node is computed
in the following way:
<ul>
<li>Subtract the fixed size overhead for
the child node (for example, its signature, version,
checksum, and so on and <em>one</em> pointer triplet
of information for the child node (because there is one
more pointer triplet than records in each internal node))
from the size of nodes for the B-tree. </li>
<li>Divide that result by the size of a record plus the
pointer triplet of information stored to reach each
child node from this node.</li>
</ul>
</p>
<p>
Note that leaf nodes do not encode any
child pointer triplets, so the maximum number of records in a
leaf node is just the node size minus the leaf node overhead,
divided by the record size.
</p>
<p>
Also note that the first level of internal nodes above the
leaf nodes do not encode the <em>Total Number of Records in Child
Node</em> value in the child pointer triplets (since it is the
same as the <em>Number of Records in Child Node</em>), so the
maximum number of records in these nodes is computed with the
equation above, but using (<em>Child Pointer</em>, <em>Number of
Records in Child Node</em>) pairs instead of triplets.
</p>
<p>
The number of
bytes used to encode this field is the least number of bytes
required to encode the maximum number of records in a child
node value for the child nodes below this level
in the B-tree.
</p>
<p>
For example, if the maximum number of child records is
123, one byte will be used to encode these values in this
node; if the maximum number of child records is
20000, two bytes will be used to encode these values in this
node; and so on. The maximum number of bytes used to
encode these values is 8 (in other words, an unsigned
64-bit integer).
</p>
</td>
</tr>
<tr>
<td><p>Total Number of Records in Child Node</p></td>
<td>
<p>This is the total number of records for the node pointed to by
the corresponding <em>Node Pointer</em> and all its children.
This field exists only in nodes whose depth in the B-tree node
is greater than 1 (in other words, the “twig”
internal nodes, just above leaf nodes, do not store this
field in their child node pointers).
</p>
<p>The number of bytes used to store this field is determined by
the maximum possible number of records able to be stored in the
child node and its descendants.
</p>
<p>
The maximum possible number of records able to be stored in a
child node and its descendants is computed iteratively, in the
following way: The maximum number of records in a leaf node
is computed, then that value is used to compute the maximum
possible number of records in the first level of internal nodes
above the leaf nodes. Multiplying these two values together
determines the maximum possible number of records in child node
pointers for the level of nodes two levels above leaf nodes.
This process is continued up to any level in the B-tree.
</p>
<p>
The number of bytes used to encode this value is computed in
the same way as for the <em>Number of Records in Child Node</em>
field.
</p>
</td>
</tr>
<tr>
<td><p>Checksum</p></td>
<td>
<p>This is the checksum for this node.
</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Version 2 B-tree Leaf Node
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan="4">Signature</td>
</tr>
<tr>
<td>Version</td>
<td>Type</td>
<td colspan="2">Record 0, 1, 2...N-1 <em>(variable size)</em></td>
</tr>
<tr>
<td colspan="4">Checksum</td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Version 2 B-tree Leaf Node
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Signature</p></td>
<td>
<p>The ASCII character string “<code>BTLF</code>“
is used to indicate the leaf node of a version 2 (v2) B-tree.
</p>
</td>
</tr>
<tr>
<td><p>Version</p></td>
<td>
<p>The version number for this B-tree leaf node.
This document describes version 0.
</p>
</td>
</tr>
<tr>
<td><p>Type</p></td>
<td>
<p>This field is the type of the B-tree node. It should always
be the same as the B-tree type in the header.
</p>
</td>
</tr>
<tr>
<td><p>Records</p></td>
<td>
<p>The size of this field is determined by the number of records
for this node and the record size (from the header). The format
of records depends on the type of B-tree.
</p>
</td>
</tr>
<tr>
<td><p>Checksum</p></td>
<td>
<p>This is the checksum for this node.
</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<p>The record layout for each stored (in other words, non-testing)
B-tree type is as follows:</p>
<div align="center">
<table class="format">
<caption>
Layout: Version 2 B-tree, Type 1 Record Layout - Indirectly
Accessed, Non-filtered, ‘Huge’ Fractal Heap Objects
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan="4"><br />Huge Object Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Huge Object Length<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Huge Object ID<sup>L</sup><br /><br /></td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
<tr>
<td> </td>
<td>
(Items marked with an ‘L’ in the above table are
of the size specified in the <a href="#SizeOfLengthsV0">Size
of Lengths</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Version 2 B-tree, Type 1 Record Layout - Indirectly
Accessed, Non-filtered, ‘Huge’ Fractal Heap Objects
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Huge Object Address</p></td>
<td>
<p>The address of the huge object in the file.
</p>
</td>
</tr>
<tr>
<td><p>Huge Object Length</p></td>
<td>
<p>The length of the huge object in the file.
</p>
</td>
</tr>
<tr>
<td><p>Huge Object ID</p></td>
<td>
<p>The heap ID for the huge object.
</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Version 2 B-tree, Type 2 Record Layout - Indirectly
Accessed, Filtered, ‘Huge’ Fractal Heap Objects
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan="4"><br />Filtered Huge Object Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Filtered Huge Object Length<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4">Filter Mask</td>
</tr>
<tr>
<td colspan="4"><br />Filtered Huge Object Memory Size<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Huge Object ID<sup>L</sup><br /><br /></td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
<tr>
<td> </td>
<td>
(Items marked with an ‘L’ in the above table are
of the size specified in the <a href="#SizeOfLengthsV0">Size
of Lengths</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Version 2 B-tree, Type 2 Record Layout - Indirectly
Accessed, Filtered, ‘Huge’ Fractal Heap Objects
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Filtered Huge Object Address</p></td>
<td>
<p>The address of the filtered huge object in the file.
</p>
</td>
</tr>
<tr>
<td><p>Filtered Huge Object Length</p></td>
<td>
<p>The length of the filtered huge object in the file.
</p>
</td>
</tr>
<tr>
<td><p>Filter Mask</p></td>
<td>
<p>A 32-bit bit field indicating which filters have been skipped for
this chunk. Each filter has an index number in the pipeline
(starting at 0, with the first filter to apply) and if that
filter is skipped, the bit corresponding to its index is set.
</p>
</td>
</tr>
<tr>
<td><p>Filtered Huge Object Memory Size</p></td>
<td>
<p>The size of the de-filtered huge object in memory.
</p>
</td>
</tr>
<tr>
<td><p>Huge Object ID</p></td>
<td>
<p>The heap ID for the huge object.
</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Version 2 B-tree, Type 3 Record Layout - Directly
Accessed, Non-filtered, ‘Huge’ Fractal Heap Objects
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan="4"><br />Huge Object Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Huge Object Length<sup>L</sup><br /><br /></td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
<tr>
<td> </td>
<td>
(Items marked with an ‘L’ in the above table are
of the size specified in the <a href="#SizeOfLengthsV0">Size
of Lengths</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Version 2 B-tree, Type 3 Record Layout - Directly
Accessed, Non-filtered, ‘Huge’ Fractal Heap Objects
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Huge Object Address</p></td>
<td>
<p>The address of the huge object in the file.
</p>
</td>
</tr>
<tr>
<td><p>Huge Object Length</p></td>
<td>
<p>The length of the huge object in the file.
</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Version 2 B-tree, Type 4 Record Layout - Directly
Accessed, Filtered, ‘Huge’ Fractal Heap Objects
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan="4"><br />Filtered Huge Object Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Filtered Huge Object Length<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4">Filter Mask</td>
</tr>
<tr>
<td colspan="4"><br />Filtered Huge Object Memory Size<sup>L</sup><br /><br /></td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
<tr>
<td> </td>
<td>
(Items marked with an ‘L’ in the above table are
of the size specified in the <a href="#SizeOfLengthsV0">Size
of Lengths</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Version 2 B-tree, Type 4 Record Layout - Directly
Accessed, Filtered, ‘Huge’ Fractal Heap Objects
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Filtered Huge Object Address</p></td>
<td>
<p>The address of the filtered huge object in the file.
</p>
</td>
</tr>
<tr>
<td><p>Filtered Huge Object Length</p></td>
<td>
<p>The length of the filtered huge object in the file.
</p>
</td>
</tr>
<tr>
<td><p>Filter Mask</p></td>
<td>
<p>A 32-bit bit field indicating which filters have been skipped for
this chunk. Each filter has an index number in the pipeline
(starting at 0, with the first filter to apply) and if that
filter is skipped, the bit corresponding to its index is set.
</p>
</td>
</tr>
<tr>
<td><p>Filtered Huge Object Memory Size</p></td>
<td>
<p>The size of the de-filtered huge object in memory.
</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Version 2 B-tree, Type 5 Record Layout - Link Name
for Indexed Group
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan="4">Hash of Name</td>
</tr>
<tr>
<td colspan="4">ID <em>(bytes 1-4)</em></td>
</tr>
<tr>
<td colspan="3">ID <em>(bytes 5-7)</em></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Version 2 B-tree, Type 5 Record Layout - Link Name
for Indexed Group
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Hash</p></td>
<td>
<p>This field is hash value of the name for the link. The hash
value is the Jenkins’ lookup3 checksum algorithm applied to
the link’s name.
</p>
</td>
</tr>
<tr>
<td><p>ID</p></td>
<td>
<p>This is a 7-byte sequence of bytes and is the heap ID for the
link record in the group’s fractal heap.</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Version 2 B-tree, Type 6 Record Layout - Creation
Order for Indexed Group
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan="4"><br />Creation Order
<em>(8 bytes)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4">ID <em>(bytes 1-4)</em></td>
</tr>
<tr>
<td colspan="3">ID <em>(bytes 5-7)</em></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Version 2 B-tree, Type 6 Record Layout - Creation
Order for Indexed Group
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Creation Order</p></td>
<td>
<p>This field is the creation order value for the link.
</p>
</td>
</tr>
<tr>
<td><p>ID</p></td>
<td>
<p>This is a 7-byte sequence of bytes and is the heap ID for the
link record in the group’s fractal heap.</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Version 2 B-tree, Type 7 Record Layout - Shared
Object Header Messages (Sub-type 0 - Message in Heap)
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan>Message Location</td>
<td colspan="3" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4">Hash</td>
</tr>
<tr>
<td colspan="4">Reference Count</td>
</tr>
<tr>
<td colspan="4"><br />Heap ID <em>(8 bytes)</em><br /><br /></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Version 2 B-tree, Type 7 Record Layout - Shared
Object Header Messages (Sub-type 0 - Message in Heap)
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Message Location</p></td>
<td>
<p>This field Indicates the location where the message is stored:
<table class="list">
<tr>
<th width="20%" align="center">Value</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center">0</td>
<td>Shared message is stored in shared message index heap.
</td>
</tr>
<tr>
<td align="center">1</td>
<td>Shared message is stored in object header.
</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Hash</p></td>
<td>
<p>This field is hash value of the shared message. The hash
value is the Jenkins’ lookup3 checksum algorithm applied to
the shared message.</p>
</td>
</tr>
<tr>
<td><p>Reference Count</p></td>
<td>
<p>The number of objects which reference this message.</p>
</td>
</tr>
<tr>
<td><p>Heap ID</p></td>
<td>
<p>This is an 8-byte sequence of bytes and is the heap ID for the
shared message in the shared message index’s fractal heap.</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Version 2 B-tree, Type 7 Record Layout - Shared
Object Header Messages (Sub-type 1 - Message in Object Header)
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan>Message Location</td>
<td colspan="3" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4">Hash</td>
</tr>
<tr>
<td>Reserved (zero)</td>
<td>Message Type</td>
<td colspan="2">Object Header Index</td>
</tr>
<tr>
<td colspan="4"><br />Object Header Address<sup>O</sup><br /><br /></td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Version 2 B-tree, Type 7 Record Layout - Shared
Object Header Messages (Sub-type 1 - Message in Object Header)
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Message Location</p></td>
<td>
<p>This field Indicates the location where the message is stored:
<table class="list">
<tr>
<th width="20%" align="center">Value</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center">0</td>
<td>Shared message is stored in shared message index heap.
</td>
</tr>
<tr>
<td align="center">1</td>
<td>Shared message is stored in object header.
</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Hash</p></td>
<td>
<p>This field is hash value of the shared message. The hash
value is the Jenkins’ lookup3 checksum algorithm applied to
the shared message.</p>
</td>
</tr>
<tr>
<td><p>Message Type</p></td>
<td>
<p>The object header message type of the shared message.</p>
</td>
</tr>
<tr>
<td><p>Object Header Index</p></td>
<td>
<p>This field indicates that the shared message is the n<sup>th</sup> message
of its type in the specified object header.</p>
</td>
</tr>
<tr>
<td><p>Object Header Address</p></td>
<td>
<p>The address of the object header containing the shared message.</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Version 2 B-tree, Type 8 Record Layout - Attribute
Name for Indexed Attributes
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan="4"><br />Heap ID <em>(8 bytes)</em><br /><br /></td>
</tr>
<tr>
<td colspan>Message Flags</td>
<td colspan="3" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4">Creation Order</td>
</tr>
<tr>
<td colspan="4">Hash of Name</td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Version 2 B-tree, Type 8 Record Layout - Attribute
Name for Indexed Attributes
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Heap ID</p></td>
<td>
<p>This is an 8-byte sequence of bytes and is the heap ID for the
attribute in the object’s attribute fractal heap.</p>
</td>
</tr>
<tr>
<td><p>Message Flags</p></td>
<td><p>The object header message flags for the attribute message.</p>
</td>
</tr>
<tr>
<td><p>Creation Order</p></td>
<td>
<p>This field is the creation order value for the attribute.
</p>
</td>
</tr>
<tr>
<td><p>Hash</p></td>
<td>
<p>This field is hash value of the name for the attribute. The hash
value is the Jenkins’ lookup3 checksum algorithm applied to
the attribute’s name.
</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Version 2 B-tree, Type 9 Record Layout - Creation
Order for Indexed Attributes
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan="4"><br />Heap ID <em>(8 bytes)</em><br /><br /></td>
</tr>
<tr>
<td colspan>Message Flags</td>
<td colspan="3" bgcolor="#DDDDDD">
<em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4">Creation Order</td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Version 2 B-tree, Type 9 Record Layout - Creation
Order for Indexed Attributes
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Heap ID</p></td>
<td>
<p>This is an 8-byte sequence of bytes and is the heap ID for the
attribute in the object’s attribute fractal heap.</p>
</td>
</tr>
<tr>
<td><p>Message Flags</p></td>
<td>
<p>The object header message flags for the attribute message.</p>
</td>
</tr>
<tr>
<td><p>Creation Order</p></td>
<td>
<p>This field is the creation order value for the attribute.
</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<a name="V2BtType10"></a>
<div align="center">
<table class="format">
<caption>
<a name="V2BtreesType10"></a>
Layout: Version 2 B-tree, Type 10 Record Layout -
Non-filtered Dataset Chunks
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan="4"><br />Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Dimension 0 Scaled Offset
<em>(8 bytes)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Dimension 1 Scaled Offset
<em>(8 bytes)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />...<br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Dimension #n Scaled Offset
<em>(8 bytes)</em><br /><br /></td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Version 2 B-tree, Type 10 Record Layout -
Non-filtered Dataset Chunks
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Address</p></td>
<td>
<p>This field is the address of the dataset chunk in the file.</p>
</td>
</tr>
<tr>
<td><p>Dimension #n Scaled Offset</p></td>
<td>
<p>This field is the scaled offset of the chunk within the
dataset. <em>n</em> is the number of dimensions for the
dataset. The first scaled offset stored in the list is for
the slowest changing dimension, and the last scaled offset
stored is for the fastest changing dimension. Scaled offset
is calculated by dividing the chunk dimension sizes into
the chunk offsets.</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
<a name="V2BtreesType11"></a>
Layout: Version 2 B-tree, Type 11 Record Layout - Filtered
Dataset Chunks
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan="4"><br />Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Chunk Size
<em>(variable size; at most 8 bytes)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4">Filter Mask</td>
</tr>
<tr>
<td colspan="4"><br />Dimension 0 Scaled Offset
<em>(8 bytes)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Dimension 1 Scaled Offset
<em>(8 bytes)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />...<br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Dimension #n Scaled Offset
<em>(8 bytes)</em><br /><br /></td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Version 2 B-tree, Type 11 Record Layout - Filtered
Dataset Chunks
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Address</p></td>
<td>
<p>This field is the address of the dataset chunk in the file.</p>
</td>
</tr>
<tr>
<td><p>Chunk Size</p></td>
<td>
<p>This field is the size of the dataset chunk in bytes.</p>
</td>
</tr>
<tr>
<td><p>Filter Mask</p></td>
<td>
<p>This field is the filter mask which indicates the filter
to skip for the dataset chunk. Each filter has an index
number in the pipeline and if that filter is skipped,
the bit corresponding to its index is set.</p>
</td>
</tr>
<tr>
<td><p>Dimension #n Scaled Offset</p></td>
<td>
<p>This field is the scaled offset of the chunk within
the dataset. <em>n</em> is the number of dimensions for
the dataset. The first scaled offset stored in the list
is for the slowest changing dimension, and the last scaled
offset stored is for the fastest changing dimension.</p>
</td>
</tr>
</table>
</div>
<h3><a name="SymbolTable">
III.B. Disk Format: Level 1B - Group Symbol Table Nodes</a></h3>
<p>A group is an object internal to the file that allows
arbitrary nesting of objects within the file (including other
groups). A group maps a set of link names in the group to a set
of relative file addresses of objects in the file. Certain metadata
for an object to which the group points can be cached in the
group’s symbol table entry in addition to being in the
object’s header.</p>
<p>An HDF5 object name space can be stored hierarchically by
partitioning the name into components and storing each
component as a link in a group. The link for a
non-ultimate component points to the group containing
the next component. The link for the last
component points to the object being named.</p>
<p>One implementation of a group is a collection of symbol table
nodes indexed by a B-tree. Each symbol table node contains entries
for one or more links. If an attempt is made to add a link to an
already full symbol table node containing 2<em>K</em> entries, then
the node is split and one node contains <em>K</em> symbols and the
other contains <em>K</em>+1 symbols.</p>
<div align="center">
<table class="format">
<caption>
Layout: Symbol Table Node (A Leaf of a B-tree)
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan="4">Signature</td>
</tr>
<tr>
<td>Version Number</td>
<td>Reserved <em>(zero)</em></td>
<td colspan="2">Number of Symbols</td>
</tr>
<tr>
<td colspan="4"><br /><br />Group Entries<br /><br /><br /></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Symbol Table Node (A Leaf of a B-tree)
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Signature</p></td>
<td>
<p>The ASCII character string “<code>SNOD</code>” is
used to indicate the
beginning of a symbol table node. This gives file
consistency checking utilities a better chance of
reconstructing a damaged file.
</p>
</td>
</tr>
<tr>
<td><p>Version Number</p></td>
<td>
<p>The version number for the symbol table node. This
document describes version 1. (There is no version ‘0’
of the symbol table node)
</p>
</td>
</tr>
<tr>
<td><p>Number of Entries</p></td>
<td>
<p>Although all symbol table nodes have the same length,
most contain fewer than the maximum possible number of
link entries. This field indicates how many entries
contain valid data. The valid entries are packed at the
beginning of the symbol table node while the remaining
entries contain undefined values.
</p>
</td>
</tr>
<tr>
<td><p>Symbol Table Entries</p></td>
<td>
<p>Each link has an entry in the symbol table node.
The format of the entry is described below.
There are 2<em>K</em> entries in each group node, where
<em>K</em> is the “Group Leaf Node K” value from the
<a href="#Superblock">superblock</a>.
</p>
</td>
</tr>
</table>
</div>
<h3><a name="SymbolTableEntry">
III.C. Disk Format: Level 1C - Symbol Table Entry </a></h3>
<p>Each symbol table entry in a symbol table node is designed
to allow for very fast browsing of stored objects.
Toward that design goal, the symbol table entries
include space for caching certain constant metadata from the
object header.</p>
<div align="center">
<table class="format">
<caption>
Layout: Symbol Table Entry
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan="4"><br />Link Name Offset<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Object Header Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4">Cache Type</td>
</tr>
<tr>
<td colspan="4">Reserved <em>(zero)</em></td>
</tr>
<tr>
<td colspan="4"><br /><br />Scratch-pad Space
<em>(16 bytes)</em><br /><br /><br /></td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Symbol Table Entry
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Link Name Offset</p></td>
<td>
<p>This is the byte offset into the group’s local
heap for the name of the link. The name is null
terminated.
</p>
</td>
</tr>
<tr>
<td><p>Object Header Address</p></td>
<td>
<p>Every object has an object header which serves as a
permanent location for the object’s metadata. In addition
to appearing in the object header, some of the object’s metadata
can be cached in the scratch-pad space.
</p>
</td>
</tr>
<tr>
<td><p>Cache Type</p></td>
<td>
<p>The cache type is determined from the object header.
It also determines the format for the scratch-pad space:
<table class="list">
<tr>
<th width="20%" align="center">Type</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center">0</td>
<td>No data is cached by the group entry. This
is guaranteed to be the case when an object header
has a link count greater than one.
</td>
</tr>
<tr>
<td align="center">1</td>
<td>Group object header metadata is cached in the
scratch-pad space. This implies that the symbol table
entry refers to another group.
</td>
</tr>
<tr>
<td align="center">2</td>
<td>The entry is a symbolic link. The first four bytes
of the scratch-pad space are the offset into the local
heap for the link value. The object header address
will be undefined.
</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Reserved</p></td>
<td>
<p>These four bytes are present so that the scratch-pad
space is aligned on an eight-byte boundary. They are
always set to zero.
</p>
</td>
</tr>
<tr>
<td><p>Scratch-pad Space</p></td>
<td>
<p>This space is used for different purposes, depending
on the value of the Cache Type field. Any metadata
about an object represented in the scratch-pad
space is duplicated in the object header for that
object.
</p>
<p>
Furthermore, no data is cached in the group
entry scratch-pad space if the object header for
the object has a link count greater than one.
</p>
</td>
</tr>
</table>
</div>
<h4>Format of the Scratch-pad Space</h4>
<p>The symbol table entry scratch-pad space is formatted
according to the value in the Cache Type field.</p>
<p>If the Cache Type field contains the value zero
<code>(0)</code> then no information is
stored in the scratch-pad space.</p>
<p>If the Cache Type field contains the value one
<code>(1)</code>, then the scratch-pad space
contains cached metadata for another object header
in the following format:</p>
<div align="center">
<table class="format">
<caption>
Layout: Object Header Scratch-pad Format
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan="4"><br />Address of B-tree<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Address of Name Heap<sup>O</sup><br /><br /></td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Object Header Scratch-pad Format
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Address of B-tree</p></td>
<td>
<p>This is the file address for the root of the
group’s B-tree.
</p>
</td>
</tr>
<tr>
<td><p>Address of Name Heap</p></td>
<td>
<p>This is the file address for the group’s local
heap, in which are stored the group’s symbol names.
</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<p>If the Cache Type field contains the value two
<code>(2)</code>, then the scratch-pad space
contains cached metadata for a symbolic link
in the following format:</p>
<div align="center">
<table class="format">
<caption>
Layout: Symbolic Link Scratch-pad Format
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan="4">Offset to Link Value</td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Symbolic Link Scratch-pad Format
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Offset to Link Value</p></td>
<td>
<p>The value of a symbolic link (that is, the name of the
thing to which it points) is stored in the local heap.
This field is the 4-byte offset into the local heap for
the start of the link value, which is null terminated.
</p>
</td>
</tr>
</table>
</div>
<h3><a name="LocalHeap">
III.D. Disk Format: Level 1D - Local Heaps</a></h3>
<p>A local heap is a collection of small pieces of data that are particular
to a single object in the HDF5 file. Objects can be
inserted and removed from the heap at any time.
The address of a heap does not change once the heap is created.
For example, a group stores addresses of objects in symbol table nodes
with the names of links stored in the group’s local heap.
</p>
<div align="center">
<table class="format">
<caption>
Layout: Local Heap
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan="4">Signature</td>
</tr>
<tr>
<td>Version</td>
<td colspan="3">Reserved <em>(zero)</em></td>
</tr>
<tr>
<td colspan="4"><br />Data Segment Size<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Offset to Head of Free-list<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Address of Data Segment<sup>O</sup><br /><br /></td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘L’ in the above table are
of the size specified in the <a href="#SizeOfLengthsV0">Size
of Lengths</a> field in the superblock.)
</td></tr>
<tr>
<td> </td>
<td>
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Local Heap
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Signature</p></td>
<td>
<p>The ASCII character string “<code>HEAP</code>”
is used to indicate the
beginning of a heap. This gives file consistency
checking utilities a better chance of reconstructing a
damaged file.
</p>
</td>
</tr>
<tr>
<td><p>Version</p></td>
<td>
<p>Each local heap has its own version number so that new
heaps can be added to old files. This document
describes version zero (0) of the local heap.
</p>
</td>
</tr>
<tr>
<td><p>Data Segment Size</p></td>
<td>
<p>The total amount of disk memory allocated for the heap
data. This may be larger than the amount of space
required by the objects stored in the heap. The extra
unused space in the heap holds a linked list of free blocks.
</p>
</td>
</tr>
<tr>
<td><p>Offset to Head of Free-list</p></td>
<td>
<p>This is the offset within the heap data segment of the
first free block (or the
<a href="#UndefinedAddress">undefined address</a> if there is no
free block). The free block contains
<a href="#SizeOfLengthsV0">Size of Lengths</a> bytes that
are the offset of the next free block (or the
value ‘1’ if this is the
last free block) followed by Size of Lengths bytes that store
the size of this free block. The size of the free block includes
the space used to store the offset of the next free block and
the size of the current block, making the minimum size of a free
block 2 * Size of Lengths.
</p>
</td>
</tr>
<tr>
<td><p>Address of Data Segment</p></td>
<td>
<p>The data segment originally starts immediately after
the heap header, but if the data segment must grow as a
result of adding more objects, then the data segment may
be relocated, in its entirety, to another part of the
file.
</p>
</td>
</tr>
</table>
</div>
<p>Objects within a local heap should be aligned on an 8-byte boundary.</p>
<h3><a name="GlobalHeap">
III.E. Disk Format: Level 1E - Global Heap</a></h3>
<p>Each HDF5 file has a global heap which stores various types of
information which is typically shared between datasets. The
global heap was designed to satisfy these goals:</p>
<ol type="A">
<li>Repeated access to a heap object must be efficient without
resulting in repeated file I/O requests. Since global heap
objects will typically be shared among several datasets, it is
probable that the object will be accessed repeatedly.</li>
<li>Collections of related global heap objects should result in
fewer and larger I/O requests. For instance, a dataset of
object references will have a global heap object for each
reference. Reading the entire set of object references
should result in a few large I/O requests instead of one small
I/O request for each reference.</li>
<li>It should be possible to remove objects from the global heap
and the resulting file hole should be eligible to be reclaimed
for other uses.</li>
</ol>
<p>The implementation of the heap makes use of the memory management
already available at the file level and combines that with a new
object called a <em>collection</em> to achieve goal B. The global heap
is the set of all collections. Each global heap object belongs to
exactly one collection, and each collection contains one or more global
heap objects. For the purposes of disk I/O and caching, a collection is
treated as an atomic object, addressing goal A.
</p>
<p>When a global heap object is deleted from a collection (which
occurs when its reference count falls to zero), objects located
after the deleted object in the collection are packed down toward
the beginning of the collection, and the collection’s
global heap object 0 is created (if possible), or its size is
increased to account for the recently freed space. There are
no gaps between objects in each collection, with the possible
exception of the final space in the collection, if it is not
large enough to hold the header for the collection’s
global heap object 0. These features address goal C.
</p>
<p>The HDF5 Library creates global heap collections as needed, so there may
be multiple collections throughout the file. The set of all of them is
abstractly called the “global heap”, although they do not actually link
to each other, and there is no global place in the file where you can
discover all of the collections. The collections are found simply by
finding a reference to one through another object in the file. For
example, data of variable-length datatype elements is stored in the
global heap and is accessed via a global heap ID. The format for
global heap IDs is described at the end of this section.
</p>
<p>For more information on global heaps for virtual datasets, see
<a href="#GlobalHeapVDS">“Disk Format: Level 1F - Global Heap
Block for Virtual Datasets.”</a></p>
<br />
<div align="center">
<table class="format">
<caption>
Layout: A Global Heap Collection
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan="4">Signature</td>
</tr>
<tr>
<td>Version</td>
<td colspan="3">Reserved (zero)</td>
</tr>
<tr>
<td colspan="4"><br />Collection Size<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Global Heap Object 1<br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Global Heap Object 2<br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />...<br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Global Heap Object <em>N</em><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Global Heap Object 0 (free space)<br /><br /></td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘L’ in the above table are
of the size specified in the <a href="#SizeOfLengthsV0">Size
of Lengths</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: A Global Heap Collection
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Signature</p></td>
<td>
<p>The ASCII character string “<code>GCOL</code>”
is used to indicate the
beginning of a collection. This gives file consistency
checking utilities a better chance of reconstructing a
damaged file.
</p>
</td>
</tr>
<tr>
<td><p>Version</p></td>
<td>
<p>Each collection has its own version number so that new
collections can be added to old files. This document
describes version one (1) of the collections (there is no
version zero (0)).
</p>
</td>
</tr>
<tr>
<td><p>Collection Size</p></td>
<td>
<p>This is the size in bytes of the entire collection
including this field. The default (and minimum)
collection size is 4096 bytes which is a typical file
system block size. This allows for 127 16-byte heap
objects plus their overhead (the collection header of 16 bytes
and the 16 bytes of information about each heap object).
</p>
</td>
</tr>
<tr>
<td><p>Global Heap Object 1 through <em>N</em></p></td>
<td>
<p>The objects are stored in any order with no
intervening unused space.
</p>
</td>
</tr>
<tr>
<td><p>Global Heap Object 0</p></td>
<td>
<p>Global Heap Object 0 (zero), when present, represents the free
space in the collection. Free space always appears at the end of
the collection. If the free space is too small to store the header
for Object 0 (described below) then the header is implied and is not
written.
<p>
The field <em>Object Size</em> for Object 0 indicates the
amount of possible free space in the collection including the 16-byte
header size of Object 0.
</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Global Heap Object
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan="2">Heap Object Index</td>
<td colspan="2">Reference Count</td>
</tr>
<tr>
<td colspan="4">Reserved (zero)</td>
</tr>
<tr>
<td colspan="4"><br />Object Size<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Object Data<br /><br /></td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘L’ in the above table are
of the size specified in the <a href="#SizeOfLengthsV0">Size
of Lengths</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Global Heap Object
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Heap Object Index</p></td>
<td>
<p>Each object has a unique identification number within a
collection. The identification numbers are chosen so that
new objects have the smallest value possible with the
exception that the identifier <code>0</code> always refers to the
object which represents all free space within the
collection.
</p>
</td>
</tr>
<tr>
<td><p>Reference Count</p></td>
<td>
<p>All heap objects have a reference count field. An
object which is referenced from some other part of the
file will have a positive reference count. The reference
count for Object 0 is always zero.
</p>
</td>
</tr>
<tr>
<td><p>Reserved</p></td>
<td>
<p>Zero padding to align next field on an 8-byte boundary.
</p>
</td>
</tr>
<tr>
<td><p>Object Size</p></td>
<td>
<p>This is the size of the object data stored for the object.
The actual storage space allocated for the object data is rounded
up to a multiple of eight.
</p>
</td>
</tr>
<tr>
<td><p>Object Data</p></td>
<td>
<p>The object data is treated as a one-dimensional array
of bytes to be interpreted by the caller.
</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<p>
<a name="GlobalHeapID"></a>
The format for the ID used to locate an object in the global heap is
described here:</p>
<div align="center">
<table class="format">
<caption>
Layout: Global Heap ID
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan="4"><br />Collection Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4">Object Index</td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Global Heap ID
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Collection Address</p></td>
<td>
<p>This field is the address of the global heap collection
where the data object is stored.
</p>
</td>
</tr>
<tr>
<td><p>ID</p></td>
<td>
<p>This field is the index of the data object within the
global heap collection.
</p>
</td>
</tr>
</table>
</div>
<h3><a name="GlobalHeapVDS"> III.F. Disk Format: Level 1F - Global
Heap Block for Virtual Datasets</a></h3>
<p>The layout for the global heap block used with virtual datasets is
described below. For more information on global heaps, see
<a href="#GlobalHeap"></a>“Disk Format: Level 1E - Global Heap.”</p>
<br />
<div align="center">
<table class="format">
<caption>
Layout: Global Heap Block for Virtual Dataset
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td>Version</td>
<td colspan="3" bgcolor="#DDDDDD"><em>This space inserted
only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4"><br />Num Entries<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Source Filename #1 <em>(variable size)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Source Dataset #1 <em>(variable
size)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Source Selection #1 <em>(variable
size)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Virtual Selection #1 <em>(variable
size)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4">.<br />.<br />.<br /></td>
</tr>
<tr>
<td colspan="4"><br />Source Filename #n <em>(variable
size)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Source Dataset #n <em>(variable
size)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Source Selection #n <em>(variable
size)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Virtual Selection #n <em>(variable
size)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4">Checksum</td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘L’ in the above table are
of the size specified in the <a href="#SizeOfLengthsV0">Size
of Lengths</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Global Heap Block for Virtual Dataset
</caption>
<tr>
<th width="40%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Version</p></td>
<td>
<p>The version number for the block; the value is 0.</p>
</td>
</tr>
<tr>
<td><p>Num Entries</p></td>
<td><p>The number of entries in the block.</p>
</td>
</tr>
<tr>
<td><p>Source Filename #n</p></td>
<td>
<p>The source file name where the source dataset is located.
</p>
</td>
</tr>
<tr>
<td><p>Source Dataset #n</p></td>
<td><p>The source dataset name that is mapped to the
virtual dataset.</p></td>
</tr>
<tr>
<td><p>Source Selection #n</p></td>
<td>
<p>The <a href="#DataspaceSEL">dataspace selection</a> in the
source dataset that is mapped to the virtual selection.
</p>
</td>
</tr>
<tr>
<td><p>Virtual Selection #n</p></td>
<td>
<p>This is the <a href="#DataspaceSEL">dataspace selection</a> in the virtual dataset that is
mapped to the source selection.
</p>
</td>
</tr>
<tr>
<td><p>Checksum</p></td>
<td>
<p>This is the checksum for the block.</p>
</td>
</tr>
</table>
</div>
<br>
<h3><a name="FractalHeap">
III.G. Disk Format: Level 1G - Fractal Heap</a></h3>
<p>
Each fractal heap consists of a header and zero or more direct and
indirect blocks (described below). The header contains general
information as well as
initialization parameters for the doubling table. The <em>Address
of Root Block</em> field in the header points to the first direct or
indirect block in the heap.
</p>
<p>
Fractal heaps are based on a data structure called a <em>doubling
table</em>. A doubling table provides a mechanism for quickly
extending an array-like data structure that minimizes the number of
empty blocks in the heap, while retaining very fast lookup of any
element within the array. More information on fractal heaps and
doubling tables can be found in the RFC
“<a href="Supplements/FractalHeap/PrivateHeap.pdf">Private
Heaps in HDF5</a>.”
</p>
<p>
The fractal heap implements the doubling table structure with
indirect and direct blocks.
Indirect blocks in the heap do not actually contain data for
objects in the heap, their “size” is abstract -
they represent the indexing structure for locating the
direct blocks in the doubling table.
Direct blocks
contain the actual data for objects stored in the heap.
</p>
<p>
All indirect blocks have a constant number of block entries in each
row, called the <em>width</em> of the doubling table
(see <em>Table Width</em> field in the header).
The number
of rows for each indirect block in the heap is determined by the
size of the block that the indirect block represents in the
doubling table (calculation of this is shown below) and is
constant, except for the “root”
indirect block, which expands and shrinks its number of rows as
needed.
</p>
<p>
Blocks in the first <em>two</em> rows of an indirect block
are <em>Starting Block Size</em> number of bytes in size.
For example, if the row <em>width</em> of the doubling table is 4,
then the first eight block entries in the
indirect block are <em>Starting Block Size</em> number of bytes in size.
The blocks in each subsequent row are twice the size of
the blocks in the previous row. In other words, blocks in
the third row are twice the <em>Starting Block Size</em>,
blocks in the fourth row are four times the
<em>Starting Block Size</em>, and so on. Entries for
blocks up to the <em>Maximum Direct Block Size</em> point to
direct blocks, and entries for blocks greater than that size
point to further indirect blocks (which have their own
entries for direct and indirect blocks).
<em>Starting Block Size</em> and
<em>Maximum Direct Block Size</em> are fields
stored in the header.
</p>
<p>
The number of rows of blocks, <em>nrows</em>, in an
indirect block is calculated by the following expression:
<br /> <br />
<em>nrows</em> = (log<sub>2</sub>(<em>block_size</em>) -
log<sub>2</sub>(<em><Starting Block Size></em>)) + 1
</p>
where <em>block_size</em> is the size of the block that the indirect block
represents in the doubling table.
For example, to represent a block with <em>block_size</em> equals to 1024,
and <em>Starting Block Size</em> equals to 256,
three rows are needed.
<p>
The maximum number of rows of direct blocks, <em>max_dblock_rows</em>,
in any indirect block of a fractal heap is given by the
following expression:
<br /> <br />
<em>max_dblock_rows</em> =
(log<sub>2</sub>(<em><Maximum Direct Block Size></em>) -
log<sub>2</sub>(<em><Starting Block Size></em>)) + 2
</p>
<p>
Using the computed values for <em>nrows</em> and
<em>max_dblock_rows</em>, along with the <em>width</em> of the
doubling table, the number of direct and indirect block entries
(<em>K</em> and <em>N</em> in the indirect block description, below)
in an indirect block can be computed:
<br /> <br />
<em>K</em> = MIN(<em>nrows</em>, <em>max_dblock_rows</em>) *
<em><Table Width></em>
<br /> <br />
If <em>nrows</em> is less than or equal to <em>max_dblock_rows</em>,
<em>N</em> is 0. Otherwise, <em>N</em> is simply computed:
<br /> <br />
<em>N</em> = <em>K</em> - (<em>max_dblock_rows</em> *
<em><Table Width></em>)
</p>
<p>
The size of indirect blocks on disk is determined by the number
of rows in the indirect block (computed above). The size of direct
blocks on disk is exactly the size of the block in the doubling
table.
</p>
<br>
<div align="center">
<table class="format">
<caption>
Layout: Fractal Heap Header
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan="4">Signature</td>
</tr>
<tr>
<td>Version</td>
<td colspan="3" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="2">Heap ID Length</td>
<td colspan="2">I/O Filters’ Encoded Length</td>
</tr>
<tr>
<td>Flags</td>
<td colspan="3" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4">Maximum Size of Managed Objects</td>
</tr>
<tr>
<td colspan="4"><br />Next Huge Object ID<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />v2 B-tree Address of Huge Objects<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Amount of Free Space in Managed Blocks<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Address of Managed Block Free Space Manager<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Amount of Managed Space in Heap<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Amount of Allocated Managed Space in Heap<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Offset of Direct Block Allocation Iterator in Managed Space<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Number of Managed Objects in Heap<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Size of Huge Objects in Heap<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Number of Huge Objects in Heap<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Size of Tiny Objects in Heap<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Number of Tiny Objects in Heap<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="2">Table Width</td>
<td colspan="2" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4"><br />Starting Block Size<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Maximum Direct Block Size<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="2">Maximum Heap Size</td>
<td colspan="2">Starting # of Rows in Root Indirect Block</td>
</tr>
<tr>
<td colspan="4"><br />Address of Root Block<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="2">Current # of Rows in Root Indirect Block</td>
<td colspan="2" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4"><br />Size of Filtered Root Direct Block <em>(optional)</em><sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4">I/O Filter Mask<em> (optional)</em></td>
</tr>
<tr>
<td colspan="4">I/O Filter Information<em> (optional, variable size)</em></td>
</tr>
<tr>
<td colspan="4">Checksum</td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘L’ in the above table are
of the size specified in the <a href="#SizeOfLengthsV0">Size
of Lengths</a> field in the superblock.)
</td></tr>
<tr>
<td> </td>
<td>
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Fractal Heap Header
</caption>
<tr>
<th width="40%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Signature</p></td>
<td>
<p>The ASCII character string “<code>FRHP</code>”
is used to indicate the
beginning of a fractal heap header. This gives file consistency
checking utilities a better chance of reconstructing a
damaged file.
</p>
</td>
</tr>
<tr>
<td><p>Version</p></td>
<td>
<p>This document describes version 0.</p>
</td>
</tr>
<tr>
<td><p>Heap ID Length</p></td>
<td>
<p>This is the length in bytes of heap object IDs for this heap.</p>
</td>
</tr>
<tr>
<td><p>I/O Filters’ Encoded Length</p></td>
<td>
<p>This is the size in bytes of the encoded <em>I/O Filter Information</em>.
</p>
</td>
</tr>
<tr>
<td><p>Flags</p></td>
<td>
<p>This field is the heap status flag and is a bit field
indicating additional information about the fractal heap.
<table class="list">
<tr>
<th width="20%" align="center">Bit(s)</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>If set, the ID value to use for huge object has wrapped
around. If the value for the <em>Next Huge Object ID</em>
has wrapped around, each new huge object inserted into the
heap will require a search for an ID value.
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>If set, the direct blocks in the heap are checksummed.
</td>
</tr>
<tr>
<td align="center"><code>2-7</code></td>
<td>Reserved</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Maximum Size of Managed Objects</p></td>
<td>
<p>This is the maximum size of managed objects allowed in the heap.
Objects greater than this this are ‘huge’ objects and will be
stored in the file directly, rather than in a direct block for
the heap.
</p>
</td>
</tr>
<tr>
<td><p>Next Huge Object ID</p></td>
<td>
<p>This is the next ID value to use for a huge object in the heap.
</p>
</td>
</tr>
<tr>
<td><p>v2 B-tree Address of Huge Objects</p></td>
<td>
<p>This is the address of the <a href="#V2Btrees">v2 B-tree</a>
used to track huge objects in the heap. The type of records
stored in the <em>v2 B-tree</em> will
be determined by whether the address and length of a huge object
can fit into a heap ID (if yes, it is a “directly” accessed
huge object) and whether there is a filter used on objects
in the heap.
</p>
</td>
</tr>
<tr>
<td><p>Amount of Free Space in Managed Blocks</p></td>
<td>
<p>This is the total amount of free space in managed direct blocks
(in bytes).
</p>
</td>
</tr>
<tr>
<td><p>Address of Managed Block Free Space Manager</p></td>
<td>
<p>This is the address of the
<em><a href="#FreeSpaceManager">Free-space Manager</a></em> for
managed blocks.
</p>
</td>
</tr>
<tr>
<td><p>Amount of Managed Space in Heap</p></td>
<td>
<p>This is the total amount of managed space in the heap (in bytes),
essentially the upper bound of the heap’s linear address space.
</p>
</td>
</tr>
<tr>
<td><p>Amount of Allocated Managed Space in Heap</p></td>
<td>
<p>This is the total amount of managed space (in bytes) actually
allocated in
the heap. This can be less than the <em>Amount of Managed Space
in Heap</em> field, if some direct blocks in the heap’s linear
address space are not allocated.
</p>
</td>
</tr>
<tr>
<td><p>Offset of Direct Block Allocation Iterator in Managed Space</p></td>
<td>
<p>This is the linear heap offset where the next direct
block should be allocated at (in bytes). This may be less than
the <em>Amount of Managed Space in Heap</em> value because the
heap’s address space is increased by a “row” of direct blocks
at a time, rather than by single direct block increments.
</p>
</td>
</tr>
<tr>
<td><p>Number of Managed Objects in Heap</p></td>
<td>
<p>This is the number of managed objects in the heap.
</p>
</td>
</tr>
<tr>
<td><p>Size of Huge Objects in Heap</p></td>
<td>
<p>This is the total size of huge objects in the heap (in bytes).
</p>
</td>
</tr>
<tr>
<td><p>Number of Huge Objects in Heap</p></td>
<td>
<p>This is the number of huge objects in the heap.
</p>
</td>
</tr>
<tr>
<td><p>Size of Tiny Objects in Heap</p></td>
<td>
<p>This is the total size of tiny objects that are packed in heap
IDs (in bytes).
</p>
</td>
</tr>
<tr>
<td><p>Number of Tiny Objects in Heap</p></td>
<td>
<p>This is the number of tiny objects that are packed in heap IDs.
</p>
</td>
</tr>
<tr>
<td><p>Table Width</p></td>
<td>
<p>This is the number of columns in the doubling table for managed
blocks. This value must be a power of two.
</p>
</td>
</tr>
<tr>
<td><p>Starting Block Size</p></td>
<td>
<p>This is the starting block size to use in the doubling table for
managed blocks (in bytes). This value must be a power of two.
</p>
</td>
</tr>
<tr>
<td><p>Maximum Direct Block Size</p></td>
<td>
<p>This is the maximum size allowed for a managed direct block.
Objects inserted into the heap that are larger than this value
(less the number of bytes of direct block prefix/suffix)
are stored as ‘huge’ objects. This value must be a power of
two.
</p>
</td>
</tr>
<tr>
<td><p>Maximum Heap Size</p></td>
<td>
<p>This is the maximum size of the heap’s linear address space for
managed objects (in bytes). The value stored is the log2 of
the actual value, that is: the number of bits of the address space.
‘Huge’ and ‘tiny’ objects are not counted in this value, since
they do not store objects in the linear address space of the
heap.
</p>
</td>
</tr>
<tr>
<td><p>Starting # of Rows in Root Indirect Block</p></td>
<td>
<p>This is the starting number of rows for the root indirect block.
A value of 0 indicates that the root indirect block will have
the maximum number of rows needed to address the heap’s <em>Maximum
Heap Size</em>.
</p>
</td>
</tr>
<tr>
<td><p>Address of Root Block</p></td>
<td>
<p>This is the address of the root block for the heap. It can
be the <a href="#UndefinedAddress">undefined address</a> if
there is no data in the heap. It either points to a direct
block (if the <em>Current # of Rows in the Root Indirect
Block</em> value is 0), or an indirect block.
</p>
</td>
</tr>
<tr>
<td><p>Current # of Rows in Root Indirect Block</p></td>
<td>
<p>This is the current number of rows in the root indirect block.
A value of 0 indicates that <em>Address of Root Block</em>
points to direct block instead of indirect block.
</p>
</td>
</tr>
<tr>
<td><p>Size of Filtered Root Direct Block</p></td>
<td>
<p>This is the size of the root direct block, if filters are
applied to heap objects (in bytes). This field is only
stored in the header if the <em>I/O Filters’ Encoded Length</em>
is greater than 0.
</p>
</td>
</tr>
<tr>
<td><p>I/O Filter Mask</p></td>
<td>
<p>This is the filter mask for the root direct block, if filters
are applied to heap objects. This mask has the same format as
that used for the filter mask in chunked raw data records in a
<a href="#V1Btrees">v1 B-tree</a>.
This field is only
stored in the header if the <em>I/O Filters’ Encoded Length</em>
is greater than 0.
</p>
</td>
</tr>
<tr>
<td><p>I/O Filter Information</p></td>
<td>
<p>This is the I/O filter information encoding direct blocks and
huge objects, if filters are applied to heap objects. This
field is encoded as a <a href="#FilterMessage">Filter Pipeline</a>
message.
The size of this field is determined by <em>I/O Filters’
Encoded Length</em>.
</p>
</td>
</tr>
<tr>
<td><p>Checksum</p></td>
<td>
<p>This is the checksum for the header.</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Fractal Heap Direct Block
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan="4">Signature</td>
</tr>
<tr>
<td>Version</td>
<td colspan="3" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4"><br />Heap Header Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4">Block Offset <em>(variable size)</em></td>
</tr>
<tr>
<td colspan="4">Checksum <em>(optional)</em></td>
</tr>
<tr>
<td colspan="4"><br />Object Data <em>(variable size)</em><br /><br /></td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Fractal Heap Direct Block
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Signature</p></td>
<td>
<p>The ASCII character string “<code>FHDB</code>”
is used to indicate the
beginning of a fractal heap direct block. This gives file consistency
checking utilities a better chance of reconstructing a
damaged file.
</p>
</td>
</tr>
<tr>
<td><p>Version</p></td>
<td>
<p>This document describes version 0.</p>
</td>
</tr>
<tr>
<td><p>Heap Header Address</p></td>
<td>
<p>This is the address for the fractal heap header that this
block belongs to. This field is principally used for file
integrity checking.
</p>
</td>
</tr>
<tr>
<td><p>Block Offset</p></td>
<td>
<p>This is the offset of the block within the fractal heap’s
address space (in bytes). The number of bytes used to encode
this field is the <em>Maximum Heap Size</em> (in the heap’s
header) divided by 8 and rounded up to the next highest integer,
for values that are not a multiple of 8. This value is
principally used for file integrity checking.
</p>
</td>
</tr>
<tr>
<td><p>Checksum</p></td>
<td>
<p>This is the checksum for the direct block.</p>
<p>This field is only present if bit 1 of <em>Flags</em> in the
heap’s header is set.</p>
</td>
</tr>
<tr>
<td><p>Object Data</p></td>
<td>
<p>This section of the direct block stores the actual data for
objects in the heap. The size of this section is determined by
the direct block’s size minus the size of the other fields
stored in the direct block (for example, the <em>Signature</em>,
<em>Version</em>, and others including the <em>Checksum</em> if it is
present).
</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Fractal Heap Indirect Block
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan="4">Signature</td>
</tr>
<tr>
<td>Version</td>
<td colspan="3" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4"><br />Heap Header Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4">Block Offset <em>(variable size)</em></td>
</tr>
<tr>
<td colspan="4"><br />Child Direct Block #0 Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Size of Filtered Direct Block #0 <em>(optional)</em> <sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4">Filter Mask for Direct Block #0 <em>(optional)</em></td>
</tr>
<tr>
<td colspan="4"><br />Child Direct Block #1 Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Size of Filtered Direct Block #1 <em>(optional)</em><sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4">Filter Mask for Direct Block #1 <em>(optional)</em></td>
</tr>
<tr>
<td colspan="4">...</td>
</tr>
<tr>
<td colspan="4"><br />Child Direct Block #K-1 Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Size of Filtered Direct Block #K-1 <em>(optional)</em><sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4">Filter Mask for Direct Block #K-1 <em>(optional)</em></td>
</tr>
<tr>
<td colspan="4"><br />Child Indirect Block #0 Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Child Indirect Block #1 Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4">...</td>
</tr>
<tr>
<td colspan="4"><br />Child Indirect Block #N-1 Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4">Checksum</td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
<tr>
<td> </td>
<td>
(Items marked with an ‘L’ in the above table are
of the size specified in the <a href="#SizeOfLengthsV0">Size
of Lengths</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Fractal Heap Indirect Block
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Signature</p></td>
<td>
<p>The ASCII character string “<code>FHIB</code>” is used to
indicate the beginning of a fractal heap indirect block. This
gives file consistency checking utilities a better chance of
reconstructing a damaged file.
</p>
</td>
</tr>
<tr>
<td><p>Version</p></td>
<td>
<p>This document describes version 0.</p>
</td>
</tr>
<tr>
<td><p>Heap Header Address</p></td>
<td>
<p>This is the address for the fractal heap header that this
block belongs to. This field is principally used for file
integrity checking.
</p>
</td>
</tr>
<tr>
<td><p>Block Offset</p></td>
<td>
<p>This is the offset of the block within the fractal heap’s
address space (in bytes). The number of bytes used to encode
this field is the <em>Maximum Heap Size</em> (in the heap’s
header) divided by 8 and rounded up to the next highest integer,
for values that are not a multiple of 8. This value is
principally used for file integrity checking.
</p>
</td>
</tr>
<tr>
<td><p>Child Direct Block #K Address</p></td>
<td>
<p>This field is the address of the child direct block.
The size of the [uncompressed] direct block can be computed by
its offset in the heap’s linear address space.
</p>
</td>
</tr>
<tr>
<td><p>Size of Filtered Direct Block #K</p></td>
<td>
<p>This is the size of the child direct block after passing through
the I/O filters defined for this heap (in bytes). If no I/O
filters are present for this heap, this field is not present.
</p>
</td>
</tr>
<tr>
<td><p>Filter Mask for Direct Block #K</p></td>
<td>
<p>This is the I/O filter mask for the filtered direct block.
This mask has the same format as that used for the filter mask
in chunked raw data records in a <a href="#V1Btrees">v1 B-tree</a>.
If no I/O filters are present for this heap, this field is not
present.
</p>
</td>
</tr>
<tr>
<td><p>Child Indirect Block #N Address</p></td>
<td>
<p>This field is the address of the child indirect block.
The size of the indirect block can be computed by
its offset in the heap’s linear address space.
</p>
</td>
</tr>
<tr>
<td><p>Checksum</p></td>
<td>
<p>This is the checksum for the indirect block.</p>
</td>
</tr>
</table>
</div>
<br />
<p>An object in the fractal heap is identified by means of a fractal heap ID,
which encodes information to locate the object in the heap.
Currently, the fractal heap stores an object in one of three ways,
depending on the object’s size:</p>
<div align="center">
<table class="list80">
<tr>
<th width="20%">Type</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center">Tiny</td>
<td>
<p>When an object is small enough to be encoded in the
heap ID, the object’s data is embedded in the fractal
heap ID itself. There are two sub-types for this type of
object: normal and extended. The sub-type for tiny heap
IDs depends on whether the heap ID is large enough to
store objects greater than 16 bytes or not. If the
heap ID length is 18 bytes or smaller, the
‘normal’ tiny heap ID form is used. If the
heap ID length is greater than 18 bytes in length, the
“extended” form is used. See the format
description below for both sub-types.
</p>
</td>
</tr>
<tr>
<td align="center">Huge</td>
<td>
<p>When the size of an object is larger than <em>Maximum Size of
Managed Objects</em> in the <em>Fractal Heap Header</em>, the
object’s data is stored on its own in the file and the object
is tracked/indexed via a version 2 B-tree. All huge objects
for a particular fractal heap use the same v2 B-tree. All huge
objects for a particular fractal heap use the same format for
their huge object IDs.
</p>
<p>Depending on whether the IDs for a heap are large enough to hold
the object’s retrieval information and whether I/O pipeline filters
are applied to the heap’s objects, 4 sub-types are derived for
huge object IDs for this heap:</p>
<div align="center">
<table class="list">
<tr>
<th align="left" width="35%">Sub-type</th>
<th align="left">Description</th>
</tr>
<tr>
<td align="left">Directly accessed, non-filtered</td>
<td>
<p>The object’s address and length are embedded in the
fractal heap ID itself and the object is directly accessed
from them. This allows the object to be accessed without
resorting to the B-tree.
</p>
</td>
</tr>
<tr>
<td align="left">Directly accessed, filtered</td>
<td>
<p>The filtered object’s address, length, filter mask and
de-filtered size are embedded in the fractal heap ID itself
and the object is accessed directly with them. This allows
the object to be accessed without resorting to the B-tree.
</p>
</td>
</tr>
<tr>
<td align="left">Indirectly accessed, non-filtered</td>
<td>
<p>The object is located by using a B-tree key embedded in
the fractal heap ID to retrieve the address and length from
the version 2 B-tree for huge objects. Then, the address
and length are used to access the object.
</p>
</td>
</tr>
<tr>
<td align="left">Indirectly accessed, filtered</td>
<td>
<p>The object is located by using a B-tree key embedded in
the fractal heap ID to retrieve the filtered object’s
address, length, filter mask and de-filtered size from the
version 2 B-tree for huge objects. Then, this information
is used to access the object.
</p>
</td>
</tr>
</table>
</div>
</td>
</tr>
<tr>
<td align="center">Managed</td>
<td>
<p>When the size of an object does not meet the above two
conditions, the object is stored and managed via the direct and
indirect blocks based on the doubling table.
</p>
</td>
</tr>
</table>
</div>
<br />
<p>The specific format for each type of heap ID is described below:
</p>
<div align="center">
<table class="format">
<caption>
Layout: Fractal Heap ID for Tiny Objects (Sub-type 1 -
‘Normal’)
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td>Version, Type, and Length</td>
<td colspan="3" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4"><br />Data <em>(variable size)</em></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Fractal Heap ID for Tiny Objects (Sub-type 1 -
‘Normal’)
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Version, Type, and Length</p></td>
<td>
<p>This is a bit field with the following definition:
<table class="list">
<tr>
<th width="20%" align="center">Bit</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>6-7</code></td>
<td>The current version of ID format. This document
describes version 0.
</td>
</tr>
<tr>
<td align="center"><code>4-5</code></td>
<td>The ID type. Tiny objects have a value of <code>2</code>.
</td>
</tr>
<tr>
<td align="center"><code>0-3</code></td>
<td>The length of the tiny object. The value stored
is one less than the actual length (since zero-length
objects are not allowed to be stored in the heap).
For example, an object of actual length 1 has an
encoded length of 0, an object of actual length 2
has an encoded length of 1, and so on.
</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Data</p></td>
<td>
<p>This is the data for the object.
</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Fractal Heap ID for Tiny Objects (Sub-type 2 -
‘Extended’)
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td>Version, Type, and Length</td>
<td>Extended Length</td>
<td colspan="2" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4">Data <em>(variable size)</em></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Fractal Heap ID for Tiny Objects (Sub-type 2 -
‘Extended’)
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Version, Type, and Length</p></td>
<td>
<p>This is a bit field with the following definition:
<table class="list">
<tr>
<th width="20%" align="center">Bit</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>6-7</code></td>
<td>The current version of ID format. This document
describes version 0.
</td>
</tr>
<tr>
<td align="center"><code>4-5</code></td>
<td>The ID type. Tiny objects have a value of <code>2</code>.
</td>
</tr>
<tr>
<td align="center"><code>0-3</code></td>
<td>These 4 bits, together with the next byte, form an
unsigned 12-bit integer for holding the length of the
object. These 4-bits are bits 8-11 of the 12-bit integer.
See description for the <em>Extended Length</em> field below.
</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Extended Length</p></td>
<td>
<p>This byte, together with the 4 bits in the previous byte,
forms an unsigned 12-bit integer for holding the length of
the tiny object. These 8 bits are bits 0-7 of the 12-bit
integer formed. The value stored is one less than the actual
length (since zero-length objects are not allowed to be
stored in the heap). For example, an object of actual length
1 has an encoded length of 0, an object of actual length
2 has an encoded length of 1, and so on.
</p>
</td>
</tr>
<tr>
<td><p>Data</p></td>
<td>
<p>This is the data for the object.
</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Fractal Heap ID for Huge Objects (Sub-types 1 and 2):
Indirectly Accessed, Non-filtered/Filtered
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td>Version and Type</td>
<td colspan="3" bgcolor="#DDDDDD"><em>This space inserted
only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4"><br />v2 B-tree Key<sup>L</sup><em> (variable size)</em><br /><br /></td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘L’ in the above table are
of the size specified in the <a href="#SizeOfLengthsV0">Size
of Lengths</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Fractal Heap ID for Huge Objects (Sub-types 1 and 2):
Indirectly Accessed, Non-filtered/Filtered
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Version and Type</p></td>
<td>
<p>This is a bit field with the following definition:
<table class="list">
<tr>
<th width="20%" align="center">Bit</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>6-7</code></td>
<td>The current version of ID format. This document
describes version 0.
</td>
</tr>
<tr>
<td align="center"><code>4-5</code></td>
<td>The ID type. Huge objects have a value of <code>1</code>.
</td>
</tr>
<tr>
<td align="center"><code>0-3</code></td>
<td>Reserved.
</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>v2 B-tree Key</p></td>
<td><p>This field is the B-tree key for retrieving the information
from the version 2 B-tree for huge objects needed to access the
object. See the description of <a href="#V2Btrees">v2 B-tree</a>
records sub-types 1 and 2 for a description of the fields. New key
values are derived from <em>Next Huge Object ID</em> in the
<em>Fractal Heap Header</em>.</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Fractal Heap ID for Huge Objects (Sub-type 3):
Directly Accessed, Non-filtered
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td>Version and Type</td>
<td colspan="3" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4"><br />Address <sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Length <sup>L</sup><br /><br /></td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
<tr>
<td> </td>
<td>
(Items marked with an ‘L’ in the above table are
of the size specified in the <a href="#SizeOfLengthsV0">Size
of Lengths</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Fractal Heap ID for Huge Objects (Sub-type 3):
Directly Accessed, Non-filtered
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Version and Type</p></td>
<td>
<p>This is a bit field with the following definition:
<table class="list">
<tr>
<th width="20%" align="center">Bit</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>6-7</code></td>
<td>The current version of ID format. This document
describes version 0.
</td>
</tr>
<tr>
<td align="center"><code>4-5</code></td>
<td>The ID type. Huge objects have a value of <code>1</code>.
</td>
</tr>
<tr>
<td align="center"><code>0-3</code></td>
<td>Reserved.
</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Address</p></td>
<td><p>This field is the address of the object in the file.</p>
</td>
</tr>
<tr>
<td><p>Length</p></td>
<td><p>This field is the length of the object in the file.</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Fractal Heap ID for Huge Objects (Sub-type 4):
Directly Accessed, Filtered
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td>Version and Type</td>
<td colspan="3" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4"><br />Address <sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Length <sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4">Filter Mask</td>
</tr>
<tr>
<td colspan="4"><br />De-filtered Size <sup>L</sup><br /><br /></td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
<tr>
<td> </td>
<td>
(Items marked with an ‘L’ in the above table are
of the size specified in the <a href="#SizeOfLengthsV0">Size
of Lengths</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Fractal Heap ID for Huge Objects (Sub-type 4):
Directly Accessed, Filtered
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Version and Type</p></td>
<td>
<p>This is a bit field with the following definition:
<table class="list">
<tr>
<th width="20%" align="center">Bit</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>6-7</code></td>
<td>The current version of ID format. This document
describes version 0.
</td>
</tr>
<tr>
<td align="center"><code>4-5</code></td>
<td>The ID type. Huge objects have a value of <code>1</code>.
</td>
</tr>
<tr>
<td align="center"><code>0-3</code></td>
<td>Reserved.
</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Address</p></td>
<td><p>This field is the address of the filtered object in the file.</p>
</td>
</tr>
<tr>
<td><p>Length</p></td>
<td><p>This field is the length of the filtered object in the file.</p>
</td>
</tr>
<tr>
<td><p>Filter Mask</p></td>
<td><p>This field is the I/O pipeline filter mask for the
filtered object in the file.</p>
</td>
</tr>
<tr>
<td><p>Filtered Size</p></td>
<td><p>This field is the size of the de-filtered object in the file.</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Fractal Heap ID for Managed Objects
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td>Version and Type</td>
<td colspan="3" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4">Offset <em>(variable size)</em></td>
</tr>
<tr>
<td colspan="4">Length <em>(variable size)</em></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Fractal Heap ID for Managed Objects
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Version and Type</p></td>
<td><p>This is a bit field with the following definition:
<table class="list">
<tr>
<th width="20%" align="center">Bit</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>6-7</code></td>
<td>The current version of ID format. This document
describes version 0.
</td>
</tr>
<tr>
<td align="center"><code>4-5</code></td>
<td>The ID type. Managed objects have a value of <code>0</code>.
</td>
</tr>
<tr>
<td align="center"><code>0-3</code></td>
<td>Reserved.
</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Offset</p></td>
<td><p>This field is the offset of the object in the heap.
This field’s size is the minimum number of bytes
necessary to encode the <em>Maximum Heap Size</em> value
(from the <em>Fractal Heap Header</em>). For example, if the
value of the <em>Maximum Heap Size</em> is less than 256 bytes,
this field is 1 byte in length, a <em>Maximum Heap Size</em>
of 256-65535 bytes uses a 2 byte length, and so on.</p></td>
</tr>
<tr>
<td><p>Length</p></td>
<td><p>This field is the length of the object in the heap. It
is determined by taking the minimum value of <em>Maximum
Direct Block Size</em> and <em>Maximum Size of Managed
Objects</em> in the <em>Fractal Heap Header</em>. Again,
the minimum number of bytes needed to encode that value is
used for the size of this field.</p></td>
</tr>
</table>
</div>
<h3><a name="FreeSpaceManager">
III.H. Disk Format: Level 1H - Free-space Manager</a></h3>
<p>
Free-space managers are used to describe space within a heap or
the entire HDF5 file that is not currently used for that heap or
file.
</p>
<p>
The <em>free-space manager header</em> contains metadata information
about the space being tracked, along with the address of the list
of <em>free space sections</em> which actually describes the free
space. The header records information about free-space sections being
tracked, creation parameters for handling free-space sections of a
client, and section information used to locate the collection of
free-space sections.
</p>
<p>
The <em>free-space section list</em> stores a collection of
free-space sections that is specific to each <em>client</em> of the
free-space manager.
For example, the fractal heap is a client of the free space manager
and uses it to track unused space within the heap. There are 4
types of section records for the fractal heap, each of which has
its own format, listed below.
</p>
<div align="center">
<table class="format">
<caption>
Layout: Free-space Manager Header
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan="4">Signature</td>
</tr>
<tr>
<td>Version</td>
<td>Client ID</td>
<td colspan="2" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4"><br />Total Space Tracked<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Total Number of Sections<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Number of Serialized Sections<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Number of Un-Serialized Sections<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="2">Number of Section Classes</td>
<td colspan="2" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="2">Shrink Percent</td>
<td colspan="2">Expand Percent</td>
</tr>
<tr>
<td colspan="2">Size of Address Space</td>
<td colspan="2" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4"><br />Maximum Section Size <sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Address of Serialized Section List<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Size of Serialized Section List Used<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Allocated Size of Serialized Section List<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4">Checksum</td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘L’ in the above table are
of the size specified in the <a href="#SizeOfLengthsV0">Size
of Lengths</a> field in the superblock.)
</td></tr>
<tr>
<td> </td>
<td>
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Free-space Manager Header
</caption>
<tr>
<th width="35%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Signature</p></td>
<td>
<p>The ASCII character string “<code>FSHD</code>”
is used to indicate the beginning of the Free-space Manager
Header. This gives file consistency checking utilities a
better chance of reconstructing a damaged file.
</p>
</td>
</tr>
<tr>
<td><p>Version</p></td>
<td>
<p>This is the version number for the Free-space Manager Header
and this document describes version 0.</p>
</td>
</tr>
<tr>
<td><p>Client ID</p></td>
<td>
<p>This is the client ID for identifying the user of this
free-space manager:
<table class="list">
<tr>
<th width="20%" align="center">ID</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>Fractal heap
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>File
</td>
</tr>
<tr>
<td align="center"><code>2+</code></td>
<td>Reserved.
</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Total Space Tracked</p></td>
<td>
<p>This is the total amount of free space being tracked, in bytes.
</p>
</td>
</tr>
<tr>
<td><p>Total Number of Sections</p></td>
<td>
<p>This is the total number of free-space sections being tracked.
</p>
</td>
</tr>
<tr>
<td><p>Number of Serialized Sections</p></td>
<td>
<p>This is the number of serialized free-space sections being
tracked.
</p>
</td>
</tr>
<tr>
<td><p>Number of Un-Serialized Sections</p></td>
<td>
<p>This is the number of un-serialized free-space sections being
managed. Un-serialized sections are created by the free-space
client when the list of sections is read in.
</p>
</td>
</tr>
<tr>
<td><p>Number of Section Classes</p></td>
<td>
<p>This is the number of section classes handled by this free space
manager for the free-space client.
</p>
</td>
</tr>
<tr>
<td><p>Shrink Percent</p></td>
<td>
<p>This is the percent of current size to shrink the allocated
serialized free-space section list.
</p>
</td>
</tr>
<tr>
<td><p>Expand Percent</p></td>
<td>
<p>This is the percent of current size to expand the allocated
serialized free-space section list.
</p>
</td>
</tr>
<tr>
<td><p>Size of Address Space</p></td>
<td>
<p>This is the size of the address space that free-space sections
are within. This is stored as the log<sub>2</sub> of the
actual value (in other words, the number of bits required
to store values within that address space).
</p>
</td>
</tr>
<tr>
<td><p>Maximum Section Size</p></td>
<td>
<p>This is the maximum size of a section to be tracked.
</p>
</td>
</tr>
<tr>
<td><p>Address of Serialized Section List</p></td>
<td>
<p>This is the address where the serialized free-space section
list is stored.
</p>
</td>
</tr>
<tr>
<td><p>Size of Serialized Section List Used</p></td>
<td>
<p>This is the size of the serialized free-space section
list used (in bytes). This value must be less than
or equal to the <em>allocated size of serialized section
list</em>, below.
</p>
</td>
</tr>
<tr>
<td><p>Allocated Size of Serialized Section List</p></td>
<td>
<p>This is the size of serialized free-space section list
actually allocated (in bytes).
</p>
</td>
</tr>
<tr>
<td><p>Checksum</p></td>
<td>
<p>This is the checksum for the free-space manager header.</p>
</td>
</tr>
</table>
</div>
<br />
<p>The free-space sections being managed are stored in a
<em>free-space section list</em>, described below. The sections
in the free-space section list are stored in the following way:
a count of the number of sections describing a particular size of
free space and the size of the free-space described (in bytes),
followed by a list of section description records; then another
section count and size, followed by the list of section
descriptions for that size; and so on.</p>
<div align="center">
<table class="format">
<caption>
Layout: Free-space Section List
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan="4">Signature</td>
</tr>
<tr>
<td>Version</td>
<td colspan="3" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4"><br />Free-space Manager Header Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4">Number of Section Records in Set #0 <em>(variable size)</em></td>
</tr>
<tr>
<td colspan="4">Size of Free-space Section Described in Record Set #0 <em>(variable size)</em></td>
</tr>
<tr>
<td colspan="4">Record Set #0 Section Record #0 Offset<em>(variable size)</em></td>
</tr>
<tr>
<td colspan="1">Record Set #0 Section Record #0 Type</td>
<td colspan="3" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4">Record Set #0 Section Record #0 Data <em>(variable size)</em></td>
</tr>
<tr>
<td colspan="4">...</td>
</tr>
<tr>
<td colspan="4">Record Set #0 Section Record #K-1 Offset<em>(variable size)</em></td>
</tr>
<tr>
<td colspan="1">Record Set #0 Section Record #K-1 Type</td>
<td colspan="3" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4">Record Set #0 Section Record #K-1 Data <em>(variable size)</em></td>
</tr>
<tr>
<td colspan="4">Number of Section Records in Set #1 <em>(variable size)</em></td>
</tr>
<tr>
<td colspan="4">Size of Free-space Section Described in Record Set #1 <em>(variable size)</em></td>
</tr>
<tr>
<td colspan="4">Record Set #1 Section Record #0 Offset<em>(variable size)</em></td>
</tr>
<tr>
<td colspan="1">Record Set #1 Section Record #0 Type</td>
<td colspan="3" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4">Record Set #1 Section Record #0 Data <em>(variable size)</em></td>
</tr>
<tr>
<td colspan="4">...</td>
</tr>
<tr>
<td colspan="4">Record Set #1 Section Record #K-1 Offset<em>(variable size)</em></td>
</tr>
<tr>
<td colspan="1">Record Set #1 Section Record #K-1 Type</td>
<td colspan="3" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4">Record Set #1 Section Record #K-1 Data <em>(variable size)</em></td>
</tr>
<tr>
<td colspan="4"><strong>...</strong></td>
</tr>
<tr>
<td colspan="4"><strong>...</strong></td>
</tr>
<tr>
<td colspan="4">Number of Section Records in Set #N-1 <em>(variable size)</em></td>
</tr>
<tr>
<td colspan="4">Size of Free-space Section Described in Record Set #N-1 <em>(variable size)</em></td>
</tr>
<tr>
<td colspan="4">Record Set #N-1 Section Record #0 Offset<em>(variable size)</em></td>
</tr>
<tr>
<td colspan="1">Record Set #N-1 Section Record #0 Type</td>
<td colspan="3" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4">Record Set #N-1 Section Record #0 Data <em>(variable size)</em></td>
</tr>
<tr>
<td colspan="4">...</td>
</tr>
<tr>
<td colspan="4">Record Set #N-1 Section Record #K-1 Offset<em>(variable size)</em></td>
</tr>
<tr>
<td colspan="1">Record Set #N-1 Section Record #K-1 Type</td>
<td colspan="3" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4">Record Set #N-1 Section Record #K-1 Data <em>(variable size)</td>
</tr>
<tr>
<td colspan="4">Checksum</td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Free-space Section List
</caption>
<tr>
<th width="35%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Signature</p></td>
<td>
<p>The ASCII character string “<code>FSSE</code>”
is used to indicate the beginning of the Free-space Section
Information. This gives file consistency checking utilities
a better chance of reconstructing a damaged file.
</p>
</td>
</tr>
<tr>
<td><p>Version</p></td>
<td>
<p>This is the version number for the Free-space Section List
and this document describes version 0.</p>
</td>
</tr>
<tr>
<td><p>Free-space Manager Header Address</p></td>
<td>
<p>This is the address of the <em>Free-space Manager Header</em>.
This field is principally used for file
integrity checking.
</p>
</td>
</tr>
<tr>
<td><p>Number of Section Records for Set #N</p></td>
<td>
<p>This is the number of free-space section records for set #N.
The length of this field is the minimum number of bytes needed
to store the <em>number of serialized sections</em> (from the
<em>free-space manager header</em>).
</p>
<p>
The number of sets of free-space section records is
determined by the <em>size of serialized section list</em> in
the <em>free-space manager header</em>.
</p>
</td>
</tr>
<tr>
<td><p>Section Size for Record Set #N</p></td>
<td>
<p>This is the size (in bytes) of the free-space section described
for <em>all</em> the section records in set #N.
</p>
<p>
The length of this field is the minimum number of bytes needed
to store the <em>maximum section size</em> (from the
<em>free-space manager header</em>).
</p>
</td>
</tr>
<tr>
<td><p>Record Set #N Section #K Offset</p></td>
<td>
<p>This is the offset (in bytes) of the free-space section within
the client for the free-space manager.
</p>
<p>
The length of this field is the minimum number of bytes needed
to store the <em>size of address space</em> (from the
<em>free-space manager header</em>).
</p>
</td>
</tr>
<tr>
<td><p>Record Set #N Section #K Type</p></td>
<td>
<p>This is the type of the section record, used to decode the
<em>record set #N section #K data</em> information. The defined
record type for <em>file</em> client is:
<table class="list">
<tr>
<th width="20%" align="center">Type</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>File’s section (a range of actual bytes in file)
</td>
</tr>
<tr>
<td align="center"><code>1+</code></td>
<td>Reserved.
</td>
</tr>
</table></p>
<p>The defined record types for a <em>fractal heap</em> client are:
<table class="list">
<tr>
<th width="20%" align="center">Type</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>Fractal heap “single” section
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>Fractal heap “first row” section
</td>
</tr>
<tr>
<td align="center"><code>2</code></td>
<td>Fractal heap “normal row” section
</td>
</tr>
<tr>
<td align="center"><code>3</code></td>
<td>Fractal heap “indirect” section
</td>
</tr>
<tr>
<td align="center"><code>4+</code></td>
<td>Reserved.
</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Record Set #N Section #K Data</p></td>
<td>
<p>This is the section-type specific information for each record
in the record set, described below.
</p>
</td>
</tr>
<tr>
<td><p>Checksum</p></td>
<td>
<p>This is the checksum for the <em>Free-space Section List</em>.
</p>
</td>
</tr>
</table>
</div>
<br />
<p>
The section-type specific data for each free-space section record is
described below:
</p>
<div align="center">
<table class="format">
<caption>
Layout: File’s Section Data Record
</caption>
<tr>
<td colspan="4"><em>No additional record data stored</em></td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Fractal Heap “Single” Section Data Record
</caption>
<tr>
<td colspan="4"><em>No additional record data stored</em></td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Fractal Heap “First Row” Section Data
Record
</caption>
<tr>
<td colspan="4"><em>Same format as “indirect”
section data</em></td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Fractal Heap “Normal Row” Section Data
Record
</caption>
<tr>
<td colspan="4"><em>No additional record data stored</em></td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Fractal Heap “Indirect” Section
Data Record
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan="4">Fractal Heap Indirect Block Offset <em>(variable size)</em></td>
</tr>
<tr>
<td colspan="2">Block Start Row</td>
<td colspan="2">Block Start Column</td>
</tr>
<tr>
<td colspan="2">Number of Blocks</td>
<td colspan="2" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Fractal Heap “Indirect” Section
Data Record
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Fractal Heap Block Offset</p></td>
<td>
<p>The offset of the indirect block in the fractal heap’s address
space containing the empty blocks.
</p>
<p>
The number of bytes used to encode this field is the minimum
number of bytes needed to encode values for the <em>Maximum
Heap Size</em> (in the fractal heap’s header).
</p>
</td>
</tr>
<tr>
<td><p>Block Start Row</p></td>
<td>
<p>This is the row that the empty blocks start in.
</p>
</td>
</tr>
<tr>
<td><p>Block Start Column</p></td>
<td>
<p>This is the column that the empty blocks start in.
</p>
</td>
</tr>
<tr>
<td><p>Number of Blocks</p></td>
<td>
<p>This is the number of empty blocks covered by the section.
</p>
</td>
</tr>
</table>
</div>
<h3><a name="SOHMTable">
III.I. Disk Format: Level 1I - Shared Object Header Message Table</a></h3>
<p>
The <em>shared object header message table</em> is used to locate
object
header messages that are shared between two or more object headers
in the file. Shared object header messages are stored and indexed
in the file in one of two ways: indexed sequentially in a
<em>shared header message list</em> or indexed with a v2 B-tree.
The shared messages themselves are either stored in a fractal
heap (when two or more objects share the message), or remain in an
object’s header (when only one object uses the message currently,
but the message can be shared in the future).
</p>
<p>
The <em>shared object header message table</em>
contains a list of shared message index headers. Each index header
records information about the version of the index format, the index
storage type, flags for the message types indexed, the number of
messages in the index, the address where the index resides,
and the fractal heap address if shared messages are stored there.
</p>
<p>
Each index can be either a list or a v2 B-tree and may transition
between those two forms as the number of messages in the index
varies. Each shared message record contains information used to
locate the shared message from either a fractal heap or an object
header. The types of messages that can be shared are: <em>Dataspace,
Datatype, Fill Value, Filter Pipeline and Attribute</em>.
</p>
<p>
The <em>shared object header message table</em> is pointed to
from a <a href="#SOHMTableMessage">shared message table</a> message
in the superblock extension for a file. This message stores the
version of the table format, along with the number of index headers
in the table.
</p>
<div align="center">
<table class="format">
<caption>
Layout: Shared Object Header Message Table
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan="4">Signature</td>
</tr>
<tr>
<td>Version for index #0</td>
<td>Index Type for index #0</td>
<td colspan="2">Message Type Flags for index #0</td>
</tr>
<tr>
<td colspan="4">Minimum Message Size for index #0</td>
</tr>
<tr>
<td colspan="2">List Cutoff for index #0</td>
<td colspan="2">v2 B-tree Cutoff for index #0</td>
</tr>
<tr>
<td colspan="2">Number of Messages for index #0</td>
<td colspan="2" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4"><br />Index Address<sup>O</sup> for index #0<br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Fractal Heap Address<sup>O</sup> for index #0<br /><br /></td>
</tr>
<tr>
<td colspan="4">...</td>
</tr>
<tr>
<td colspan="4">...</td>
</tr>
<tr>
<td>Version for index #N-1</td>
<td>Index Type for index #N-1</td>
<td colspan="2">Message Type Flags for index #N-1</td>
</tr>
<tr>
<td colspan="4">Minimum Message Size for index #N-1</td>
</tr>
<tr>
<td colspan="2">List Cutoff for index #N-1</td>
<td colspan="2">v2 B-tree Cutoff for index #N-1</td>
</tr>
<tr>
<td colspan="2">Number of Messages for index #N-1</td>
<td colspan="2" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4"><br />Index Address<sup>O</sup> for index #N-1<br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Fractal Heap Address<sup>O</sup> for index #N-1<br /><br /></td>
</tr>
<tr>
<td colspan="4">Checksum</td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Shared Object Header Message Table
</caption>
<tr>
<th width="35%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Signature</p></td>
<td>
<p>The ASCII character string “<code>SMTB</code>”
is used to indicate the beginning of the Shared Object
Header Message table. This gives file consistency checking
utilities a better chance of reconstructing a damaged file.
</p>
</td>
</tr>
<tr>
<td><p>Version for index #N</p></td>
<td>
<p>This is the version number for the list of shared object header message
indexes and this document describes version 0.</p>
</td>
</tr>
<tr>
<td><p>Index Type for index #N</p></td>
<td>
<p>The type of index can be an unsorted list or a v2 B-tree.
</p>
</td>
</tr>
<tr>
<td><p>Message Type Flags for index #N</p></td>
<td>
<p>This field indicates the type of messages tracked in the index,
as follows:
<table class="list">
<tr>
<th width="20%" align="center">Bits</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>If set, the index tracks <em>Dataspace Messages</em>.
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>If set, the message tracks <em>Datatype Messages</em>.
</td>
</tr>
<tr>
<td align="center"><code>2</code></td>
<td>If set, the message tracks <em>Fill Value Messages</em>.
</td>
</tr>
<tr>
<td align="center"><code>3</code></td>
<td>If set, the message tracks <em>Filter Pipeline Messages</em>.
</td>
</tr>
<tr>
<td align="center"><code>4</code></td>
<td>If set, the message tracks <em>Attribute Messages</em>.
</td>
</tr>
<tr>
<td align="center"><code>5-15</code></td>
<td>Reserved (zero).
</td>
</tr>
</table></p>
<p>
An index can track more than one type of message, but each type
of message can only by in one index.
</p>
</td>
</tr>
<tr>
<td><p>Minimum Message Size for index #N</p></td>
<td>
<p>This is the message size sharing threshold for the index.
If the encoded size of the message is less than this value, the
message is not shared.
</p>
</td>
</tr>
<tr>
<td><p>List Cutoff for index #N</p></td>
<td>
<p>This is the cutoff value for the indexing of messages to
switch from a list to a v2 B-tree. If the number of messages
is greater than this value, the index should be a v2 B-tree.
</p>
</td>
</tr>
<tr>
<td><p>v2 B-tree Cutoff for index #N</p></td>
<td>
<p>This is the cutoff value for the indexing of messages
to switch from a v2 B-tree back to a list. If the number
of messages is less than this value, the index should be
a list.
</p>
</td>
</tr>
<tr>
<td><p>Number of Messages for index #N</p></td>
<td>
<p>The number of shared messages being tracked for the index.
</p>
</td>
</tr>
<tr>
<td><p>Index Address for index #N</p></td>
<td>
<p>This field is the address of the list or v2 B-tree where the
index nodes reside.
</p>
</td>
</tr>
<tr>
<td><p>Fractal Heap Address for index #N</p></td>
<td>
<p>This field is the address of the fractal heap if shared messages
are stored there.
</p>
</td>
</tr>
<tr>
<td><p>Checksum</p></td>
<td>
<p>This is the checksum for the table.</p>
</td>
</tr>
</table>
</div>
<br />
<p>
Shared messages are indexed either with a <em>shared message record
list</em>, described below, or using a v2 B-tree (using record type 7).
The number of records in the <em>shared message record list</em> is
determined in the index’s entry in the <em>shared object header message
table</em>.
</p>
<div align="center">
<table class="format">
<caption>
Layout: Shared Message Record List
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan="4">Signature</td>
</tr>
<tr>
<td colspan="4">Shared Message Record #0</td>
</tr>
<tr>
<td colspan="4">Shared Message Record #1</td>
</tr>
<tr>
<td colspan="4">...</td>
</tr>
<tr>
<td colspan="4">Shared Message Record #N-1</td>
</tr>
<tr>
<td colspan="4">Checksum</td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Shared Message Record List
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Signature</p></td>
<td>
<p>The ASCII character string “<code>SMLI</code>”
is used to indicate the beginning of a list of index nodes.
This gives file consistency checking utilities a better
chance of reconstructing a damaged file.
</p>
</td>
</tr>
<tr>
<td><p>Shared Message Record #N</p></td>
<td>
<p>The record for locating the shared message, either in the
fractal heap for the index, or an object header (see format for
<em>index nodes</em> below).
</p>
</td>
</tr>
<tr>
<td><p>Checksum</p></td>
<td>
<p>This is the checksum for the list.
</p>
</td>
</tr>
</table>
</div>
<br />
<p>
The record for each shared message in an index is stored in one
of the following forms:
</p>
<div align="center">
<table class="format">
<caption>
Layout: Shared Message Record for Messages Stored in a
Fractal Heap
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td>Message Location</td>
<td colspan="3" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4">Hash Value</td>
</tr>
<tr>
<td colspan="4">Reference Count</td>
</tr>
<tr>
<td colspan="4"><br />Fractal Heap ID<br /><br /></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Shared Message Record for Messages Stored in a
Fractal Heap
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Message Location</p></td>
<td>
<p>This has a value of 0 indicating that the message is stored in
the heap.
</p>
</td>
</tr>
<tr>
<td><p>Hash Value</p></td>
<td>
<p>This is the hash value for the message.
</p>
</td>
</tr>
<tr>
<td><p>Reference Count</p></td>
<td>
<p>This is the number of times the message is used in the file.
</p>
</td>
</tr>
<tr>
<td><p>Fractal Heap ID</p></td>
<td>
<p>This is an 8-byte fractal heap ID for the message as stored in
the fractal heap for the index.
</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Shared Message Record for Messages Stored in an
Object Header
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td>Message Location</td>
<td colspan="3" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4">Hash Value</td>
</tr>
<tr>
<td>Reserved</td>
<td>Message Type</td>
<td colspan="2">Creation Index</td>
</tr>
<tr>
<td colspan="4"><br />Object Header Address<sup>O</sup><br /><br /></td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Shared Message Record for Messages Stored in an
Object Header
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Message Location</p></td>
<td>
<p>This has a value of 1 indicating that the message is stored in
an object header.
</p>
</td>
</tr>
<tr>
<td><p>Hash Value</p></td>
<td>
<p>This is the hash value for the message.
</p>
</td>
</tr>
<tr>
<td><p>Message Type</p></td>
<td>
<p>This is the message type in the object header.
</p>
</td>
</tr>
<tr>
<td><p>Creation Index</p></td>
<td>
<p>This is the creation index of the message within the object
header.
</p>
</td>
</tr>
<tr>
<td><p>Object Header Address</p></td>
<td>
<p>This is the address of the object header where the message is
located.
</p>
</td>
</tr>
</table>
</div>
<h2><a name="DataObject">
IV. Disk Format: Level 2 - Data Objects </a></h2>
<p>Data objects contain the “real” user-visible information in the file.
These objects compose the scientific data and other information which
are generally thought of as “data” by the end-user. All the
other information in the file is provided as a framework for
storing and accessing these data objects.
</p>
<p>A data object is composed of header and data
information. The header information contains the information
needed to interpret the data information for the object as
well as additional “metadata” or pointers to additional
“metadata” used to describe or annotate each object.
</p>
<h3><a name="ObjectHeader">
IV.A. Disk Format: Level 2A - Data Object Headers</a></h3>
<p>The header information of an object is designed to encompass
all of the information about an object, except for the data itself.
This information includes the dataspace, the datatype, information
about how the data is stored on disk (in external files, compressed,
broken up in blocks, and so on), as well as other information used
by the library to speed up access to the data objects or maintain
a file’s integrity. Information stored by user applications
as attributes is also stored in the object’s header. The header
of each object is not necessarily located immediately prior to the
object’s data in the file and in fact may be located in any
position in the file. The order of the messages in an object header
is not significant.</p>
<p>Object headers are composed of a prefix and a set of messages. The
prefix contains the information needed to interpret the messages and
a small amount of metadata about the object, and the messages contain
the majority of the metadata about the object.
</p>
<h3><a name="ObjectHeaderPrefix">
IV.A.1. Disk Format: Level 2A1 - Data Object Header Prefix</a></h3>
<h4><a name="V1ObjectHeaderPrefix">
IV.A.1.a. Version 1 Data Object Header Prefix</a></h4>
<p>Header messages are aligned on 8-byte boundaries for version 1
object headers.
</p>
<div align="center">
<table class="format">
<caption>
Layout: Version 1 Object Header
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td>Version</td>
<td>Reserved (zero)</td>
<td colspan="2">Total Number of Header Messages</td>
</tr>
<tr>
<td colspan="4">Object Reference Count</td>
</tr>
<tr>
<td colspan="4">Object Header Size</td>
</tr>
<tr>
<td colspan="4">Reserved (zero)</td>
</tr>
<tr>
<td colspan="2">Header Message Type #1</td>
<td colspan="2">Size of Header Message Data #1</td>
</tr>
<tr>
<td>Header Message #1 Flags</td>
<td colspan="3">Reserved (zero)</td>
</tr>
<tr>
<td colspan="4"><br />Header Message Data #1<br /><br /></td>
</tr>
<tr>
<td colspan="4">.<br />.<br />.<br /></td>
</tr>
<tr>
<td colspan="2">Header Message Type #n</td>
<td colspan="2">Size of Header Message Data #n</td>
</tr>
<tr>
<td>Header Message #n Flags</td>
<td colspan="3">Reserved (zero)</td>
</tr>
<tr>
<td colspan="4"><br />Header Message Data #n<br /><br /></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Version 1 Object Header
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Version</p></td>
<td>
<p>This value is used to determine the format of the
information in the object header. When the format of the
object header is changed, the version number
is incremented and can be used to determine how the
information in the object header is formatted. This
is version one (1) (there was no version zero (0)) of the
object header.
</p>
</td>
</tr>
<tr>
<td><p>Total Number of Header Messages</p></td>
<td>
<p>This value determines the total number of messages listed in
object headers for this object. This value includes the messages
in continuation messages for this object.
</p>
</td>
</tr>
<tr>
<td><p>Object Reference Count</p></td>
<td>
<p>This value specifies the number of “hard links” to this object
within the current file. References to the object from external
files, “soft links” in this file and object references in this
file are not tracked.
</p>
</td>
</tr>
<tr>
<td><p>Object Header Size</p></td>
<td>
<p>This value specifies the number of bytes of header message data
following this length field that contain object header messages
for this object header. This value does not include the size of
object header continuation blocks for this object elsewhere in the
file.
</p>
</td>
</tr>
<tr>
<td><p>Header Message #n Type</p></td>
<td>
<p>This value specifies the type of information included in the
following header message data. The message types for
header messages are defined in sections below.
</p>
</td>
</tr>
<tr>
<td><p>Size of Header Message #n Data</p></td>
<td>
<p>This value specifies the number of bytes of header
message data following the header message type and length
information for the current message. The size includes
padding bytes to make the message a multiple of eight
bytes.
</p>
</td>
</tr>
<tr>
<td><p>Header Message #n Flags</p></td>
<td>
<p>This is a bit field with the following definition:
<table class="list">
<tr>
<th width="20%" align="center">Bit</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>If set, the message data is constant. This is used
for messages like the datatype message of a dataset.
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>If set, the message is <em>shared</em> and stored
in another location than the object header. The Header
Message Data field contains a Shared Message
(described in the <a href="#ObjectHeaderMessages">Data Object Header Messages</a>
section below)
and the Size of Header Message Data field
contains the size of that Shared Message.
</td>
</tr>
<tr>
<td align="center"><code>2</code></td>
<td>If set, the message should not be shared.
</td>
</tr>
<tr>
<td align="center"><code>3</code></td>
<td>If set, the HDF5 decoder should fail to open this object
if it does not understand the message’s type and the file
is open with permissions allowing write access to the file.
(Normally, unknown messages can just be ignored by HDF5
decoders)
</td>
</tr>
<tr>
<td align="center"><code>4</code></td>
<td>If set, the HDF5 decoder should set bit 5 of this
message’s flags (in other words, this bit field)
if it does not understand the message’s type
and the object is modified in any way. (Normally,
unknown messages can just be ignored by HDF5
decoders)
</td>
</tr>
<tr>
<td align="center"><code>5</code></td>
<td>If set, this object was modified by software that did not
understand this message.
(Normally, unknown messages should just be ignored by HDF5
decoders) (Can be used to invalidate an index or a similar
feature)
</td>
</tr>
<tr>
<td align="center"><code>6</code></td>
<td>If set, this message is shareable.
</td>
</tr>
<tr>
<td align="center"><code>7</code></td>
<td>If set, the HDF5 decoder should always fail to open this
object if it does not understand the message’s type (whether
it is open for read-only or read-write access). (Normally,
unknown messages can just be ignored by HDF5 decoders)
</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Header Message #n Data</p></td>
<td>
<p>The format and length of this field is determined by the
header message type and size respectively. Some header
message types do not require any data and this information
can be eliminated by setting the length of the message to
zero. The data is padded with enough zeroes to make the
size a multiple of eight.
</p>
</td>
</tr>
</table>
</div>
<h4><a name="V2ObjectHeaderPrefix">
IV.A.1.b. Version 2 Data Object Header Prefix</a></h4>
<p>Note that the “total number of messages” field has been dropped from
the data object header prefix in this version. The number of messages
in the data object header is just determined by the messages encountered
in all the object header blocks.</p>
<p>Note also that the fields and messages in this version of data object
headers have <em>no</em> alignment or padding bytes inserted - they are
stored packed together.</p>
<div align="center">
<table class="format">
<caption>
Layout: Version 2 Object Header
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan="4">Signature</td>
</tr>
<tr>
<td>Version</td>
<td>Flags</td>
<td colspan="2" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4">Access time <em>(optional)</em></td>
</tr>
<tr>
<td colspan="4">Modification Time <em>(optional)</em></td>
</tr>
<tr>
<td colspan="4">Change Time <em>(optional)</em></td>
</tr>
<tr>
<td colspan="4">Birth Time <em>(optional)</em></td>
</tr>
<tr>
<td colspan="2">Maximum # of compact attributes <em>(optional)</em></td>
<td colspan="2">Minimum # of dense attributes <em>(optional)</em></td>
</tr>
<tr>
<td>Size of Chunk #0 <em>(variable size)</em></td>
<td colspan="3" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td>Header Message Type #1</td>
<td colspan="2">Size of Header Message Data #1</td>
<td>Header Message #1 Flags</td>
</tr>
<tr>
<td colspan="2">Header Message #1 Creation Order <em>(optional)</em></td>
<td colspan="2" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4"><br />Header Message Data #1<br /><br /></td>
</tr>
<tr>
<td colspan="4">.<br />.<br />.<br /></td>
</tr>
<tr>
<td>Header Message Type #n</td>
<td colspan="2">Size of Header Message Data #n</td>
<td>Header Message #n Flags</td>
</tr>
<tr>
<td colspan="2">Header Message #n Creation Order <em>(optional)</em></td>
<td colspan="2" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4"><br />Header Message Data #n<br /><br /></td>
</tr>
<tr>
<td colspan="4">Gap <em>(optional, variable size)</em></td>
</tr>
<tr>
<td colspan="4">Checksum</td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Version 2 Object Header
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Signature</p></td>
<td>
<p>The ASCII character string “<code>OHDR</code>”
is used to indicate the beginning of an object header. This
gives file consistency checking utilities a better chance
of reconstructing a damaged file.
</p>
</td>
</tr>
<tr>
<td><p>Version</p></td>
<td>
<p>This field has a value of 2 indicating version 2 of the object header.
</p>
</td>
</tr>
<tr>
<td><p>Flags</p></td>
<td>
<p>This field is a bit field indicating additional information
about the object header.
<table class="list">
<tr>
<th width="20%" align="center">Bit(s)</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0-1</code></td>
<td>This two bit field determines the size of the
<em>Size of Chunk #0</em> field. The values are:
<table class="list">
<tr>
<th width="20%" align="center">Value</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>The <em>Size of Chunk #0</em> field is 1 byte.
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>The <em>Size of Chunk #0</em> field is 2 bytes.
</td>
</tr>
<tr>
<td align="center"><code>2</code></td>
<td>The <em>Size of Chunk #0</em> field is 4 bytes.
</td>
</tr>
<tr>
<td align="center"><code>3</code></td>
<td>The <em>Size of Chunk #0</em> field is 8 bytes.
</td>
</tr>
</table>
</td>
</tr>
<tr>
<td align="center"><code>2</code></td>
<td>If set, attribute creation order is tracked.</td>
</tr>
<tr>
<td align="center"><code>3</code></td>
<td>If set, attribute creation order is indexed.</td>
</tr>
<tr>
<td align="center"><code>4</code></td>
<td>If set, non-default attribute storage phase change
values are stored.</td>
</tr>
<tr>
<td align="center"><code>5</code></td>
<td>If set, access, modification, change and birth times
are stored.</td>
</tr>
<tr>
<td align="center"><code>6-7</code></td>
<td>Reserved</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Access Time</p></td>
<td>
<p>This 32-bit value represents the number of seconds after the
UNIX epoch when the object’s raw data was last accessed
(in other words, read or written).
</p>
<p>This field is present if bit 5 of <em>flags</em> is set.
</p>
</td>
</tr>
<tr>
<td><p>Modification Time</p></td>
<td>
<p>This 32-bit value represents the number of seconds after
the UNIX epoch when the object’s raw data was last
modified (in other words, written).
</p>
<p>This field is present if bit 5 of <em>flags</em> is set.
</p>
</td>
</tr>
<tr>
<td><p>Change Time</p></td>
<td>
<p>This 32-bit value represents the number of seconds after the
UNIX epoch when the object’s metadata was last changed.
</p>
<p>This field is present if bit 5 of <em>flags</em> is set.
</p>
</td>
</tr>
<tr>
<td><p>Birth Time</p></td>
<td>
<p>This 32-bit value represents the number of seconds after the
UNIX epoch when the object was created.
</p>
<p>This field is present if bit 5 of <em>flags</em> is set.
</p>
</td>
</tr>
<tr>
<td><p>Maximum # of compact attributes</p></td>
<td>
<p>This is the maximum number of attributes to store in the compact
format before switching to the indexed format.
</p>
<p>This field is present if bit 4 of <em>flags</em> is set.
</p>
</td>
</tr>
<tr>
<td><p>Minimum # of dense attributes</p></td>
<td>
<p>This is the minimum number of attributes to store in the indexed
format before switching to the compact format.
</p>
<p>This field is present if bit 4 of <em>flags</em> is set.
</p>
</td>
</tr>
<tr>
<td><p>Size of Chunk #0</p></td>
<td>
<p>
This unsigned value specifies the number of bytes of header
message data following this field that contain object header
information.
</p>
<p>
This value does not include the size of object header
continuation blocks for this object elsewhere in the file.
</p>
<p>
The length of this field varies depending on bits 0 and 1 of
the <em>flags</em> field.
</p>
</td>
</tr>
<tr>
<td><p>Header Message #n Type</p></td>
<td>
<p>Same format as version 1 of the object header, described above.
</p>
</td>
</tr>
<tr>
<td><p>Size of Header Message #n Data</p></td>
<td>
<p>This value specifies the number of bytes of header
message data following the header message type and length
information for the current message. The size of messages
in this version does <em>not</em> include any padding bytes.
</p>
</td>
</tr>
<tr>
<td><p>Header Message #n Flags</p></td>
<td>
<p>Same format as version 1 of the object header, described above.
</p>
</td>
</tr>
<tr>
<td><p>Header Message #n Creation Order</p></td>
<td>
<p>This field stores the order that a message of a given type
was created in.
</p>
<p>This field is present if bit 2 of <em>flags</em> is set.
</p>
</td>
</tr>
<tr>
<td><p>Header Message #n Data</p></td>
<td>
<p>Same format as version 1 of the object header, described above.
</p>
</td>
</tr>
<tr>
<td><p>Gap</p></td>
<td>
<p>A gap in an object header chunk is inferred by the end of the
messages for the chunk before the beginning of the chunk’s
checksum. Gaps are always smaller than the size of an
object header message prefix (message type + message size +
message flags).
</p>
<p>Gaps are formed when a message (typically an attribute message)
in an earlier chunk is deleted and a message from a later
chunk that does not quite fit into the free space is moved
into the earlier chunk.
</p>
</td>
</tr>
<tr>
<td><p>Checksum</p></td>
<td>
<p>This is the checksum for the object header chunk.
</p>
</td>
</tr>
</table>
</div>
<p>The header message types and the message data associated with
them compose the critical “metadata” about each object. Some
header messages are required for each object while others are
optional. Some optional header messages may also be repeated
several times in the header itself, the requirements and number
of times allowed in the header will be noted in each header
message description below.
</p>
<h3><a name="ObjectHeaderMessages">
IV.A.2. Disk Format: Level 2A2 - Data Object Header Messages</a></h3>
<p>Data object header messages are small pieces of metadata that are
stored in the data object header for each object in an HDF5 file.
Data object header messages provide the metadata required to describe
an object and its contents, as well as optional pieces of metadata
that annotate the meaning or purpose of the object.
</p>
<p>Data object header messages are either stored directly in the data
object header for the object or are shared between multiple objects
in the file. When a message is shared, a flag in the <em>Message Flags</em>
indicates that the actual <em>Message Data</em>
portion of that message is stored in another location (such as another
data object header, or a heap in the file) and the <em>Message Data</em>
field contains the information needed to locate the actual information
for the message.
</p>
<p>
The format of shared message data is described here:</p>
<div align="center">
<table class="format">
<caption>
Layout: Shared Message (Version 1)
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td>Version</td>
<td>Type</td>
<td colspan="2">Reserved (zero)</td>
</tr>
<tr>
<td colspan="4">Reserved (zero)</td>
</tr>
<tr>
<td colspan="4"><br />Address<sup>O</sup><br /><br /></td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Shared Message (Version 1)
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Version</p></td>
<td><p>The version number is used when there are changes in the format
of a shared object message and is described here:
<table class="list">
<tr>
<th width="20%" align="center">Version</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>Never used.</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>Used by the library before version 1.6.1.
</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Type</p></td>
<td><p>The type of shared message location:
<table class="list">
<tr>
<th width="20%" align="center">Value</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>Message stored in another object’s header (a <em>committed</em>
message).
</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Address</p></td>
<td><p>The address of the object header
containing the message to be shared.</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Shared Message (Version 2)
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td>Version</td>
<td>Type</td>
<td colspan="2" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4"><br />Address<sup>O</sup><br /><br /></td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Shared Message (Version 2)
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Version</p></td>
<td><p>The version number is used when there are changes in the format
of a shared object message and is described here:
<table class="list">
<tr>
<th width="20%" align="center">Version</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>2</code></td>
<td>Used by the library of version 1.6.1 and after.
</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Type</p></td>
<td><p>The type of shared message location:
<table class="list">
<tr>
<th width="20%" align="center">Value</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>Message stored in another object’s header (a <em>committed</em>
message).
</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Address</p></td>
<td><p>The address of the object header
containing the message to be shared.</p></td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Shared Message (Version 3)
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td>Version</td>
<td>Type</td>
<td colspan="2" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4">Location <em>(variable size)</em></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Shared Message (Version 3)
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Version</p></td>
<td><p>The version number indicates changes in the format of shared
object message and is described here:
<table class="list">
<tr>
<th width="20%" align="center">Version</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>3</code></td>
<td>Used by the library of version 1.8 and after. In this
version, the <em>Type</em> field can indicate that
the message is stored in the fractal heap.
</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Type</p></td>
<td><p>The type of shared message location:
<table class="list">
<tr>
<th width="20%" align="center">Value</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>Message is not shared and is not shareable.
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>Message stored in file’s <em>shared object header message</em>
heap (a <em>shared</em> message).
</td>
</tr>
<tr>
<td align="center"><code>2</code></td>
<td>Message stored in another object’s header (a <em>committed</em>
message).
</td>
</tr>
<tr>
<td align="center"><code>3</code></td>
<td>Message stored is not shared, but is shareable.
</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Location</p></td>
<td><p>This field contains either a <a href="#SizeOfOffsetsV0">
<em>Size of Offsets</em></a>-bytes address of the object header
containing the message to be shared, or an 8-byte fractal heap
ID for the message in the file’s <em>shared object header
message</em> heap.
</p>
</td>
</tr>
</table>
</div>
<p>The following is a list of currently defined header messages:
</p>
<h4><a name="NILMessage">IV.A.2.a. The NIL Message</a></h4>
<!-- start msgdesc table -->
<center>
<table class="msgdesc">
<tr><td colspan="2"><b>Header Message Name:</b> NIL</td></tr>
<tr><td colspan="2"><b>Header Message Type:</b> 0x0000</td></tr>
<tr><td colspan="2"><b>Length:</b> Varies</td></tr>
<tr><td colspan="2"><b>Status:</b> Optional; may be repeated.</td></tr>
<tr><td><b>Description:</b></td>
<td>The NIL message is used to indicate a message which is to be
ignored when reading the header messages for a data object.
[Possibly one which has been deleted for some reason.]
</td></tr>
<tr><td colspan="2"><b>Format of Data:</b> Unspecified</td></tr>
</table></center>
<!-- end msgdesc table -->
<h4><a name="DataspaceMessage">IV.A.2.b. The Dataspace Message</a></h4>
<!-- start msgdesc table -->
<center>
<table class="msgdesc">
<tr><td colspan="2"><b>Header Message Name:</b> Dataspace</td></tr>
<tr><td colspan="2"><b>Header Message Type:</b> 0x0001</td></tr>
<tr><td colspan="2"><b>Length:</b> Varies according to the number of
dimensions, as described in the following table.</td></tr>
<tr><td colspan="2"><b>Status:</b> Required for dataset objects;
may not be repeated.</td></tr>
<tr><td><b>Description:</b></td>
<td>The dataspace message describes the number of dimensions (in
other words, “rank”) and size of each dimension that
the data object has. This message is only used for datasets which
have a simple, rectilinear, array-like layout; datasets requiring
a more complex layout are not yet supported.
</td>
</tr>
<tr><td colspan="2"><b>Format of Data:</b> See the tables
below.</td></tr>
</table></center>
<!-- end msgdesc table -->
<div align="center">
<table class="format">
<caption>
Layout: Dataspace Message - Version 1
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td>Version</td>
<td>Dimensionality</td>
<td>Flags</td>
<td>Reserved</td>
</tr>
<tr>
<td colspan="4">Reserved</td>
</tr>
<tr>
<td colspan="4"><br />Dimension #1 Size<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4">.<br />.<br />.<br /></td>
</tr>
<tr>
<td colspan="4"><br />Dimension #n Size<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Dimension #1 Maximum Size<sup>L</sup> <em>(optional)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4">.<br />.<br />.<br /></td>
</tr>
<tr>
<td colspan="4"><br />Dimension #n Maximum Size<sup>L</sup> <em>(optional)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Permutation Index #1<sup>L</sup> <em>(optional)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4">.<br />.<br />.<br /></td>
</tr>
<tr>
<td colspan="4"><br />Permutation Index #n<sup>L</sup> <em>(optional)</em><br /><br /></td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘L’ in the above table are
of the size specified in the <a href="#SizeOfLengthsV0">Size
of Lengths</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Dataspace Message - Version 1
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Version</p></td>
<td>
<p>This value is used to determine the format of the
Dataspace Message. When the format of the
information in the message is changed, the version number
is incremented and can be used to determine how the
information in the object header is formatted. This
document describes version one (1) (there was no version
zero (0)).
</p>
</td>
</tr>
<tr>
<td><p>Dimensionality</p></td>
<td>
<p>This value is the number of dimensions that the data
object has.
</p>
</td>
</tr>
<tr>
<td><p>Flags</p></td>
<td>
<p>This field is used to store flags to indicate the
presence of parts of this message. Bit 0 (the least
significant bit) is used to indicate that maximum
dimensions are present. Bit 1 is used to indicate that
permutation indices are present.
</p>
</td>
</tr>
<tr>
<td><p>Dimension #n Size</p></td>
<td>
<p>This value is the current size of the dimension of the
data as stored in the file. The first dimension stored in
the list of dimensions is the slowest changing dimension
and the last dimension stored is the fastest changing
dimension.
</p>
</td>
</tr>
<tr>
<td><p>Dimension #n Maximum Size</p></td>
<td>
<p>This value is the maximum size of the dimension of the
data as stored in the file. This value may be the special
“<a href="#UnlimitedDim">unlimited</a>” size which indicates
that the data may expand along this dimension indefinitely.
If these values are not stored, the maximum size of each
dimension is assumed to be the dimension’s current size.
</p>
</td>
</tr>
<tr>
<td><p>Permutation Index #n</p></td>
<td>
<p>This value is the index permutation used to map
each dimension from the canonical representation to an
alternate axis for each dimension. If these values are
not stored, the first dimension stored in the list of
dimensions is the slowest changing dimension and the last
dimension stored is the fastest changing dimension.
</p>
</td>
</tr>
</table>
</div>
<br />
<p>Version 2 of the dataspace message dropped the optional
permutation index value support, as it was never implemented in the
HDF5 Library:</p>
<div align="center">
<table class="format">
<caption>
Layout: Dataspace Message - Version 2
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td>Version</td>
<td>Dimensionality</td>
<td>Flags</td>
<td>Type</td>
</tr>
<tr>
<td colspan="4"><br />Dimension #1 Size<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4">.<br />.<br />.<br /></td>
</tr>
<tr>
<td colspan="4"><br />Dimension #n Size<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Dimension #1 Maximum Size<sup>L</sup> <em>(optional)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4">.<br />.<br />.<br /></td>
</tr>
<tr>
<td colspan="4"><br />Dimension #n Maximum Size<sup>L</sup> <em>(optional)</em><br /><br /></td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘L’ in the above table are
of the size specified in the <a href="#SizeOfLengthsV0">Size
of Lengths</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Dataspace Message - Version 2
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Version</p></td>
<td>
<p>This value is used to determine the format of the
Dataspace Message. This field should be ‘2’ for version 2
format messages.
</p>
</td>
</tr>
<tr>
<td><p>Dimensionality</p></td>
<td>
<p>This value is the number of dimensions that the data object has.
</p>
</td>
</tr>
<tr>
<td><p>Flags</p></td>
<td>
<p>This field is used to store flags to indicate the
presence of parts of this message. Bit 0 (the least
significant bit) is used to indicate that maximum
dimensions are present.
</p>
</td>
</tr>
<tr>
<td><p>Type</p></td>
<td>
<p>This field indicates the type of the dataspace:
<table class="list">
<tr>
<th width="20%" align="center">Value</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>A <em>scalar</em> dataspace; in other words,
a dataspace with a single, dimensionless element.
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>A <em>simple</em> dataspace; in other words,
a dataspace with a rank greater than 0 and an
appropriate number of dimensions.
</td>
</tr>
<tr>
<td align="center"><code>2</code></td>
<td>A <em>null</em> dataspace; in other words,
a dataspace with no elements.
</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Dimension #n Size</p></td>
<td>
<p>This value is the current size of the dimension of the
data as stored in the file. The first dimension stored in
the list of dimensions is the slowest changing dimension
and the last dimension stored is the fastest changing
dimension.
</p>
</td>
</tr>
<tr>
<td><p>Dimension #n Maximum Size</p></td>
<td>
<p>This value is the maximum size of the dimension of the
data as stored in the file. This value may be the special
“<a href="#UnlimitedDim">unlimited</a>” size which indicates
that the data may expand along this dimension indefinitely.
If these values are not stored, the maximum size of each
dimension is assumed to be the dimension’s current size.
</p>
</td>
</tr>
</table>
</div>
<!--
<h4><a name="DataSpaceMessage">Header Message Name: Complex Dataspace (Fiber Bundle?)</a></h4>
<!-- start msgdesc table --
<center>
<table class="msgdesc">
<p><b>Header Message Name: ???????</b></td></tr>
<b>Header Message Type: </b>0x0002<br />
<b>Length:</b> Varies</td></tr>
<b>Status:</b> One of the <em>Simple Dataspace</em> or
<em>Complex Dataspace</em> messages is required (but not both) and may
not be repeated.<br /> <b>Description:</b> The
<em>Dataspace</em> message describes space that the dataset is
mapped onto in a more comprehensive way than the <em>Simple
Dimensionality</em> message is capable of handling. The
dataspace of a dataset encompasses the type of coordinate system
used to locate the dataset’s elements as well as the structure and
regularity of the coordinate system. The dataspace also
describes the number of dimensions which the dataset inhabits as
well as a possible higher dimensional space in which the dataset
is located within.
<br />
<p><b>Format of Data:</b></p>
<center>
<table border cellpadding="4" width="80%">
<caption align="bottom">
<b>HDF5 Dataspace Message Layout</b>
</caption>
<tr align="center">
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr align="center">
<td colspan="4">Mesh Type</td>
</tr>
<tr align="center">
<td colspan="4">Logical Dimensionality</td>
</tr>
</table>
</center>
<br />
<dl>
<dt>The elements of the dimensionality message are described below:
<dd>
<dl>
<dt>Mesh Type: (unsigned 32-bit integer)
<dd>This value indicates whether the grid is
polar/spherical/cartesion,
structured/unstructured and regular/irregular. <br />
The mesh type value is broken up as follows: <br />
<br />
<center>
<table border cellpadding="4" width="80%">
<caption align="bottom">
<b>HDF5 Mesh-type Layout</b>
</caption>
<tr align="center">
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr align="center">
<td colspan="1">Mesh Embedding</td>
<td colspan="1">Coordinate System</td>
<td colspan="1">Structure</td>
<td colspan="1">Regularity</td>
</tr>
</table>
</center>
The following are the definitions of mesh-type bytes:
<dl>
<dt>Mesh Embedding
<dd>This value indicates whether the dataset dataspace
is located within
another dataspace or not:
<dl> <dl>
<dt><STANDALONE>
<dd>The dataset mesh is self-contained and is not
embedded in another mesh.
<dt><EMBEDDED>
<dd>The dataset’s dataspace is located within
another dataspace, as
described in information below.
</dl> </dl>
<dt>Coordinate System
<dd>This value defines the type of coordinate system
used for the mesh:
<dl> <dl>
<dt><POLAR>
<dd>The last two dimensions are in polar
coordinates, higher dimensions are
cartesian.
<dt><SPHERICAL>
<dd>The last three dimensions are in spherical
coordinates, higher dimensions
are cartesian.
<dt><CARTESIAN>
<dd>All dimensions are in cartesian coordinates.
</dl> </dl>
<dt>Structure
<dd>This value defines the locations of the grid-points
on the axes:
<dl> <dl>
<dt><STRUCTURED>
<dd>All grid-points are on integral, sequential
locations, starting from 0.
<dt><UNSTRUCTURED>
<dd>Grid-points locations in each dimension are
explicitly defined and
may be of any numeric datatype.
</dl> </dl>
<dt>Regularity
<dd>This value defines the locations of the dataset
points on the grid:
<dl> <dl>
<dt><REGULAR>
<dd>All dataset elements are located at the
grid-points defined.
<dt><IRREGULAR>
<dd>Each dataset element has a particular
grid-location defined.
</dl> </dl>
</dl>
<p>The following grid combinations are currently allowed:</p>
<dl> <dl>
<dt><POLAR-STRUCTURED-REGULAR>
<dt><SPHERICAL-STRUCTURED-REGULAR>
<dt><CARTESIAN-STRUCTURED-REGULAR>
<dt><POLAR-UNSTRUCTURED-REGULAR>
<dt><SPHERICAL-UNSTRUCTURED-REGULAR>
<dt><CARTESIAN-UNSTRUCTURED-REGULAR>
<dt><CARTESIAN-UNSTRUCTURED-IRREGULAR>
</dl> </dl>
All of the above grid types can be embedded within another
dataspace.
<br /> <br />
<dt>Logical Dimensionality: (unsigned 32-bit integer)
<dd>This value is the number of dimensions that the dataset occupies.
<br />
<center>
<table border cellpadding="4" width="80%">
<caption align="bottom">
<b>HDF5 Dataspace Embedded Dimensionality Information</b>
</caption>
<tr align="center">
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr align="center">
<td colspan="4">Embedded Dimensionality</td>
</tr>
<tr align="center">
<td colspan="4">Embedded Dimension Size #1</td>
</tr>
<tr align="center">
<td colspan="4">.<br />.<br />.<br /></td>
</tr>
<tr align="center">
<td colspan="4">Embedded Dimension Size #n</td>
</tr>
<tr align="center">
<td colspan="4">Embedded Origin Location #1</td>
</tr>
<tr align="center">
<td colspan="4">.<br />.<br />.<br /></td>
</tr>
<tr align="center">
<td colspan="4">Embedded Origin Location #n</td>
</tr>
</table>
</center>
<dt>Embedded Dimensionality: (unsigned 32-bit integer)
<dd>This value is the number of dimensions of the space the
dataset is located within: in other words, a planar dataset
located within a 3-D space, a 3-D dataset
which is a subset of another 3-D space, and so on.
<dt>Embedded Dimension Size: (unsigned 32-bit integer)
<dd>These values are the sizes of the dimensions of the
embedded dataspace
that the dataset is located within.
<dt>Embedded Origin Location: (unsigned 32-bit integer)
<dd>These values comprise the location of the dataset’s
origin within the embedded dataspace.
</dl>
</dl>
[Comment: need some way to handle different orientations of the
dataset dataspace
within the embedded dataspace]<br />
<br />
<center>
<table border cellpadding="4" width="80%">
<caption align="bottom">
<b>HDF5 Dataspace Structured/Regular Grid Information</b>
</caption>
<tr align="center">
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr align="center">
<td colspan="4">Logical Dimension Size #1</td>
</tr>
<tr align="center">
<td colspan="4">Logical Dimension Maximum #1</td>
</tr>
<tr align="center">
<td colspan="4">.<br />.<br />.<br /></td>
</tr>
<tr align="center">
<td colspan="4">Logical Dimension Size #n</td>
</tr>
<tr align="center">
<td colspan="4">Logical Dimension Maximum #n</td>
</tr>
</table>
</center>
<br />
<dl>
<dt>The elements of the dimensionality message are described below:
<dd>
<dl>
<dt>Logical Dimension Size #n: (unsigned 32-bit integer)
<dd>This value is the current size of the dimension of the
data as stored in
the file. The first dimension stored in the list of
dimensions is the slowest
changing dimension and the last dimension stored is the
fastest changing
dimension.
<dt>Logical Dimension Maximum #n: (unsigned 32-bit integer)
<dd>This value is the maximum size of the dimension of the
data as stored in
the file. This value may be the special value
<UNLIMITED> which
indicates that the data may expand along this dimension
indefinitely.
</dl>
</dl>
<br />
<center>
<table border cellpadding="4" width="80%">
<caption align="bottom">
<b>HDF5 Dataspace Structured/Irregular Grid Information</b>
</caption>
<tr align="center">
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr align="center">
<td colspan="4"># of Grid Points in Dimension #1</td>
</tr>
<tr align="center">
<td colspan="4">.<br />.<br />.<br /></td>
</tr>
<tr align="center">
<td colspan="4"># of Grid Points in Dimension #n</td>
</tr>
<tr align="center">
<td colspan="4">Datatype of Grid Point Locations</td>
</tr>
<tr align="center">
<td colspan="4">Location of Grid Points in Dimension #1</td>
</tr>
<tr align="center">
<td colspan="4">.<br />.<br />.<br /></td>
</tr>
<tr align="center">
<td colspan="4">Location of Grid Points in Dimension #n</td>
</tr>
</table>
</center>
<br />
<center>
<table border cellpadding="4" width="80%">
<caption align="bottom">
<b>HDF5 Dataspace Unstructured Grid Information</b>
</caption>
<tr align="center">
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr align="center">
<td colspan="4"># of Grid Points</td>
</tr>
<tr align="center">
<td colspan="4">Datatype of Grid Point Locations</td>
</tr>
<tr align="center">
<td colspan="4">Grid Point Locations<br />.<br />.<br /></td>
</tr>
</table>
</center>
-->
<h4><a name="LinkInfoMessage">IV.A.2.c. The Link Info Message</a></h4>
<!-- start msgdesc table -->
<center>
<table class="msgdesc">
<tr><td colspan="2"><b>Header Message Name:</b> Link Info</td></tr>
<tr><td colspan="2"><b>Header Message Type:</b> 0x002 </td></tr>
<tr><td colspan="2"><b>Length:</b> Varies </td></tr>
<tr><td colspan="2"><b>Status:</b> Optional; may not be
repeated. </td></tr>
<tr><td><b>Description:</b></td>
<td>The link info message tracks variable information about the
current state of the links for a “new style”
group’s behavior. Variable information will be stored in
this message and constant information will be stored in the
<a href="#GroupInfoMessage">Group Info</a> message.
</td></tr>
<tr><td colspan="2"><b>Format of Data:</b> See the tables
below.</td></tr>
</table></center>
<!-- end msgdesc table -->
<div align="center">
<table class="format">
<caption>
Layout: Link Info
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td>Version</td>
<td>Flags</td>
<td colspan="2" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4"><br />Maximum Creation Index <em>(8 bytes, optional)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Fractal Heap Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Address of v2 B-tree for Name Index<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Address of v2 B-tree for Creation Order Index<sup>O</sup> <em>(optional)</em><br /><br /></td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Link Info
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Version</p></td>
<td>
<p>The version number for this message. This document describes
version 0.</p>
</td>
</tr>
<tr>
<td><p>Flags</p></td>
<td><p>This field determines various optional aspects of the link
info message:
<table class="list">
<tr>
<th width="20%" align="center">Bit</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>If set, creation order for the links is tracked.
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>If set, creation order for the links is indexed.
</td>
</tr>
<tr>
<td align="center"><code>2-7</code></td>
<td>Reserved</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Maximum Creation Index</p></td>
<td><p>This 64-bit value is the maximum creation order index value
stored for a link in this group.</p>
<p>This field is present if bit 0 of <em>flags</em> is set.</p>
</td>
</tr>
<tr>
<td><p>Fractal Heap Address</p></td>
<td>
<p>
This is the address of the fractal heap to store dense links.
Each link stored in the fractal heap is stored as a
<a href="#LinkMessage">Link Message</a>.
</p>
<p>
If there are no links in the group, or the group’s links
are stored “compactly” (as object header messages), this
value will be the <a href="#UndefinedAddress">undefined
address</a>.
</p>
</td>
</tr>
<tr>
<td><p>Address of v2 B-tree for Name Index</p></td>
<td><p>This is the address of the version 2 B-tree to index names of links.</p>
<p>If there are no links in the group, or the group’s links
are stored “compactly” (as object header messages), this
value will be the <a href="#UndefinedAddress">undefined
address</a>.
</p>
</td>
</tr>
<tr>
<td><p>Address of v2 B-tree for Creation Order Index</p></td>
<td><p>This is the address of the version 2 B-tree to index creation order of links.</p>
<p>If there are no links in the group, or the group’s links
are stored “compactly” (as object header messages), this
value will be the <a href="#UndefinedAddress">undefined
address</a>.
</p>
<p>This field exists if bit 1 of <em>flags</em> is set.</p>
</td>
</tr>
</table>
</div>
<h4><a name="DatatypeMessage">IV.A.2.d. The Datatype Message</a></h4>
<!-- start msgdesc table -->
<center>
<table class="msgdesc">
<tr><td colspan="2"><b>Header Message Name:</b> Datatype</td></tr>
<tr><td colspan="2"><b>Header Message Type:</b> 0x0003
</td></tr>
<tr><td colspan="2"><b>Length:</b> Variable</td></tr>
<tr><td colspan="2"><b>Status:</b> Required for dataset or committed
datatype (formerly named datatype) objects; may not be repeated.
</td></tr>
<tr><td><b>Description:</b></td>
<td><p>The datatype message defines the datatype for each element
of a dataset or a common datatype for sharing between multiple
datasets. A datatype can describe an atomic type like a fixed-
or floating-point type or more complex types like a C struct
(compound datatype), array (array datatype), or C++ vector
(variable-length datatype).</p>
<p>Datatype messages that are part of a dataset object do not
describe how elements are related to one another; the dataspace
message is used for that purpose. Datatype messages that are part of
a committed datatype (formerly named datatype) message describe
a common datatype that can be shared by multiple datasets in the
file.</p>
</td></tr>
<tr><td colspan="2"><b>Format of Data:</b> See the tables
below.</td></tr>
</table></center>
<!-- end msgdesc table -->
<div align="center">
<table class="format">
<caption>
Layout: Datatype Message
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td>Class and Version</td>
<td>Class Bit Field, Bits 0-7</td>
<td>Class Bit Field, Bits 8-15</td>
<td>Class Bit Field, Bits 16-23</td>
</tr>
<tr>
<td colspan="4">Size</td>
</tr>
<tr>
<td colspan="4"><br /><br />Properties<br /><br /><br /></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Datatype Message
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Class and Version</p></td>
<td>
<p>The version of the datatype message and the datatype’s class
information are packed together in this field. The version
number is packed in the top 4 bits of the field and the class
is contained in the bottom 4 bits.
</p>
<p>The version number information is used for changes in the
format of the datatype message and is described here:
<table class="list">
<tr>
<th width="20%" align="center">Version</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>Never used
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>Used by early versions of the library to encode
compound datatypes with explicit array fields.
See the compound datatype description below for
further details.
</td>
</tr>
<tr>
<td align="center"><code>2</code></td>
<td>Used when an array datatype needs to be encoded.
</td>
</tr>
<tr>
<td align="center"><code>3</code></td>
<td>Used when a VAX byte-ordered type needs to be
encoded. Packs various other datatype classes more
efficiently also.
</td>
</tr>
<tr>
<td align="center"><code>4</code></td>
<td>Used to encode the revised reference datatype.
</td>
</tr>
</table></p>
<p>The class of the datatype determines the format for the class
bit field and properties portion of the datatype message, which
are described below. The
following classes are currently defined:
<table class="list">
<tr>
<th width="20%" align="center">Value</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td><a href="#ClassFixedPoint">Fixed-Point</a></td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td><a href="#ClassFloatingPoint">Floating-Point</a></td>
</tr>
<tr>
<td align="center"><code>2</code></td>
<td> <a href="#ClassTime">Time</a></td>
</tr>
<tr>
<td align="center"><code>3</code></td>
<td><a href="#ClassString">String</a></td>
</tr>
<tr>
<td align="center"><code>4</code></td>
<td><a href="#ClassBitField">Bit field</a></td>
</tr>
<tr>
<td align="center"><code>5</code></td>
<td><a href="#ClassOpaque">Opaque</a></td>
</tr>
<tr>
<td align="center"><code>6</code></td>
<td><a href="#ClassCompound">Compound</a></td>
</tr>
<tr>
<td align="center"><code>7</code></td>
<td><a href="#ClassReference">Reference</a></td>
</tr>
<tr>
<td align="center"><code>8</code></td>
<td><a href="#ClassEnum">Enumerated</a></td>
</tr>
<tr>
<td align="center"><code>9</code></td>
<td><a href="#ClassVarLen">Variable-Length</a></td>
</tr>
<tr>
<td align="center"><code>10</code></td>
<td><a href="#ClassArray">Array</a></td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Class Bit Fields</p></td>
<td>
<p>The information in these bit fields is specific to each datatype
class and is described below. All bits not defined for a
datatype class are set to zero.
</p>
</td>
</tr>
<tr>
<td><p>Size</p></td>
<td>
<p>The size of a datatype element in bytes.
</p>
</td>
</tr>
<tr>
<td><p>Properties</p></td>
<td>
<p>This variable-sized sequence of bytes encodes information
specific to each datatype class and is described for each class
below. If there is no property information specified for a
datatype class, the size of this field is zero bytes.
</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<a name="ClassFixedPoint"></a>
<p>Class specific information for the Fixed-point Numbers class
(Class 0):</p>
<div align="center">
<table class="desc">
<caption>
Bits: Fixed-point Bit Field Description
</caption>
<tr>
<th width="10%">Bits</th>
<th>Meaning</th>
</tr>
<tr>
<td><p>0</p></td>
<td><p><b>Byte Order.</b> If zero, byte order is little-endian;
otherwise, byte order is big endian.</p></td>
</tr>
<tr>
<td><p>1, 2</p></td>
<td><p><b>Padding type.</b> Bit 1 is the lo_pad bit and bit 2
is the hi_pad bit. If a datum has unused bits at either
end, then the lo_pad or hi_pad bit is copied to those
locations.</p></td>
</tr>
<tr>
<td><p>3</p></td>
<td><p><b>Signed.</b> If this bit is set then the fixed-point
number is in 2’s complement form.</p></td>
</tr>
<tr>
<td><p>4-23</p></td>
<td><p>Reserved (zero).</p></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="format">
<caption>
Layout: Fixed-point Property Description
</caption>
<tr>
<th width="25%">Byte</th>
<th width="25%">Byte</th>
<th width="25%">Byte</th>
<th width="25%">Byte</th>
</tr>
<tr>
<td colspan="2">Bit Offset</td>
<td colspan="2">Bit Precision</td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Fixed-point Property Description
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Bit Offset</p></td>
<td>
<p>The bit offset of the first significant bit of the fixed-point
value within the datatype. The bit offset specifies the number
of bits “to the right of” the value (which are set to the
lo_pad bit value).
</p>
</td>
</tr>
<tr>
<td><p>Bit Precision</p></td>
<td>
<p>The number of bits of precision of the fixed-point value
within the datatype. This value, combined with the datatype
element’s size and the Bit Offset field specifies the number
of bits “to the left of” the value (which are set to the
hi_pad bit value).
</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<a name="ClassFloatingPoint"></a>
<p>Class specific information for the Floating-point Numbers class
(Class 1):</p>
<div align="center">
<table class="desc">
<caption>
Bits: Floating-point Bit Field Description
</caption>
<tr>
<th width="10%">Bits</th>
<th>Meaning</th>
</tr>
<tr>
<td><p>0, 6</p></td>
<td><p><b>Byte Order.</b> These two non-contiguous bits specify the
“endianness” of the bytes in the datatype element.
<table class="list">
<tr>
<th width="10%" align="center">Bit 6</th>
<th width="10%" align="center">Bit 0</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td align="center"><code>0</code></td>
<td>Byte order is little-endian
</td>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td align="center"><code>1</code></td>
<td>Byte order is big-endian
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td align="center"><code>0</code></td>
<td>Reserved
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td align="center"><code>1</code></td>
<td>Byte order is VAX-endian
</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>1, 2, 3</p></td>
<td><p><b>Padding type.</b> Bit 1 is the low bits pad type, bit 2
is the high bits pad type, and bit 3 is the internal bits
pad type. If a datum has unused bits at either end or between
the sign bit, exponent, or mantissa, then the value of bit
1, 2, or 3 is copied to those locations.</p></td>
</tr>
<tr>
<td><p>4-5</p></td>
<td><p><b>Mantissa Normalization.</b> This 2-bit bit field specifies
how the most significant bit of the mantissa is managed.
<table class="list">
<tr>
<th width="20%" align="center">Value</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>No normalization
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>The most significant bit of the mantissa is always set
(except for 0.0).
</td>
</tr>
<tr>
<td align="center"><code>2</code></td>
<td>The most significant bit of the mantissa is not stored,
but is implied to be set.
</td>
</tr>
<tr>
<td align="center"><code>3</code></td>
<td>Reserved.
</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>7</p></td>
<td><p>Reserved (zero).</p></td>
</tr>
<tr>
<td><p>8-15</p></td>
<td><p><b>Sign Location.</b> This is the bit position of the sign
bit. Bits are numbered with the least significant bit zero.</p></td>
</tr>
<tr>
<td><p>16-23</p></td>
<td><p>Reserved (zero).</p></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="format">
<caption>
Layout: Floating-point Property Description
</caption>
<tr>
<th width="25%">Byte</th>
<th width="25%">Byte</th>
<th width="25%">Byte</th>
<th width="25%">Byte</th>
</tr>
<tr>
<td colspan="2">Bit Offset</td>
<td colspan="2">Bit Precision</td>
</tr>
<tr>
<td>Exponent Location</td>
<td>Exponent Size</td>
<td>Mantissa Location</td>
<td>Mantissa Size</td>
</tr>
<tr>
<td colspan="4">Exponent Bias</td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Floating-point Property Description
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Bit Offset</p></td>
<td>
<p>The bit offset of the first significant bit of the floating-point
value within the datatype. The bit offset specifies the number
of bits “to the right of” the value.
</p>
</td>
</tr>
<tr>
<td><p>Bit Precision</p></td>
<td>
<p>The number of bits of precision of the floating-point value
within the datatype.
</p>
</td>
</tr>
<tr>
<td><p>Exponent Location</p></td>
<td>
<p>The bit position of the exponent field. Bits are numbered with
the least significant bit number zero.
</p>
</td>
</tr>
<tr>
<td><p>Exponent Size</p></td>
<td>
<p>The size of the exponent field in bits.
</p>
</td>
</tr>
<tr>
<td><p>Mantissa Location</p></td>
<td>
<p>The bit position of the mantissa field. Bits are numbered with
the least significant bit number zero.
</p>
</td>
</tr>
<tr>
<td><p>Mantissa Size</p></td>
<td>
<p>The size of the mantissa field in bits.
</p>
</td>
</tr>
<tr>
<td><p>Exponent Bias</p></td>
<td>
<p>The bias of the exponent field.
</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<a name="ClassTime"></a>
<p>Class specific information for the Time class (Class 2):</p>
<div align="center">
<table class="desc">
<caption>
Bits: Time Bit Field Description
</caption>
<tr>
<th width="10%">Bits</th>
<th>Meaning</th>
</tr>
<tr>
<td><p>0</p></td>
<td><p><b>Byte Order.</b> If zero, byte order is little-endian;
otherwise, byte order is big endian.</p></td>
</tr>
<tr>
<td><p>1-23</p></td>
<td><p>Reserved (zero).</p></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="format">
<caption>
Layout: Time Property Description
</caption>
<tr>
<th width="25%">Byte</th>
<th width="25%">Byte</th>
</tr>
<tr>
<td colspan="2">Bit Precision</td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Time Property Description
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Bit Precision</p></td>
<td>
<p>The number of bits of precision of the time value.
</p>
</td>
</tr>
</table>
</div>
<br />
<a name="ClassString"></a>
<p>Class specific information for the Strings class (Class 3):</p>
<div align="center">
<table class="desc">
<caption>
Bits: String Bit Field Description
</caption>
<tr>
<th width="10%">Bits</th>
<th>Meaning</th>
</tr>
<tr>
<td><p>0-3</p></td>
<td><p><b>Padding type.</b> This four-bit value determines the
type of padding to use for the string. The values are:
<table class="list">
<tr>
<th width="20%" align="center">Value</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>Null Terminate: A zero byte marks the end of the
string and is guaranteed to be present after
converting a long string to a short string. When
converting a short string to a long string the value is
padded with additional null characters as necessary.
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>Null Pad: Null characters are added to the end of
the value during conversions from short values to long
values but conversion in the opposite direction simply
truncates the value.
</td>
</tr>
<tr>
<td align="center"><code>2</code></td>
<td>Space Pad: Space characters are added to the end of
the value during conversions from short values to long
values but conversion in the opposite direction simply
truncates the value. This is the Fortran
representation of the string.
</td>
</tr>
<tr>
<td align="center"><code>3-15</code></td>
<td>Reserved
</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>4-7</p></td>
<td><p><b>Character Set.</b> The character set used to
encode the string.
<table class="list">
<tr>
<th width="20%" align="center">Value</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>ASCII character set encoding
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>UTF-8 character set encoding
</td>
</tr>
<tr>
<td align="center"><code>2-15</code></td>
<td>Reserved
</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>8-23</p></td>
<td><p>Reserved (zero).</p></td>
</tr>
</table>
</div>
<p>There are no properties defined for the string class.
</p>
<br />
<br />
<a name="ClassBitField"></a>
<p>Class specific information for the Bit Fields class (Class 4):</p>
<div align="center">
<table class="desc">
<caption>
Bits: Bitfield Bit Field Description
</caption>
<tr>
<th width="10%">Bits</th>
<th>Meaning</th>
</tr>
<tr>
<td><p>0</p></td>
<td><p><b>Byte Order.</b> If zero, byte order is little-endian;
otherwise, byte order is big endian.</p></td>
</tr>
<tr>
<td><p>1, 2</p></td>
<td><p><b>Padding type.</b> Bit 1 is the lo_pad type and bit 2
is the hi_pad type. If a datum has unused bits at either
end, then the lo_pad or hi_pad bit is copied to those
locations.</p></td>
</tr>
<tr>
<td><p>3-23</p></td>
<td><p>Reserved (zero).</p></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="format">
<caption>
Layout: Bit Field Property Description
</caption>
<tr>
<th width="25%">Byte</th>
<th width="25%">Byte</th>
<th width="25%">Byte</th>
<th width="25%">Byte</th>
</tr>
<tr>
<td colspan="2">Bit Offset</td>
<td colspan="2">Bit Precision</td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Bit Field Property Description
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Bit Offset</p></td>
<td>
<p>The bit offset of the first significant bit of the bit field
within the datatype. The bit offset specifies the number
of bits “to the right of” the value.
</p>
</td>
</tr>
<tr>
<td><p>Bit Precision</p></td>
<td>
<p>The number of bits of precision of the bit field
within the datatype.
</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<a name="ClassOpaque"></a>
<p>Class specific information for the Opaque class (Class 5):</p>
<div align="center">
<table class="desc">
<caption>
Bits: Opaque Bit Field Description
</caption>
<tr>
<th width="10%">Bits</th>
<th>Meaning</th>
</tr>
<tr>
<td><p>0-7</p></td>
<td><p>Length of ASCII tag in bytes.</p></td>
</tr>
<tr>
<td><p>8-23</p></td>
<td><p>Reserved (zero).</p></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="format">
<caption>
Layout: Opaque Property Description
</caption>
<tr>
<th width="25%">Byte</th>
<th width="25%">Byte</th>
<th width="25%">Byte</th>
<th width="25%">Byte</th>
</tr>
<tr>
<td colspan="4"><br />ASCII Tag<br />
<br /></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Opaque Property Description
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>ASCII Tag</p></td>
<td>
<p>This NUL-terminated string provides a description for the
opaque type. It is NUL-padded to a multiple of 8 bytes.
</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<a name="ClassCompound"></a>
<p>Class specific information for the Compound class (Class 6):</p>
<div align="center">
<table class="desc">
<caption>
Bits: Compound Bit Field Description
</caption>
<tr>
<th width="10%">Bits</th>
<th>Meaning</th>
</tr>
<tr>
<td><p>0-15</p></td>
<td><p><b>Number of Members.</b> This field contains the number
of members defined for the compound datatype. The member
definitions are listed in the Properties field of the data
type message.</p></td>
</tr>
<tr>
<td><p>16-23</p></td>
<td><p>Reserved (zero).</p></td>
</tr>
</table>
</div>
<p>The Properties field of a compound datatype is a list of the
member definitions of the compound datatype. The member
definitions appear one after another with no intervening bytes.
The member types are described with a (recursively) encoded datatype
message.</p>
<p>Note that the property descriptions are different for different
versions of the datatype version. Additionally note that the version
0 datatype encoding is deprecated and has been replaced with later
encodings in versions of the HDF5 Library from the 1.4 release
onward.</p>
<div align="center">
<table class="format">
<caption>
Layout: Compound Properties Description for Datatype Version 1
</caption>
<tr>
<th width="25%">Byte</th>
<th width="25%">Byte</th>
<th width="25%">Byte</th>
<th width="25%">Byte</th>
</tr>
<tr>
<td colspan="4"><br />Name<br /><br /></td>
</tr>
<tr>
<td colspan="4">Byte Offset of Member</td>
</tr>
<tr>
<td>Dimensionality</td>
<td colspan="3">Reserved (zero)</td>
</tr>
<tr>
<td colspan="4">Dimension Permutation</td>
</tr>
<tr>
<td colspan="4">Reserved (zero)</td>
</tr>
<tr>
<td colspan="4">Dimension #1 Size (required)</td>
</tr>
<tr>
<td colspan="4">Dimension #2 Size (required)</td>
</tr>
<tr>
<td colspan="4">Dimension #3 Size (required)</td>
</tr>
<tr>
<td colspan="4">Dimension #4 Size (required)</td>
</tr>
<tr>
<td colspan="4"><br />Member Type Message<br /><br /></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Compound Properties Description for Datatype Version 1
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Name</p></td>
<td>
<p>This NUL-terminated string provides a description for the
opaque type. It is NUL-padded to a multiple of 8 bytes.
</p>
</td>
</tr>
<tr>
<td><p>Byte Offset of Member</p></td>
<td>
<p>This is the byte offset of the member within the datatype.
</p>
</td>
</tr>
<tr>
<td><p>Dimensionality</p></td>
<td>
<p>If set to zero, this field indicates a scalar member. If set
to a value greater than zero, this field indicates that the
member is an array of values. For array members, the size of
the array is indicated by the ‘Size of Dimension n’ field in
this message.
</p>
</td>
</tr>
<tr>
<td><p>Dimension Permutation</p></td>
<td>
<p>This field was intended to allow an array field to have
its dimensions permuted, but this was never implemented.
This field should always be set to zero.
</p>
</td>
</tr>
<tr>
<td><p>Dimension #n Size</p></td>
<td>
<p>This field is the size of a dimension of the array field as
stored in the file. The first dimension stored in the list of
dimensions is the slowest changing dimension and the last
dimension stored is the fastest changing dimension.
</p>
</td>
</tr>
<tr>
<td><p>Member Type Message</p></td>
<td>
<p>This field is a datatype message describing the datatype of
the member.
</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Compound Properties Description for Datatype Version 2
</caption>
<tr>
<th width="25%">Byte</th>
<th width="25%">Byte</th>
<th width="25%">Byte</th>
<th width="25%">Byte</th>
</tr>
<tr>
<td colspan="4"><br />Name<br /><br /></td>
</tr>
<tr>
<td colspan="4">Byte Offset of Member</td>
</tr>
<tr>
<td colspan="4"><br />Member Type Message<br /><br /></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Compound Properties Description for Datatype Version 2
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Name</p></td>
<td>
<p>This NUL-terminated string provides a description for the
opaque type. It is NUL-padded to a multiple of 8 bytes.
</p>
</td>
</tr>
<tr>
<td><p>Byte Offset of Member</p></td>
<td>
<p>This is the byte offset of the member within the datatype.
</p>
</td>
</tr>
<tr>
<td><p>Member Type Message</p></td>
<td>
<p>This field is a datatype message describing the datatype of
the member.
</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Compound Properties Description for Datatype Version 3
</caption>
<tr>
<th width="25%">Byte</th>
<th width="25%">Byte</th>
<th width="25%">Byte</th>
<th width="25%">Byte</th>
</tr>
<tr>
<td colspan="4"><br />Name<br /><br /></td>
</tr>
<tr>
<td colspan="4">Byte Offset of Member <em>(variable size)</em></td>
</tr>
<tr>
<td colspan="4"><br />Member Type Message<br /><br /></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Compound Properties Description for Datatype Version 3
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Name</p></td>
<td><p>This NUL-terminated string provides a description for the
opaque type. It is <em>not</em> NUL-padded to a multiple of 8
bytes.</p></td>
</tr>
<tr>
<td><p>Byte Offset of Member</p></td>
<td><p>This is the byte offset of the member within the datatype.
The field size is the minimum number of bytes necessary,
based on the size of the datatype element. For example, a
datatype element size of less than 256 bytes uses a 1 byte
length, a datatype element size of 256-65535 bytes uses a
2 byte length, and so on.</p></td>
</tr>
<tr>
<td><p>Member Type Message</p></td>
<td><p>This field is a datatype message describing the datatype of
the member.</p></td>
</tr>
</table>
</div>
<br />
<br />
<a name="ClassReference"></a>
<p>Class specific information for the Reference class (Class 7):</p>
<div align="center">
<table class="desc">
<caption>
Bits: Reference Bit Field Description for Datatype Version < 4
</caption>
<tr>
<th width="10%">Bits</th>
<th>Meaning</th>
</tr>
<tr>
<td><p>0-3</p></td>
<td><p><b>Type.</b> This four-bit value contains the reference types which are supported for
backward compatibility. The values defined are:
<table class="list">
<tr>
<th width="20%" align="center">Value</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>Object Reference (H5R_OBJECT1): A reference to another object in this
HDF5 file.
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>Dataset Region Reference (H5R_DATASET_REGION1): A reference to a region within
a dataset in this HDF5 file.
</td>
</tr>
<tr>
<td align="center"><code>2-15</code></td>
<td>Reserved
</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>4-23</p></td>
<td><p>Reserved (zero).</p></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Bits: Reference Bit Field Description for Datatype Version 4
</caption>
<tr>
<th width="10%">Bits</th>
<th>Meaning</th>
</tr>
<tr>
<td><p>0-3</p></td>
<td><p><b>Type.</b> This four-bit value contains the revised reference types.
The values defined are:
<table class="list">
<tr>
<th width="20%" align="center">Value</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>2</code></td>
<td>Object Reference (H5R_OBJECT2): A reference to another object
in this file or an external file.
</td>
</tr>
<tr>
<td align="center"><code>3</code></td>
<td>Dataset Region Reference (H5R_DATASET_REGION2): A reference to a region within
a dataset in this file or an external file.
</td>
</tr>
<tr>
<td align="center"><code>4</code></td>
<td>Attribute Reference (H5R_ATTR): A reference to an attribute attached to an
object in this file or an external file.
</td>
</tr>
<tr>
<td align="center"><code>5-15</code></td>
<td>Reserved
</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>4-7</p></td>
<td><p><b>Version.</b> This four-bit value contains the version for encoding
the revised reference types. The values defined are:
<table class="list">
<tr>
<th width="20%" align="center">Value</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>Unused
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>The version for encoding the revised reference types: Object Reference (2),
Dataset Region Reference (3) and Attribute Reference (4).
</td>
</tr>
<tr>
<td align="center"><code>2-15</code></td>
<td>Reserved
</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>8-23</p></td>
<td><p>Reserved (zero).</p></td>
</tr>
</table>
</div>
<p>There are no properties defined for the reference class.
</p>
<br />
<br />
<a name="ClassEnum"></a>
<p>Class specific information for the Enumeration class (Class 8):</p>
<div align="center">
<table class="desc">
<caption>
Bits: Enumeration Bit Field Description
</caption>
<tr>
<th width="10%">Bits</th>
<th>Meaning</th>
</tr>
<tr>
<td><p>0-15</p></td>
<td><p><b>Number of Members.</b> The number of name/value
pairs defined for the enumeration type.</p></td>
</tr>
<tr>
<td><p>16-23</p></td>
<td><p>Reserved (zero).</p></td>
</tr>
</table>
</div>
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Enumeration Property Description for Datatype Versions
1 and 2
</caption>
<tr>
<th width="25%">Byte</th>
<th width="25%">Byte</th>
<th width="25%">Byte</th>
<th width="25%">Byte</th>
</tr>
<tr>
<td colspan="4"><br />Base Type<br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Names<br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Values<br /><br /></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Enumeration Property Description for Datatype Versions
1 and 2
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Base Type</p></td>
<td>
<p>Each enumeration type is based on some parent type, usually an
integer. The information for that parent type is described
recursively by this field.
</p>
</td>
</tr>
<tr>
<td><p>Names</p></td>
<td>
<p>The name for each name/value pair. Each name is stored as a null
terminated ASCII string in a multiple of eight bytes. The names
are in no particular order.
</p>
</td>
</tr>
<tr>
<td><p>Values</p></td>
<td>
<p>The list of values in the same order as the names. The values
are packed (no inter-value padding) and the size of each value
is determined by the parent type.
</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Enumeration Property Description for Datatype Version 3
</caption>
<tr>
<th width="25%">Byte</th>
<th width="25%">Byte</th>
<th width="25%">Byte</th>
<th width="25%">Byte</th>
</tr>
<tr>
<td colspan="4"><br />Base Type<br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Names<br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Values<br /><br /></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Enumeration Property Description for Datatype Version 3
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Base Type</p></td>
<td>
<p>Each enumeration type is based on some parent type, usually an
integer. The information for that parent type is described
recursively by this field.
</p>
</td>
</tr>
<tr>
<td><p>Names</p></td>
<td>
<p>The name for each name/value pair. Each name is stored as a null
terminated ASCII string, <em>not</em> padded to a multiple of
eight bytes. The names are in no particular order.
</p>
</td>
</tr>
<tr>
<td><p>Values</p></td>
<td>
<p>The list of values in the same order as the names. The values
are packed (no inter-value padding) and the size of each value
is determined by the parent type.
</p>
</td>
</tr>
</table>
</div>
<br />
<a name="ClassVarLen"></a>
<p>Class specific information for the Variable-length class (Class 9):</p>
<div align="center">
<table class="desc">
<caption>
Bits: Variable-length Bit Field Description
</caption>
<tr>
<th width="10%">Bits</th>
<th>Meaning</th>
</tr>
<tr>
<td><p>0-3</p></td>
<td><p><b>Type.</b> This four-bit value contains the type of
variable-length datatype described. The values defined are:
<table class="list">
<tr>
<th width="20%" align="center">Value</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>Sequence: A variable-length sequence of any datatype.
Variable-length sequences do not have padding or
character set information.
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>String: A variable-length sequence of characters.
Variable-length strings have padding and character set
information.
</td>
</tr>
<tr>
<td align="center"><code>2-15</code></td>
<td>Reserved
</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>4-7</p></td>
<td><p><b>Padding type.</b> (variable-length string only)
This four-bit value determines the type of padding
used for variable-length strings. The values are the same
as for the string padding type, as follows:
<table class="list">
<tr>
<th width="20%" align="center">Value</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>Null terminate: A zero byte marks the end of a string
and is guaranteed to be present after converting a long
string to a short string. When converting a short string
to a long string, the value is padded with additional null
characters as necessary.
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>Null pad: Null characters are added to the end of the
value during conversion from a short string to a longer
string. Conversion from a long string to a shorter string
simply truncates the value.
</td>
</tr>
<tr>
<td align="center"><code>2</code></td>
<td>Space pad: Space characters are added to the end of the
value during conversion from a short string to a longer
string. Conversion from a long string to a shorter string
simply truncates the value. This is the Fortran
representation of the string.
</td>
</tr>
<tr>
<td align="center"><code>3-15</code></td>
<td>Reserved
</td>
</tr>
</table></p>
<p>This value is set to zero for variable-length sequences.</p>
</td>
</tr>
<tr>
<td><p>8-11</p></td>
<td><p><b>Character Set.</b> (variable-length string only)
This four-bit value specifies the character set
to be used for encoding the string:
<table class="list">
<tr>
<th width="20%" align="center">Value</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>ASCII character set encoding
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>UTF-8 character set encoding
</td>
</tr>
<tr>
<td align="center"><code>2-15</code></td>
<td>Reserved
</td>
</tr>
</table></p>
<p>This value is set to zero for variable-length sequences.</p>
</td>
</tr>
<tr>
<td><p>12-23</p></td>
<td><p>Reserved (zero).</p></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="format">
<caption>
Layout: Variable-length Property Description
</caption>
<tr>
<th width="25%">Byte</th>
<th width="25%">Byte</th>
<th width="25%">Byte</th>
<th width="25%">Byte</th>
</tr>
<tr>
<td colspan="4"><br />Base Type<br /><br /></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Variable-length Property Description
</caption>
<tr>
<th width="10%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Base Type</p></td>
<td>
<p>Each variable-length type is based on some parent type. The
information for that parent type is described recursively by
this field.
</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<a name="ClassArray"></a>
<p>Class specific information for the Array class (Class 10):</p>
<p>There are no bit fields defined for the array class.
</p>
<p>Note that the dimension information defined in the property for this
datatype class is independent of dataspace information for a dataset.
The dimension information here describes the dimensionality of the
information within a data element (or a component of an element, if the
array datatype is nested within another datatype) and the dataspace for a
dataset describes the size and locations of the elements in a dataset.
</p>
<div align="center">
<table class="format">
<caption>
Layout: Array Property Description for Datatype Version 2
</caption>
<tr>
<th width="25%">Byte</th>
<th width="25%">Byte</th>
<th width="25%">Byte</th>
<th width="25%">Byte</th>
</tr>
<tr>
<td>Dimensionality</td>
<td colspan="3">Reserved (zero)</td>
</tr>
<tr>
<td colspan="4">Dimension #1 Size</td>
</tr>
<tr>
<td colspan="4">.<br />.<br />.<br /></td>
</tr>
<tr>
<td colspan="4">Dimension #n Size</td>
</tr>
<tr>
<td colspan="4">Permutation Index #1</td>
</tr>
<tr>
<td colspan="4">.<br />.<br />.<br /></td>
</tr>
<tr>
<td colspan="4">Permutation Index #n</td>
</tr>
<tr>
<td colspan="4"><br />Base Type<br /><br /></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Array Property Description for Datatype Version 2
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Dimensionality</p></td>
<td>
<p>This value is the number of dimensions that the array has.
</p>
</td>
</tr>
<tr>
<td><p>Dimension #n Size</p></td>
<td>
<p>This value is the size of the dimension of the array
as stored in the file. The first dimension stored in
the list of dimensions is the slowest changing dimension
and the last dimension stored is the fastest changing
dimension.
</p>
</td>
</tr>
<tr>
<td><p>Permutation Index #n</p></td>
<td>
<p>This value is the index permutation used to map
each dimension from the canonical representation to an
alternate axis for each dimension. Currently, dimension
permutations are not supported, and these indices should
be set to the index position minus one. In other words,
the first dimension should be set to 0, the second dimension
should be set to 1, and so on.
</p>
</td>
</tr>
<tr>
<td><p>Base Type</p></td>
<td>
<p>Each array type is based on some parent type. The
information for that parent type is described recursively by
this field.
</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Array Property Description for Datatype Version 3
</caption>
<tr>
<th width="25%">Byte</th>
<th width="25%">Byte</th>
<th width="25%">Byte</th>
<th width="25%">Byte</th>
</tr>
<tr>
<td>Dimensionality</td>
<td colspan="3" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4">Dimension #1 Size</td>
</tr>
<tr>
<td colspan="4">.<br />.<br />.<br /></td>
</tr>
<tr>
<td colspan="4">Dimension #n Size</td>
</tr>
<tr>
<td colspan="4"><br />Base Type<br /><br /></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Array Property Description for Datatype Version 3
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Dimensionality</p></td>
<td>
<p>This value is the number of dimensions that the array has.
</p>
</td>
</tr>
<tr>
<td><p>Dimension #n Size</p></td>
<td>
<p>This value is the size of the dimension of the array
as stored in the file. The first dimension stored in
the list of dimensions is the slowest changing dimension
and the last dimension stored is the fastest changing
dimension.
</p>
</td>
</tr>
<tr>
<td><p>Base Type</p></td>
<td>
<p>Each array type is based on some parent type. The
information for that parent type is described recursively by
this field.
</p>
</td>
</tr>
</table>
</div>
<h4><a name="OldFillValueMessage">IV.A.2.e. The Data Storage -
Fill Value (Old) Message</a></h4>
<!-- start msgdesc table -->
<center>
<table class="msgdesc">
<tr><td colspan="2"><b>Header Message Name:</b> Fill Value
(old)</td></tr>
<tr><td colspan="2"><b>Header Message Type:</b> 0x0004</td></tr>
<tr><td colspan="2"><b>Length:</b> Varies</td></tr>
<tr><td colspan="2"><b>Status:</b> Optional; may not be
repeated.</td></tr>
<tr><td><b>Description:</b></td>
<td><p>The fill value message stores a single data value which
is returned to the application when an uninitialized data element
is read from a dataset. The fill value is interpreted with the
same datatype as the dataset. If no fill value message is present
then a fill value of all zero bytes is assumed.</p>
<p>This fill value message is deprecated in favor of the
“new” fill value message (Message Type 0x0005) and
is only written to the file for forward compatibility with
versions of the HDF5 Library before the 1.6.0 version.
Additionally, it only appears for datasets with a user-defined
fill value (as opposed to the library default fill value or an
explicitly set “undefined” fill value).</p>
</td></tr>
<tr><td colspan="2"><b>Format of Data:</b> See the tables
below.</td></tr>
</table></center>
<!-- end msgdesc table -->
<div align="center">
<table class="format">
<caption>
Layout: Fill Value Message (Old)
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td colspan="4">Size</td>
</tr>
<tr>
<td colspan="4"><br />Fill Value <em>(optional, variable size)</em><br /><br /></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Fill Value Message (Old)
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Size</p></td>
<td>
<p>This is the size of the Fill Value field in bytes.
</p>
</td>
</tr>
<tr>
<td><p>Fill Value</p></td>
<td>
<p>The fill value. The bytes of the fill value are interpreted
using the same datatype as for the dataset.
</p>
</td>
</tr>
</table>
</div>
<h4><a name="FillValueMessage">IV.A.2.f. The Data Storage -
Fill Value Message</a></h4>
<!-- start msgdesc table -->
<center>
<table class="msgdesc">
<tr><td colspan="2"><b>Header Message Name:</b> Fill
Value</td></tr>
<tr><td colspan="2"><b>Header Message Type:</b> 0x0005</td></tr>
<tr><td colspan="2"><b>Length:</b> Varies</td></tr>
<tr><td colspan="2"><b>Status:</b> Required for dataset objects;
may not be repeated.</td></tr>
<tr><td><b>Description:</b></td>
<td>The fill value message stores a single data value which is
returned to the application when an uninitialized data element
is read from a dataset. The fill value is interpreted with the
same datatype as the dataset.</td></tr>
<tr><td colspan="2"><b>Format of Data:</b> See the tables
below.</td></tr>
</table></center>
<!-- end msgdesc table -->
<div align="center">
<table class="format">
<caption>
Layout: Fill Value Message - Versions 1 and 2
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td>Version</td>
<td>Space Allocation Time</td>
<td>Fill Value Write Time</td>
<td>Fill Value Defined</td>
</tr>
<tr>
<td colspan="4">Size <em>(optional)</em></td>
</tr>
<tr>
<td colspan="4"><br />Fill Value <em>(optional, variable size)</em><br /><br /></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Fill Value Message - Versions 1 and 2
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Version</p></td>
<td>
<p>The version number information is used for changes in the
format of the fill value message and is described here:
<table class="list">
<tr>
<th width="20%" align="center">Version</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>Never used
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>Initial version of this message.
</td>
</tr>
<tr>
<td align="center"><code>2</code></td>
<td>In this version, the Size and Fill Value fields are
only present if the Fill Value Defined field is set
to 1.
</td>
</tr>
<tr>
<td align="center"><code>3</code></td>
<td>This version packs the other fields in the message
more efficiently than version 2.
</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Space Allocation Time</p></td>
<td>
<p>When the storage space for the dataset’s raw data will be
allocated. The allowed values are:
<table class="list">
<tr>
<th width="20%" align="center">Value</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>Not used.
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>Early allocation. Storage space for the entire dataset
should be allocated in the file when the dataset is
created.
</td>
</tr>
<tr>
<td align="center"><code>2</code></td>
<td>Late allocation. Storage space for the entire dataset
should not be allocated until the dataset is written
to.
</td>
</tr>
<tr>
<td align="center"><code>3</code></td>
<td>Incremental allocation. Storage space for the
dataset should not be allocated until the portion
of the dataset is written to. This is currently
used in conjunction with chunked data storage for
datasets.
</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Fill Value Write Time</p></td>
<td>
<p>At the time that storage space for the dataset’s raw data is
allocated, this value indicates whether the fill value should
be written to the raw data storage elements. The allowed values
are:
<table class="list">
<tr>
<th width="20%" align="center">Value</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>On allocation. The fill value is always written to
the raw data storage when the storage space is allocated.
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>Never. The fill value should never be written to
the raw data storage.
</td>
</tr>
<tr>
<td align="center"><code>2</code></td>
<td>Fill value written if set by user. The fill value
will be written to the raw data storage when the storage
space is allocated only if the user explicitly set
the fill value. If the fill value is the library
default or is undefined, it will not be written to
the raw data storage.
</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Fill Value Defined</p></td>
<td>
<p>This value indicates if a fill value is defined for this
dataset. If this value is 0, the fill value is undefined.
If this value is 1, a fill value is defined for this dataset.
For version 2 or later of the fill value message, this value
controls the presence of the Size and Fill Value fields.
</p>
</td>
</tr>
<tr>
<td><p>Size</p></td>
<td>
<p>This is the size of the Fill Value field in bytes. This field
is not present if the Version field is greater than 1,
and the Fill Value Defined field is set to 0.
</p>
</td>
</tr>
<tr>
<td><p>Fill Value</p></td>
<td>
<p>The fill value. The bytes of the fill value are interpreted
using the same datatype as for the dataset. This field is
not present if the Version field is greater than 1,
and the Fill Value Defined field is set to 0.
</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Fill Value Message - Version 3
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td>Version</td>
<td>Flags</td>
<td colspan="2" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4">Size <em>(optional)</em></td>
</tr>
<tr>
<td colspan="4"><br />Fill Value <em>(optional, variable size)</em><br /><br /></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Fill Value Message - Version 3
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Version</p></td>
<td>
<p>The version number information is used for changes in the
format of the fill value message and is described here:
<table class="list">
<tr>
<th width="20%" align="center">Version</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>Never used
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>Initial version of this message.
</td>
</tr>
<tr>
<td align="center"><code>2</code></td>
<td>In this version, the Size and Fill Value fields are
only present if the Fill Value Defined field is set
to 1.
</td>
</tr>
<tr>
<td align="center"><code>3</code></td>
<td>This version packs the other fields in the message
more efficiently than version 2.
</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Flags</p></td>
<td>
<p>When the storage space for the dataset’s raw data will be
allocated. The allowed values are:
<table class="list">
<tr>
<th width="20%" align="center">Bits</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0-1</code></td>
<td>Space Allocation Time, with the same
values as versions 1 and 2 of the message.
</td>
</tr>
<tr>
<td align="center"><code>2-3</code></td>
<td>Fill Value Write Time, with the same
values as versions 1 and 2 of the message.
</td>
</tr>
<tr>
<td align="center"><code>4</code></td>
<td>Fill Value Undefined, indicating that the fill
value has been marked as “undefined” for this dataset.
Bits 4 and 5 cannot both be set.
</td>
</tr>
<tr>
<td align="center"><code>5</code></td>
<td>Fill Value Defined, with the same values as
versions 1 and 2 of the message.
Bits 4 and 5 cannot both be set.
</td>
</tr>
<tr>
<td align="center"><code>6-7</code></td>
<td>Reserved (zero).
</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Size</p></td>
<td>
<p>This is the size of the Fill Value field in bytes. This field
is not present if the Version field is greater than 1,
and the Fill Value Defined flag is set to 0.
</p>
</td>
</tr>
<tr>
<td><p>Fill Value</p></td>
<td>
<p>The fill value. The bytes of the fill value are interpreted
using the same datatype as for the dataset. This field is
not present if the Version field is greater than 1,
and the Fill Value Defined flag is set to 0.
</p>
</td>
</tr>
</table>
</div>
<h4><a name="LinkMessage">IV.A.2.g. The Link Message</a></h4>
<!-- start msgdesc table -->
<center>
<table class="msgdesc">
<tr><td colspan="2"><b>Header Message Name:</b> Link</td></tr>
<tr><td colspan="2"><b>Header Message Type:</b> 0x0006</td></tr>
<tr><td colspan="2"><b>Length:</b> Varies </td></tr>
<tr><td colspan="2"><b>Status:</b> Optional; may be
repeated. </td></tr>
<tr><td><b>Description:</b></td>
<td><p>This message encodes the information for a link in a
group’s object header, when the group is storing its links
“compactly”, or in the group’s fractal heap,
when the group is storing its links “densely”.</p>
<p>A group is storing its links compactly when the fractal heap
address in the <em><a href="#LinkInfoMessage">Link Info
Message</a></em> is set to the “undefined address”
value.</p></td></tr>
<tr><td colspan="2"><b>Format of Data:</b> See the tables
below.</td></tr>
</table></center>
<!-- end msgdesc table -->
<div align="center">
<table class="format">
<caption>
Layout: Link Message
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td>Version</td>
<td>Flags</td>
<td>Link type <em>(optional)</em></td>
<td bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4"><br />Creation Order <em>(8 bytes, optional)</em><br /><br /></td>
</tr>
<tr>
<td>Link Name Character Set <em>(optional)</em></td>
<td>Length of Link Name (variable size)</td>
<td colspan="2" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4">Link Name (variable size)</td>
</tr>
<tr>
<td colspan="4"><br />Link Information (variable size)<br /><br /></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Link Message
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Version</p></td>
<td><p>The version number for this message. This document describes version 1.</p>
</td>
</tr>
<tr>
<td><p>Flags</p></td>
<td><p>This field contains information about the link and controls
the presence of other fields below.
<table class="list">
<tr>
<th width="20%" align="center">Bits</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0-1</code></td>
<td>Determines the size of the <em>Length of Link Name</em>
field.
<table class="list">
<tr>
<th width="20%" align="center">Value</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>The size of the <em>Length of Link Name</em>
field is 1 byte.
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>The size of the <em>Length of Link Name</em>
field is 2 bytes.
</td>
</tr>
<tr>
<td align="center"><code>2</code></td>
<td>The size of the <em>Length of Link Name</em>
field is 4 bytes.
</td>
</tr>
<tr>
<td align="center"><code>3</code></td>
<td>The size of the <em>Length of Link Name</em>
field is 8 bytes.
</td>
</tr>
</table>
</td>
</tr>
<tr>
<td align="center"><code>2</code></td>
<td>Creation Order Field Present: if set, the <em>Creation
Order</em> field is present. If not set, creation order
information is not stored for links in this group.
</td>
</tr>
<tr>
<td align="center"><code>3</code></td>
<td>Link Type Field Present: if set, the link is not
a hard link and the <em>Link Type</em> field is present.
If not set, the link is a hard link.
</td>
</tr>
<tr>
<td align="center"><code>4</code></td>
<td>Link Name Character Set Field Present: if set, the
link name is not represented with the ASCII character
set and the <em>Link Name Character Set</em> field is
present. If not set, the link name is represented with
the ASCII character set.
</td>
</tr>
<tr>
<td align="center"><code>5-7</code></td>
<td>Reserved (zero).
</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Link type</p></td>
<td><p>This is the link class type and can be one of the following
values:
<table class="list">
<tr>
<th width="20%" align="center">Value</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>A hard link (should never be stored in the file)
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>A soft link.
</td>
</tr>
<tr>
<td align="center"><code>2-63</code></td>
<td>Reserved for future HDF5 internal use.
</td>
</tr>
<tr>
<td align="center"><code>64</code></td>
<td>An external link.
</td>
</tr>
<tr>
<td align="center"><code>65-255</code></td>
<td>Reserved, but available for user-defined link types.
</td>
</tr>
</table></p>
<p>This field is present if bit 3 of <em>Flags</em> is set.</p>
</td>
</tr>
<tr>
<td><p>Creation Order</p></td>
<td><p>This 64-bit value is an index of the link’s creation time within
the group. Values start at 0 when the group is created an increment
by one for each link added to the group. Removing a link from a
group does not change existing links’ creation order field.
</p>
<p>This field is present if bit 2 of <em>Flags</em> is set.</p>
</td>
</tr>
<tr>
<td><p>Link Name Character Set</p></td>
<td><p>This is the character set for encoding the link’s name:
<table class="list">
<tr>
<th width="20%" align="center">Value</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>ASCII character set encoding (this should never be stored
in the file)
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>UTF-8 character set encoding
</td>
</tr>
</table></p>
<p>This field is present if bit 4 of <em>Flags</em> is set.</p>
</td>
</tr>
<tr>
<td><p>Length of link name</p></td>
<td><p>This is the length of the link’s name. The size of this field
depends on bits 0 and 1 of <em>Flags</em>.</p>
</td>
</tr>
<tr>
<td><p>Link name</p></td>
<td><p>This is the name of the link, non-NULL terminated.</p>
</td>
</tr>
<tr>
<td><p>Link information</p></td>
<td><p>The format of this field depends on the <em>link type</em>.</p>
<p>For <b>hard</b> links, the field is formatted as follows:
<table class="list">
<tr>
<td width="20%"><i><a href="#SizeOfOffsetsV0">
Size of Offsets</a></i> bytes:</td>
<td width="80%">The address of the object header for the object that the
link points to.
</td>
</tr>
</table>
</p>
<p>
For <b>soft</b> links, the field is formatted as follows:
<table class="list">
<tr>
<td width="20%">Bytes 1-2:</td>
<td width="80%">Length of soft link value.</td>
</tr>
<tr>
<td><em>Length of soft link value</em> bytes:</td>
<td>A non-NULL-terminated string storing the value of the
soft link.
</td>
</tr>
</table>
</p>
<p>
For <b>external</b> links, the field is formatted as follows:
<table class="list">
<tr>
<td width="20%">Bytes 1-2:</td>
<td width="80%">Length of external link value.</td>
</tr>
<tr>
<td><em>Length of external link value</em> bytes:</td>
<td>The first byte contains the version number in the
upper 4 bits and flags in the lower 4 bits for the external
link. Both version and flags are defined to be zero in
this document. The remaining bytes consist of two
NULL-terminated strings, with no padding between them.
The first string is the name of the HDF5 file containing
the object linked to and the second string is the full path
to the object linked to, within the HDF5 file’s
group hierarchy.
</td>
</tr>
</table>
</p>
<p>
For <b>user-defined</b> links, the field is formatted as follows:
<table class="list">
<tr>
<td width="20%">Bytes 1-2:</td>
<td width="80%">Length of user-defined data.</td>
</tr>
<tr>
<td><em>Length of user-defined link value</em> bytes:</td>
<td>The data supplied for the user-defined link type.</td>
</tr>
</table>
</p>
</td>
</tr>
</table>
</div>
<h4><a name="ExternalFileListMessage">IV.A.2.h. The Data Storage -
External Data Files Message</a></h4>
<!-- start msgdesc table -->
<center>
<table class="msgdesc">
<tr><td colspan="2"><b>Header Message Name:</b> External
Data Files</td></tr>
<tr><td colspan="2"><b>Header Message Type:</b> 0x0007</td></tr>
<tr><td colspan="2"><b>Length:</b> Varies</td></tr>
<tr><td colspan="2"><b>Status:</b> Optional; may not be
repeated.</td></tr>
<tr><td><b>Description:</b></td>
<td>The external data storage message indicates that the data
for an object is stored outside the HDF5 file. The filename of
the object is stored as a Universal Resource Location (URL) of
the actual filename containing the data. An external file list
record also contains the byte offset of the start of the data
within the file and the amount of space reserved in the file
for that data.</td></tr>
<tr><td colspan="2"><b>Format of Data:</b> See the tables
below.</td></tr>
</table></center>
<!-- end msgdesc table -->
<div align="center">
<table class="format">
<caption>
Layout: External File List Message
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td>Version</td>
<td colspan="3">Reserved (zero)</td>
</tr>
<tr>
<td colspan="2">Allocated Slots</td>
<td colspan="2">Used Slots</td>
</tr>
<tr>
<td colspan="4"><br />Heap Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Slot Definitions...<br /><br /></td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: External File List Message
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Version</p></td>
<td>
<p>The version number information is used for changes in the format of
External Data Storage Message and is described here:
<table class="list">
<tr>
<th width="20%" align="center">Version</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>Never used.</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>The current version used by the library.</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Allocated Slots</p></td>
<td>
<p>The total number of slots allocated in the message. Its value must be at least as
large as the value contained in the Used Slots field. (The current library simply
uses the number of Used Slots for this message)</p>
</td>
</tr>
<tr>
<td><p>Used Slots</p></td>
<td>
<p>The number of initial slots which contains valid information.</p>
</td>
</tr>
<tr>
<td><p>Heap Address</p></td>
<td>
<p>This is the address of a local heap which contains the names for the external
files (The local heap information can be found in Disk Format Level 1D in this
document). The name at offset zero in the heap is always the empty string.</p>
</td>
</tr>
<tr>
<td><p>Slot Definitions</p></td>
<td>
<p>The slot definitions are stored in order according to the array addresses they
represent.</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: External File List Slot
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td colspan="4"><br />Name Offset in Local Heap<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Offset in External Data File<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Data Size in External File<sup>L</sup><br /><br /></td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘L’ in the above table are
of the size specified in the <a href="#SizeOfLengthsV0">Size
of Lengths</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: External File List Slot
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Name Offset in Local Heap</p></td>
<td>
<p>The byte offset within the local name heap for the name
of the file. File names are stored as a URL which has a
protocol name, a host name, a port number, and a file
name:
<code><em>protocol</em>:<em>port</em>//<em>host</em>/<em>file</em></code>.
If the protocol is omitted then “file:” is assumed. If
the port number is omitted then a default port for that
protocol is used. If both the protocol and the port
number are omitted then the colon can also be omitted. If
the double slash and host name are omitted then
“localhost” is assumed. The file name is the only
mandatory part, and if the leading slash is missing then
it is relative to the application’s current working
directory (the use of relative names is not
recommended).
</p>
</td>
</tr>
<tr>
<td><p>Offset in External Data File</p></td>
<td>
<p>This is the byte offset to the start of the data in the
specified file. For files that contain data for a single
dataset this will usually be zero.</p>
</td>
</tr>
<tr>
<td><p>Data Size in External File</p></td>
<td>
<p>This is the total number of bytes reserved in the
specified file for raw data storage. For a file that
contains exactly one complete dataset which is not
extendable, the size will usually be the exact size of the
dataset. However, by making the size larger one allows
HDF5 to extend the dataset. The size can be set to a value
larger than the entire file since HDF5 will read zeroes
past the end of the file without failing.</p>
</td>
</tr>
</table>
</div>
<h4><a name="LayoutMessage">IV.A.2.i. The Data Layout Message</a></h4>
<!-- start msgdesc table -->
<center>
<table class="msgdesc">
<tr><td colspan="2"><b>Header Message Name:</b> Data Layout</td></tr>
<tr><td colspan="2"><b>Header Message Type:</b> 0x0008</td></tr>
<tr><td colspan="2"><b>Length:</b> Varies</td></tr>
<tr><td colspan="2"><b>Status:</b> Required for datasets; may not
be repeated.</td></tr>
<tr><td><b>Description:</b></td>
<td>The Data Layout message
describes how the elements of a multi-dimensional array are stored
in the HDF5 file. Four types of data layout are supported:
<ol>
<li>Contiguous: The array is stored in one contiguous area of
the file. This layout requires that the size of the array be
constant: data manipulations such as chunking, compression,
checksums, or encryption are not permitted. The message stores
the total storage size of the array. The offset of an element
from the beginning of the storage area is computed as in a C
array.</li>
<li>Chunked: The array domain is regularly decomposed into
chunks, and each chunk is allocated and stored separately. This
layout supports arbitrary element traversals, compression,
encryption, and checksums (these features are described
in other messages). The message stores the size of a chunk
instead of the size of the entire array; the storage size of
the entire array can be calculated by traversing the chunk index
that stores the chunk addresses.</li>
<li>Compact: The array is stored in one contiguous block as
part of this object header message.</li>
<li>Virtual: This is only supported for version 4 of the Data
Layout message. The message stores information that is used to
locate the global heap collection containing the Virtual Dataset
(VDS) mapping information. The mapping associates the VDS to
the source dataset elements that are stored across a collection
of HDF5 files.</li>
</ol></td></tr>
<tr><td colspan="2"><b>Format of Data:</b> See the tables
below.</td></tr>
</table></center>
<!-- end msgdesc table -->
<div align="center">
<table class="format">
<caption>
Layout: Data Layout Message (Versions 1 and 2)
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td>Version</td>
<td>Dimensionality</td>
<td>Layout Class</td>
<td>Reserved <em>(zero)</em></td>
</tr>
<tr>
<td colspan="4">Reserved <em>(zero)</em></td>
</tr>
<tr>
<td colspan="4"><br />Data Address<sup>O</sup> <em>(optional)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4">Dimension 1 Size</td>
</tr>
<tr>
<td colspan="4">Dimension 2 Size</td>
</tr>
<tr>
<td colspan="4">...</td>
</tr>
<tr>
<td colspan="4">Dimension #n Size</td>
</tr>
<tr>
<td colspan="4">Dataset Element Size <em>(optional)</em></td>
</tr>
<tr>
<td colspan="4">Compact Data Size <em>(optional)</em></td>
</tr>
<tr>
<td colspan="4"><br />Compact Data... <em>(variable size, optional)</em><br /><br /></td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Data Layout Message (Versions 1 and 2)
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Version</p></td>
<td>
<p>The version number information is used for changes in the format of the data
layout message and is described here:
<table class="list">
<tr>
<th width="20%" align="center">Version</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>Never used.</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>Used by version 1.4 and before of the library to encode layout information.
Data space is always allocated when the data set is created.</td>
</tr>
<tr>
<td align="center"><code>2</code></td>
<td>Used by version 1.6.[0,1,2] of the library to encode layout information.
Data space is allocated only when it is necessary.</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Dimensionality</p></td>
<td><p>An array has a fixed dimensionality. This field
specifies the number of dimension size fields later in the
message. The value stored for chunked storage is 1 greater than
the number of dimensions in the dataset’s dataspace.
For example, 2 is stored for a 1 dimensional dataset.
</p>
</td>
</tr>
<tr>
<td><p>Layout Class</p></td>
<td><p>The layout class specifies the type of storage for the data
and how the other fields of the layout message are to be
interpreted.
<table class="list">
<tr>
<th width="20%" align="center">Value</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>Compact Storage
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>Contiguous Storage
</td>
</tr>
<tr>
<td align="center"><code>2</code></td>
<td>Chunked Storage
</td>
</tr>
</table>
</p>
</td>
</tr>
<tr>
<td><p>Data Address</p></td>
<td><p>For contiguous storage, this is the address of the raw
data in the file. For chunked storage this is the address
of the <a href="#V1Btrees">v1 B-tree</a> that is used to look up the addresses of the
chunks. This field is not present for compact storage.
If the version for this message is greater than 1, the address
may have the “undefined address” value, to indicate that
storage has not yet been allocated for this array.</p>
</td>
</tr>
<tr>
<td><p>Dimension #n Size</p></td>
<td><p>For contiguous and compact storage the dimensions define
the entire size of the array while for chunked storage they define
the size of a single chunk. In all cases, they are in units of
array elements (not bytes). The first dimension stored in the list
of dimensions is the slowest changing dimension and the last
dimension stored is the fastest changing dimension.
</p>
</td>
</tr>
<tr>
<td><p>Dataset Element Size</p></td>
<td><p>The size of a dataset element, in bytes. This field is only
present for chunked storage.
</p>
</td>
</tr>
<tr>
<td><p>Compact Data Size</p></td>
<td><p>This field is only present for compact data storage.
It contains the size of the raw data for the dataset array, in
bytes.</p>
</td>
</tr>
<tr>
<td><p>Compact Data</p></td>
<td><p>This field is only present for compact data storage.
It contains the raw data for the dataset array.</p>
</td>
</tr>
</table>
</div>
<br />
<p>Version 3 of this message re-structured the format into specific
properties that are required for each layout class.</p>
<div align="center">
<table class="format">
<caption>
Layout: Data Layout Message (Version 3)
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td>Version</td>
<td>Layout Class</td>
<td colspan="2" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4"><br />Properties <em>(variable size)</em><br /><br /></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Data Layout Message (Version 3)
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Version</p></td>
<td>
<p>The version number information is used for changes in the format of layout message
and is described here:
<table class="list">
<tr>
<th width="20%" align="center">Version</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>3</code></td>
<td>Used by the version 1.6.3 and later of the library to store properties
for each layout class.</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Layout Class</p></td>
<td><p>The layout class specifies the type of storage for the data
and how the other fields of the layout message are to be
interpreted.
<table class="list">
<tr>
<th width="20%" align="center">Value</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>Compact Storage
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>Contiguous Storage
</td>
</tr>
<tr>
<td align="center"><code>2</code></td>
<td>Chunked Storage
</td>
</tr>
</table>
</p>
</td>
</tr>
<tr>
<td><p>Properties</p></td>
<td><p>This variable-sized field encodes information specific to each
layout class and is described below. If there is no property
information specified for a layout class, the size of this field
is zero bytes.</p></td>
</tr>
</table>
</div>
<br />
<a name="CompactStorage"></a>
<p>Class-specific information for compact storage (layout class 0): (Note: The dimensionality information
is in the Dataspace message)</p>
<div align="center">
<table class="format">
<caption>
Layout: Compact Storage Property Description
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td colspan="2">Size</td>
<td colspan="2" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4"><br />Raw Data... <em>(variable size)</em><br /><br /></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Compact Storage Property Description
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Size</p></td>
<td><p>This field contains the size of the raw data for the dataset
array, in bytes.
</p>
</td>
</tr>
<tr>
<td><p>Raw Data</p></td>
<td><p>This field contains the raw data for the dataset array.</p></td>
</tr>
</table>
</div>
<br />
<a name="ContiguousStorage"></a>
<p>Class-specific information for contiguous storage (layout class 1):
(Note: The dimensionality information is in the Dataspace message)</p>
<div align="center">
<table class="format">
<caption>
Layout: Contiguous Storage Property Description
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td colspan="4"><br />Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Size<sup>L</sup><br /><br /></td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
<tr>
<td> </td>
<td>
(Items marked with an ‘L’ in the above table are
of the size specified in the <a href="#SizeOfLengthsV0">Size
of Lengths</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Contiguous Storage Property Description
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Address</p></td>
<td><p>This is the address of the raw data in the file.
The address may have the “undefined address” value, to indicate
that storage has not yet been allocated for this array.</p></td>
</tr>
<tr>
<td><p>Size</p></td>
<td><p>This field contains the size allocated to store the raw data,
in bytes.
</p>
</td>
</tr>
</table>
</div>
<br />
<p>Class-specific information for chunked storage (layout class 2):</p>
<div align="center">
<table class="format">
<caption>
Layout: Chunked Storage Property Description
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td>Dimensionality</td>
<td colspan="3" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4"><br />Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4">Dimension 0 Size</td>
</tr>
<tr>
<td colspan="4">Dimension 1 Size</td>
</tr>
<tr>
<td colspan="4">...</td>
</tr>
<tr>
<td colspan="4">Dimension #n Size</td>
</tr>
<tr>
<td colspan="4">Dataset Element Size</td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Chunked Storage Property Description
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Dimensionality</p></td>
<td><p>A chunk has a fixed dimensionality. This field specifies
the number of dimension size fields later in the message.</p></td>
</tr>
<tr>
<td><p>Address</p></td>
<td><p>This is the address of the <a href="#V1Btrees">v1 B-tree</a>
that is used to look up the
addresses of the chunks that actually store portions of the array
data. The address may have the “undefined address” value, to
indicate that storage has not yet been allocated for this array.</p></td>
</tr>
<tr>
<td><p>Dimension #n Size</p></td>
<td><p>These values define the dimension size of a single chunk, in
units of array elements (not bytes). The first dimension stored in
the list of dimensions is the slowest changing dimension and the
last dimension stored is the fastest changing dimension.
</p>
</td>
</tr>
<tr>
<td><p>Dataset Element Size</p></td>
<td><p>The size of a dataset element, in bytes.
</p>
</td>
</tr>
</table>
</div>
<br />
<p><a name="DataLayoutV4">
Version 4</a> of this message is similar to version 3 but has
additional information for the virtual layout class as well as
indexing information for the chunked layout class.</p>
<div align="center">
<table class="format">
<caption>
Layout: Data Layout Message (Version 4)
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td>Version</td>
<td>Layout Class</td>
<td colspan="2" bgcolor="#DDDDDD"><em>This space inserted
only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4"><br />Properties <em>(variable size)</em><br /><br /></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Data Layout Message (Version 4)
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Version</p></td>
<td>
<p>The value for this field is 4 and is used by version 1.10.0
and later of the library to store properties for each layout
class and indexing information for the chunked layout.
</p>
</td>
</tr>
<tr>
<td><p>Layout Class</p></td>
<td><p>The layout class specifies the type of storage for the data
and how the other fields of the layout message are to be
interpreted.
<table class="list">
<tr>
<th width="20%" align="center">Value</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>Compact Storage
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>Contiguous Storage
</td>
</tr>
<tr>
<td align="center"><code>2</code></td>
<td>Chunked Storage
</td>
</tr>
<tr>
<td align="center"><code>3</code></td>
<td>Virtual Storage
</td>
</tr>
</table>
</p>
</td>
</tr>
<tr>
<td><p>Properties</p></td>
<td><p>This variable-sized field encodes information specific to a
layout class as follows:
<table class="list">
<tr>
<th align="left" width="20%">Layout Class</th>
<th align="left" width="80%">Description</th>
</tr>
<tr>
<td align="left">Compact Storage</td>
<td>See <a href="#CompactStorage">Compact Storage
Property Description</i></a> for the version 3
Data Layout message.
</td>
</tr>
<tr>
<td align="left">Contiguous Storage</td>
<td>See <a href="#ContiguousStorage">Contiguous Storage
Property Description</i></a> for the version 3
Data Layout message.
</td>
</tr>
<tr>
<td align="left">Chunked Storage</td>
<td>See <a href="#ChunkedStorage">Chunked Storage
Property Description</i></a> below.
</td>
</tr>
<tr>
<td align="left">Virtual Storage</td>
<td>See <a href="#VirtualStorage">Virtual Storage
Property Description</i></a> below.
</td>
</tr>
</table>
</p></td>
</tr>
</table>
</div>
<br />
<a name="ChunkedStorage"></a>
<p>Class-specific information for chunked storage (layout
class 2):</p>
<div align="center">
<table class="format">
<caption>
Layout: Chunked Storage Property Description
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td>Flags</td>
<td>Dimensionality</td>
<td>Dimension Size Encoded Length</td>
<td colspan="1" bgcolor="#DDDDDD"><em>This space inserted to align table nicely</em></td>
</tr>
<tr>
<td colspan="4">Dimension 0 Size <em>(variable size)</em></td>
</tr>
<tr>
<td colspan="4">Dimension 1 Size <em>(variable size)</em></td>
</tr>
<tr>
<td colspan="4">...</td>
</tr>
<tr>
<td colspan="4">Dimension #n Size <em>(variable size)</em></td>
</tr>
<tr>
<td>Chunk Indexing Type</td>
<td colspan="3" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4">Indexing Type Information <em>(variable size)</em></td>
</tr>
<tr>
<td colspan="4"><br />Address<sup>O</sup><br /><br /></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Chunked Storage Property Description
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Flags</p></td>
<td><p>This is the chunked layout feature flag:</p>
<table class="list">
<tr>
<th width="55%" align="left">Value</th>
<th width="45%" align="left">Description</th>
</tr>
<tr>
<td align="left"><code>DONT_FILTER_PARTIAL_BOUND_CHUNKS (bit 0)</code></td>
<td>Do not apply filter to a partial edge chunk.
</td>
</tr>
<tr>
<td align="left"><code>SINGLE_INDEX_WITH_FILTER (bit 1)</code></td>
<td>A filtered chunk for <i>Single Chunk</i> indexing.
</td>
</tr>
</table>
</td>
</tr>
<tr>
<td><p>Dimensionality</p></td>
<td><p>A chunk has fixed dimension. This field specifies
the number of <em>Dimension Size</em> fields later in the message.</p></td>
</tr>
<tr>
<td><p>Dimension Size Encoded Length</p></td>
<td>
<p>This is the size in bytes used to encode <em>Dimension Size</em>.
</p>
</td>
</tr>
<tr>
<td><p>Dimension #n Size</p></td>
<td><p>These values define the dimension size of a single chunk, in
units of array elements (not bytes). The first dimension stored in
the list of dimensions is the slowest changing dimension and the
last dimension stored is the fastest changing dimension.
</p>
</td>
</tr>
<tr>
<td><p>Chunk Indexing Type</p></td>
<td><p>There are five indexing types used to look up addresses
of the chunks. For more information on each type, see
<a href="#AppendixC">“Appendix C: Types of Indexes for
Dataset Chunks.”</a>
<table class="list">
<tr>
<th width="20%" align="center">Value</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td><a href="#SingleChunk"><i>Single Chunk</i></a> indexing type.
</td>
</tr>
<tr>
<td align="center"><code>2</code></td>
<td><a href="#Implicit"><i>Implicit</i></a> indexing type.
</td>
</tr>
<tr>
<td align="center"><code>3</code></td>
<td><a href="#FixedArray"><i>Fixed Array</i></a> indexing type.
</td>
</tr>
<tr>
<td align="center"><code>4</code></td>
<td><a href="#ExtensibleArray"><i>Extensible Array</i></a> indexing type.
</td>
</tr>
<tr>
<td align="center"><code>5</code></td>
<td><a href="#V2Btrees"><i>Version 2 B-tree</i></a> indexing type.
</td>
</tr>
</table>
</p>
</td>
</tr>
<tr>
<td><p>Indexing Type Information</p></td>
<td><p>This variable-sized field encodes information specific to
an indexing type. More information on what is encoded with
each type can be found below this table.
<ul>
<li>See <a href="#IndexInfoSingle"><i>Single Chunk</i></a> below.</li>
<li>See <a href="#IndexInfoImplicit"><i>Implicit</i></a> below.</li>
<li>See <a href="#IndexInfoFixed"><i>Fixed Array</i></a> below.</li>
<li>See <a href="#IndexInfoExtensible"><i>Extensible Array</i></a> below.</li>
<li>See <a href="#IndexInfoV2Btrees"><i>Version 2 B-tree</i></a> below.</li>
</ul>
</p>
</td>
</tr>
<tr>
<td><p>Address</p></td>
<td><p>This is the address specific to an indexing type.
The address may be undefined if the chunk or index storage is not allocated yet.
<table class="list">
<tr>
<th width="40%" align="left">Value</th>
<th width="60%" align="left">Description</th>
</tr>
<tr>
<td align="left"><i>Single Chunk index</i></td>
<td align="left">Address of the single chunk.</td>
</td>
</tr>
<tr>
<td align="left"><i>Implicit index</i></td>
<td align="left">Address of the array of dataset chunks.</td>
</td>
</tr>
<tr>
<td align="left"><i>Fixed Array index</i></td>
<td align="left">Address of the index.</td>
</tr>
<tr>
<td align="left"><i>Extensible Array index</i></td>
<td align="left">Address of the index.</td>
</td>
</tr>
<tr>
<td align="left"><i>Version 2 B-tree index</i></td>
<td align="left">Address of the index.</td>
</td>
</tr>
</table>
</p>
</td>
</tr>
</table>
</div>
<br />
<ol>
<li>
<a name="IndexInfoSingle"></a>
Index-specific information for <i>Single Chunk</i>:
</li>
<p>The following information exists only when the chunk is filtered.
In other words, when <code>DONT_FILTER_PARTIAL_BOUND_CHUNKS</code>
(bit 0) is enabled in the field <em>flags</em>.</p>
<div align="center">
<table class="format">
<caption>
Layout: Single Chunk Indexing Information
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td colspan="4"><br />Size of filtered chunk<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4">Filters for chunk</td>
</tr>
</table>
<table class="note">
<tr>
<td width="55%"> </td>
<td width="45%"> <!-- width is slightly different: these
tables are part of an ordered list; see <ol> tags. -->
(Items marked with an ‘L’ in the above table are
of the size specified in the <a href="#SizeOfLengthsV0">Size
of Lengths</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Single Chunk Indexing Information
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Size of filtered chunk</p></td>
<td><p>This field is the size of a filtered chunk.</p></td>
</tr>
<tr>
<td><p>Filters for chunk</p></td>
<td><p>This field contains filters for the chunk.</p></td>
</tr>
</table>
</div>
</p>
<br />
<li>
<a name="IndexInfoImplicit"></a>
Index-specific information for <i>Implicit</i>:
</li>
<div align="center">
<table class="format">
<caption>
Layout: Implicit Indexing Information
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td colspan="4" bgcolor="#DDDDDD">
<em>No specific indexing information</em></td>
</tr>
</table>
</div>
<br />
<li>
<a name="IndexInfoFixed"></a>
Index-specific information for <i>Fixed Array</i>:
</li>
<div align="center">
<table class="format">
<caption>
Layout: Fixed Array Indexing Information
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td colspan="1">Page Bits</td>
<td colspan="3" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Fixed Array Indexing Information
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Page Bits</p></td>
<td><p>This field contains the number of bits needed to store the
maximum number of elements in a data block page.</p></td>
</tr>
</table>
</div>
</p>
<br />
<li>
<a name="IndexInfoExtensible"></a>
Index-specific information for <i>Extensible Array</i>:
</li>
<div align="center">
<table class="format">
<caption>
Layout: Extensible Array Indexing Information
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td>Max Bits</td>
<td>Index Elements</td>
<td>Min Pointers</td>
<td>Min Elements</td>
</tr>
<td colspan="2">Page Bits</td>
<td colspan="2" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Extensible Array Indexing Information
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Max Bits</p></td>
<td><p>This field contains the number of bits needed to store the maximum number of elements
in the array.
</p>
</td>
</tr>
<tr>
<td><p>Index Elements</p></td>
<td><p>This field contains the number of elements to store in the
index block.
</p>
</td>
</tr>
<tr>
<td><p>Min Pointers</p></td>
<td><p>This field contains the minimum number of data block pointers
for a superblock.
</p>
</td>
</tr>
<tr>
<td><p>Min Elements</p></td>
<td><p>This field contains the minimum number of elements per data block.
</p>
</td>
</tr>
<tr>
<td><p>Page Bits</p></td>
<td><p>This field contains the number of bits needed to store the
maximum number of elements in a data block page.
</p>
</td>
</tr>
</table>
</div>
</p>
<br />
<li>
<a name="IndexInfoV2Btrees"></a>
Index-specific information for <i>Version 2 B-tree</i>:
</li>
<div align="center">
<table class="format">
<caption>
Layout: Version 2 B-tree Indexing Information
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td colspan="4">Node Size</td>
</tr>
<tr>
<td>Split Percent</td>
<td>Merge Percent</td>
<td colspan="2" bgcolor="#DDDDDD">
<em>This space inserted only to align table nicely</em></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Version 2 B-tree Indexing Information
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Node Size</p></td>
<td><p>This field is the size in bytes of a B-tree node.
</p>
</td>
</tr>
<tr>
<td><p>Split Percent</p></td>
<td><p>This field is the percentage full of a B-tree node at which to split the node.</p></td>
</tr>
<tr>
<td><p>Merge Percent</p></td>
<td><p>This field is the percentage full of a B-tree node at which to merge the node.</p></td>
</tr>
</table>
</div>
</ol>
<br />
<a name="VirtualStorage"></a>
<p>
Class-specific information for virtual storage (layout class 3):</p>
<div align="center">
<table class="format">
<caption>
Layout: Virtual Storage Property Description
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td colspan="4"><br />Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4">Index</td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Virtual Storage Property Description
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Address</p></td>
<td><p>This is the address of the global heap collection where
the VDS mapping entries are stored.
See <a href="#GlobalHeapVDS">“Disk Format: Level 1F -
Global Heap Block for Virtual Datasets.”</a>
</p></td>
</tr>
<tr>
<td><p>Index</p></td>
<td><p>This is the index of the data object within the global heap collection.
</p>
</td>
</tr>
</table>
</div>
<h4><a name="BogusMessage">IV.A.2.j. The Bogus Message</a></h4>
<!-- start msgdesc table -->
<center>
<table class="msgdesc">
<tr><td colspan="2"><b>Header Message Name:</b> Bogus</td></tr>
<tr><td colspan="2"><b>Header Message Type:</b> 0x0009</td></tr>
<tr><td colspan="2"><b>Length:</b> 4 bytes</td></tr>
<tr><td colspan="2"><b>Status:</b> For testing only; should never
be stored in a valid file.</td></tr>
<tr><td><b>Description:</b></td>
<td>This message is used for testing the HDF5 Library’s
response to an “unknown” message type and should
never be encountered in a valid HDF5 file.</td></tr>
<tr><td colspan="2"><b>Format of Data:</b> See the tables
below.</td></tr>
</table></center>
<!-- end msgdesc table -->
<div align="center">
<table class="format">
<caption>
Layout: Bogus Message
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td colspan="4">Bogus Value</td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Bogus Message
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Bogus Value</p></td>
<td>
<p>This value should always be: <code>0xdeadbeef</code>.</p>
</td>
</tr>
</table>
</div>
<h4><a name="GroupInfoMessage">IV.A.2.k. The Group Info Message
</a></h4>
<!-- start msgdesc table -->
<center>
<table class="msgdesc">
<tr><td colspan="2"><b>Header Message Name:</b> Group Info</td></tr>
<tr><td colspan="2"><b>Header Message Type:</b> 0x000A</td></tr>
<tr><td colspan="2"><b>Length:</b> Varies</td></tr>
<tr><td colspan="2"><b>Status:</b> Optional; may not be
repeated.</td></tr>
<tr><td><b>Description:</b></td>
<td><p>This message stores information for the constants defining
a “new style” group’s behavior. Constant
information will be stored in this message and variable
information will be stored in the
<a href="#LinkInfoMessage">Link Info</a> message.</p>
<p>Note: the “estimated entry” information below is
used when determining the size of the object header for the
group when it is created.</p></td></tr>
<tr><td colspan="2"><b>Format of Data:</b> See the tables
below.</td></tr>
</table></center>
<!-- end msgdesc table -->
<div align="center">
<table class="format">
<caption>
Layout: Group Info Message
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td>Version</td>
<td>Flags</td>
<td colspan="2">Link Phase Change: Maximum Compact Value <em>(optional)</em></td>
</tr>
<tr>
<td colspan="2">Link Phase Change: Minimum Dense Value <em>(optional)</em></td>
<td colspan="2">Estimated Number of Entries <em>(optional)</em></td>
</tr>
<tr>
<td colspan="2">Estimated Link Name Length of Entries <em>(optional)</em></td>
<td colspan="2" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Group Info Message
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Version</p></td>
<td><p>The version number for this message. This document describes version 0.</p>
</td>
</tr>
<tr>
<td><p>Flags</p></td>
<td><p>This is the group information flag with the following definition:
<table class="list">
<tr>
<th width="20%" align="center">Bit</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>If set, link phase change values are stored.
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>If set, the estimated entry information is non-default
and is stored.
</td>
</tr>
<tr>
<td align="center"><code>2-7</code></td>
<td>Reserved</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Link Phase Change: Maximum Compact Value</p></td>
<td><p>The is the maximum number of links to store “compactly” (in
the group’s object header).</p>
<p>This field is present if bit 0 of <em>Flags</em> is set.</p>
</td>
</tr>
<tr>
<td><p>Link Phase Change: Minimum Dense Value</p></td>
<td><p>This is the minimum number of links to store “densely” (in
the group’s fractal heap). The fractal heap’s address is
located in the <a href="#LinkInfoMessage">Link Info</a>
message.</p>
<p>This field is present if bit 0 of <em>Flags</em> is set.</p>
</td>
</tr>
<tr>
<td><p>Estimated Number of Entries</p></td>
<td><p>This is the estimated number of entries in groups.</p>
<p>If this field is not present, the default value of <code>4</code>
will be used for the estimated number of group entries.</p>
<p>This field is present if bit 1 of <em>Flags</em> is set.</p>
</td>
</tr>
<tr>
<td><p>Estimated Link Name Length of Entries</p></td>
<td><p>This is the estimated length of entry name.</p>
<p>If this field is not present, the default value of <code>8</code>
will be used for the estimated link name length of group entries.</p>
<p>This field is present if bit 1 of <em>Flags</em> is set.</p>
</td>
</tr>
</table>
</div>
<!-- </p> -->
<h4><a name="FilterMessage">IV.A.2.l. The Data Storage - Filter
Pipeline Message</a></h4>
<!-- start msgdesc table -->
<center>
<table class="msgdesc">
<tr><td colspan="2"><b>Header Message Name:</b>
Data Storage - Filter Pipeline</td></tr>
<tr><td colspan="2"><b>Header Message Type:</b> 0x000B</td></tr>
<tr><td colspan="2"><b>Length:</b> Varies</td></tr>
<tr><td colspan="2"><b>Status:</b> Optional; may not be
repeated.</td></tr>
<tr><td><b>Description:</b></td>
<td><p>This message describes the filter pipeline which should
be applied to the data stream by providing filter identification
numbers, flags, a name, and client data.</p>
<p>This message may be present in the object headers of both
dataset and group objects. For datasets, it specifies the
filters to apply to raw data. For groups, it specifies the
filters to apply to the group’s fractal heap. Currently,
only datasets using chunked data storage use the filter
pipeline on their raw data.</p></td></tr>
<tr><td colspan="2"><b>Format of Data:</b> See the tables
below.</td></tr>
</table></center>
<!-- end msgdesc table -->
<div align="center">
<table class="format">
<caption>
Layout: Filter Pipeline Message - Version 1
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td>Version</td>
<td>Number of Filters</td>
<td colspan="2">Reserved (zero)</td>
</tr>
<tr>
<td colspan="4">Reserved (zero)</td>
</tr>
<tr>
<td colspan="4"><br />Filter Description List <em>(variable size)</em><br /><br /></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Filter Pipeline Message - Version 1
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Version</p></td>
<td><p>The version number for this message. This table
describes version 1.</p></td>
</tr>
<tr>
<td><p>Number of Filters</p></td>
<td><p>The total number of filters described in this
message. The maximum possible number of filters in a
message is 32.</p></td>
</tr>
<tr>
<td><p>Filter Description List</p></td>
<td><p>A description of each filter. A filter description
appears in the next table.</p></td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Filter Description - Version 1
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td colspan="2">Filter Identification Value</td>
<td colspan="2">Name Length</td>
</tr>
<tr>
<td colspan="2">Flags</td>
<td colspan="2">Number Client Data Values</td>
</tr>
<tr>
<td colspan="4"><br />Name <em>(variable size, optional)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Client Data <em>(variable size, optional)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4">Padding <em>(variable size, optional)</em></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Filter Description - Version 1
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Filter Identification Value</p></td>
<td>
<p>
This value, often referred to as a filter identifier,
is designed to be a unique identifier for the filter.
Values from zero through 32,767 are reserved for filters
supported by The HDF Group in the HDF5 Library and for
filters requested and supported by third parties.
Filters supported by The HDF Group are documented immediately
below. Information on 3rd-party filters can be found at
The HDF Group’s
<a href="http://www.hdfgroup.org/services/contributions.html">
Contributions</a> page.</p>
<p>
To request a filter identifier, please contact
The HDF Group’s Help Desk at
<img src="Graphics/help.png" valign="middle" height="14"
alt="The HDF Group Help Desk">.
You will be asked to provide the following information:</p>
<ol>
<li>Contact information for the developer requesting the
new identifier</li>
<li>A short description of the new filter</li>
<li>Links to any relevant information, including licensing
information</li>
</ol>
<p>
Values from 32768 to 65535 are reserved for non-distributed uses
(for example, internal company usage) or for application usage
when testing a feature. The HDF Group does not track or document
the use of the filters with identifiers from this range.</p>
<p>
The filters currently in library version 1.8.0 are
listed below:
<table class="list">
<tr>
<th width="20%" align="center">Identification</th>
<th width="15%" align="left">Name</th>
<th width="65%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>N/A</td>
<td>Reserved</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>deflate</td>
<td>GZIP deflate compression</td>
</tr>
<tr>
<td align="center"><code>2</code></td>
<td>shuffle</td>
<td>Data element shuffling</td>
</tr>
<tr>
<td align="center"><code>3</code></td>
<td>fletcher32</td>
<td>Fletcher32 checksum</td>
</tr>
<tr>
<td align="center"><code>4</code></td>
<td>szip</td>
<td>SZIP compression</td>
</tr>
<tr>
<td align="center"><code>5</code></td>
<td>nbit</td>
<td>N-bit packing</td>
</tr>
<tr>
<td align="center"><code>6</code></td>
<td>scaleoffset</td>
<td>Scale and offset encoded values</td>
</tr>
</table>
</p></td>
</tr>
<tr>
<td><p>Name Length</p></td>
<td><p>Each filter has an optional null-terminated ASCII name
and this field holds the length of the name including the
null termination padded with nulls to be a multiple of
eight. If the filter has no name then a value of zero is
stored in this field.</p></td>
</tr>
<tr>
<td><p>Flags</p></td>
<td><p>The flags indicate certain properties for a filter. The
bit values defined so far are:
<table class="list">
<tr>
<th width="20%" align="center">Bit</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>If set then the filter is an optional filter.
During output, if an optional filter fails it will be
silently skipped in the pipeline.</td>
</tr>
<tr>
<td align="center"><code>1-15</code></td>
<td>Reserved (zero)</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Number of Client Data Values</p></td>
<td><p>Each filter can store integer values to control
how the filter operates. The number of entries in the
<em>Client Data</em> array is stored in this field.</p></td>
</tr>
<tr>
<td><p>Name</p></td>
<td><p>If the <em>Name Length</em> field is non-zero then it will
contain the size of this field, padded to a multiple of eight. This
field contains a null-terminated, ASCII character string to serve
as a comment/name for the filter.</p></td>
</tr>
<tr>
<td><p>Client Data</p></td>
<td><p>This is an array of four-byte integers which will be
passed to the filter function. The <em>Client Data Number</em> of
Values determines the number of elements in the array.</p></td>
</tr>
<tr>
<td><p>Padding</p></td>
<td><p>Four bytes of zeroes are added to the message at this
point if the Client Data Number of Values field contains
an odd number.</p></td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Filter Pipeline Message - Version 2
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td>Version</td>
<td>Number of Filters</td>
<td colspan="2" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4"><br />Filter Description List <em>(variable size)</em><br /><br /></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Filter Pipeline Message - Version 2
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Version</p></td>
<td><p>The version number for this message. This table
describes version 2.</p></td>
</tr>
<tr>
<td><p>Number of Filters</p></td>
<td><p>The total number of filters described in this
message. The maximum possible number of filters in a
message is 32.</p></td>
</tr>
<tr>
<td><p>Filter Description List</p></td>
<td><p>A description of each filter. A filter description
appears in the next table.</p></td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Filter Description - Version 2
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td colspan="2">Filter Identification Value</td>
<td colspan="2">Name Length <em>(optional)</em></td>
</tr>
<tr>
<td colspan="2">Flags</td>
<td colspan="2">Number Client Data Values</td>
</tr>
<tr>
<td colspan="4"><br />Name <em>(variable size, optional)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Client Data <em>(variable size, optional)</em><br /><br /></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Filter Description - Version 2
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Filter Identification Value</p></td>
<td>
<p>
This value, often referred to as a filter identifier,
is designed to be a unique identifier for the filter.
Values from zero through 32,767 are reserved for filters
supported by The HDF Group in the HDF5 Library and for
filters requested and supported by third parties.
Filters supported by The HDF Group are documented immediately
below. Information on 3rd-party filters can be found at
The HDF Group’s
<a href="http://www.hdfgroup.org/services/contributions.html">
Contributions</a> page.</p>
<p>
To request a filter identifier, please contact
The HDF Group’s Help Desk at
<img src="Graphics/help.png" valign="middle" height="14"
alt="The HDF Group Help Desk">.
You will be asked to provide the following information:</p>
<ol>
<li>Contact information for the developer requesting the
new identifier</li>
<li>A short description of the new filter</li>
<li>Links to any relevant information, including licensing
information</li>
</ol>
<p>
Values from 32768 to 65535 are reserved for non-distributed uses
(for example, internal company usage) or for application usage
when testing a feature. The HDF Group does not track or document
the use of the filters with identifiers from this range.</p>
<p>
The filters currently in library version 1.8.0 are
listed below:
<table class="list">
<tr>
<th width="20%" align="center">Identification</th>
<th width="15%" align="left">Name</th>
<th width="65%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>N/A</td>
<td>Reserved</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>deflate</td>
<td>GZIP deflate compression</td>
</tr>
<tr>
<td align="center"><code>2</code></td>
<td>shuffle</td>
<td>Data element shuffling</td>
</tr>
<tr>
<td align="center"><code>3</code></td>
<td>fletcher32</td>
<td>Fletcher32 checksum</td>
</tr>
<tr>
<td align="center"><code>4</code></td>
<td>szip</td>
<td>SZIP compression</td>
</tr>
<tr>
<td align="center"><code>5</code></td>
<td>nbit</td>
<td>N-bit packing</td>
</tr>
<tr>
<td align="center"><code>6</code></td>
<td>scaleoffset</td>
<td>Scale and offset encoded values</td>
</tr>
</table>
</p></td>
</tr>
<tr>
<td><p>Name Length</p></td>
<td><p>Each filter has an optional null-terminated ASCII name
and this field holds the length of the name including the
null termination padded with nulls to be a multiple of
eight. If the filter has no name then a value of zero is
stored in this field.</p>
<p>Filters with IDs less than 256 (in other words, filters
that are defined in this format documentation) do not store
the <em>Name Length</em> or <em>Name</em> fields.
</p>
</td>
</tr>
<tr>
<td><p>Flags</p></td>
<td><p>The flags indicate certain properties for a filter. The
bit values defined so far are:
<table class="list">
<tr>
<th width="20%" align="center">Bit</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>If set then the filter is an optional filter.
During output, if an optional filter fails it will be
silently skipped in the pipeline.</td>
</tr>
<tr>
<td align="center"><code>1-15</code></td>
<td>Reserved (zero)</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Number of Client Data Values</p></td>
<td><p>Each filter can store integer values to control
how the filter operates. The number of entries in the
<em>Client Data</em> array is stored in this field.</p></td>
</tr>
<tr>
<td><p>Name</p></td>
<td><p>If the <em>Name Length</em> field is non-zero, then it will
contain the size of this field, <em>not</em> padded to a multiple
of eight. This field contains a <em>non-</em>null-terminated,
ASCII character string to serve as a comment/name for the filter.
</p>
<p>Filters that are defined in this format documentation
such as deflate and shuffle do not store the <em>Name
Length</em> or <em>Name</em> fields.
</p>
</td>
</tr>
<tr>
<td><p>Client Data</p></td>
<td><p>This is an array of four-byte integers which will be
passed to the filter function. The Client Data Number of
Values</em> determines the number of elements in the array.</p>
</td>
</tr>
</table>
</div>
<h4><a name="AttributeMessage">IV.A.2.m. The Attribute Message</a></h4>
<!-- start msgdesc table -->
<center>
<table class="msgdesc">
<tr><td colspan="2"><b>Header Message Name:</b> Attribute</td></tr>
<tr><td colspan="2"><b>Header Message Type:</b> 0x000C</td></tr>
<tr><td colspan="2"><b>Length:</b> Varies</td></tr>
<tr><td colspan="2"><b>Status:</b> Optional; may be
repeated.</td></tr>
<tr><td><b>Description:</b></td>
<td><p>The <em>Attribute</em> message is used to store objects
in the HDF5 file which are used as attributes, or
“metadata” about the current object. An attribute
is a small dataset; it has a name, a datatype, a dataspace, and
raw data. Since attributes are stored in the object header, they
should be relatively small (in other words, less than 64KB).
They can be associated with any type of object which has an
object header (groups, datasets, or committed (named)
datatypes).</p>
<p>In 1.8.x versions of the library, attributes can be larger
than 64KB. See the
<a href="UG/HDF5_Users_Guide-Responsive%20HTML5/index.html#t=HDF5_Users_Guide%2FAttributes%2FHDF5_Attributes.htm%3Frhtocid%3Dtoc8.2_1%23TOC_8_5_Special_Issuesbc-13">
“Special Issues”</a> section of the Attributes chapter
in the <cite>HDF5 User’s Guide</cite> for more information.</p>
<p>Note: Attributes on an object must have unique names:
the HDF5 Library currently enforces this by causing the
creation of an attribute with a duplicate name to fail.
Attributes on different objects may have the same name,
however.</p></td></tr>
<tr><td colspan="2"><b>Format of Data:</b> See the tables
below.</td></tr>
</table></center>
<!-- end msgdesc table -->
<div align="center">
<table class="format">
<caption>
Layout: Attribute Message (Version 1)
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td>Version</td>
<td>Reserved (zero)</td>
<td colspan="2">Name Size</td>
</tr>
<tr>
<td colspan="2">Datatype Size</td>
<td colspan="2">Dataspace Size</td>
</tr>
<tr>
<td colspan="4"><br />Name <em>(variable size)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Datatype <em>(variable size)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Dataspace <em>(variable size)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Data <em>(variable size)</em><br /><br /></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Attribute Message (Version 1)
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Version</p></td>
<td><p>The version number information is used for changes in the format of the
attribute message and is described here:
<table class="list">
<tr>
<th width="20%" align="center">Version</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>Never used.</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>Used by the library before version 1.6 to encode attribute message.
This version does not support shared datatypes.</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Name Size</p></td>
<td><p>The length of the attribute name in bytes including the
null terminator. Note that the <em>Name</em> field below may
contain additional padding not represented by this
field.</p></td>
</tr>
<tr>
<td><p>Datatype Size</p></td>
<td><p>The length of the datatype description in the <em>Datatype</em>
field below. Note that the <em>Datatype</em> field may contain
additional padding not represented by this field.</p></td>
</tr>
<tr>
<td><p>Dataspace Size</p></td>
<td><p>The length of the dataspace description in the <em>Dataspace</em>
field below. Note that the <em>Dataspace</em> field may contain
additional padding not represented by this field.</p></td>
</tr>
<tr>
<td><p>Name</p></td>
<td><p>The null-terminated attribute name. This field is
padded with additional null characters to make it a
multiple of eight bytes.</p></td>
</tr>
<tr>
<td><p>Datatype</p></td>
<td><p>The datatype description follows the same format as
described for the datatype object header message. This
field is padded with additional zero bytes to make it a
multiple of eight bytes.</p></td>
</tr>
<tr>
<td><p>Dataspace</p></td>
<td><p>The dataspace description follows the same format as
described for the dataspace object header message. This
field is padded with additional zero bytes to make it a
multiple of eight bytes.</p></td>
</tr>
<tr>
<td><p>Data</p></td>
<td><p>The raw data for the attribute. The size is determined
from the datatype and dataspace descriptions. This
field is <em>not</em> padded with additional bytes.</p></td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Attribute Message (Version 2)
</caption>
<tr align="center">
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td>Version</td>
<td>Flags</td>
<td colspan="2">Name Size</td>
</tr>
<tr>
<td colspan="2">Datatype Size</td>
<td colspan="2">Dataspace Size</td>
</tr>
<tr>
<td colspan="4"><br />Name <em>(variable size)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Datatype <em>(variable size)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Dataspace <em>(variable size)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Data <em>(variable size)</em><br /><br /></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Attribute Message (Version 2)
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Version</p></td>
<td><p>The version number information is used for changes in the
format of the attribute message and is described here:
<table class="list">
<tr>
<th width="20%" align="center">Version</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>2</code></td>
<td>Used by the library of version 1.6.x and after to encode
attribute messages.
This version supports shared datatypes. The fields of
name, datatype, and dataspace are not padded with
additional bytes of zero.
</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Flags</p></td>
<td><p>This bit field contains extra information about
interpreting the attribute message:
<table class="list">
<tr>
<th width="20%" align="center">Bit</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>If set, datatype is shared.</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>If set, dataspace is shared.</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Name Size</p></td>
<td><p>The length of the attribute name in bytes including the
null terminator.</p></td>
</tr>
<tr>
<td><p>Datatype Size</p></td>
<td><p>The length of the datatype description in the <em>Datatype</em>
field below.</p></td>
</tr>
<tr>
<td><p>Dataspace Size</p></td>
<td><p>The length of the dataspace description in the <em>Dataspace</em>
field below.</p></td>
</tr>
<tr>
<td><p>Name</p></td>
<td><p>The null-terminated attribute name. This field is <em>not</em>
padded with additional bytes.</p></td>
</tr>
<tr>
<td><p>Datatype</p></td>
<td><p>The datatype description follows the same format as
described for the datatype object header message.
</p>
<p>If the
<em>Flag</em> field indicates this attribute’s datatype is
shared, this field will contain a “shared message” encoding
instead of the datatype encoding.
</p>
<p>This field is <em>not</em> padded with additional bytes.
</p>
</td>
</tr>
<tr>
<td><p>Dataspace</p></td>
<td><p>The dataspace description follows the same format as
described for the dataspace object header message.
</p>
<p>If the
<em>Flag</em> field indicates this attribute’s dataspace is
shared, this field will contain a “shared message” encoding
instead of the dataspace encoding.
</p>
<p>This field is <em>not</em> padded with additional bytes.</p>
</td>
</tr>
<tr>
<td><p>Data</p></td>
<td><p>The raw data for the attribute. The size is determined
from the datatype and dataspace descriptions.
</p>
<p>This field is <em>not</em> padded with additional zero bytes.
</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Attribute Message (Version 3)
</caption>
<tr align="center">
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td>Version</td>
<td>Flags</td>
<td colspan="2">Name Size</td>
</tr>
<tr>
<td colspan="2">Datatype Size</td>
<td colspan="2">Dataspace Size</td>
</tr>
<tr>
<td>Name Character Set Encoding</td>
<td colspan="3" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4"><br />Name <em>(variable size)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Datatype <em>(variable size)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Dataspace <em>(variable size)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Data <em>(variable size)</em><br /><br /></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Attribute Message (Version 3)
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Version</p></td>
<td><p>The version number information is used for changes in the
format of the attribute message and is described here:
<table class="list">
<tr>
<th width="20%" align="center">Version</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>3</code></td>
<td>Used by the library of version 1.8.x and after to
encode attribute messages.
This version supports attributes with non-ASCII names.
</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Flags</p></td>
<td><p>This bit field contains extra information about
interpreting the attribute message:
<table class="list">
<tr>
<th width="20%" align="center">Bit</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>If set, datatype is shared.</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>If set, dataspace is shared.</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Name Size</p></td>
<td><p>The length of the attribute name in bytes including the
null terminator.</p></td>
</tr>
<tr>
<td><p>Datatype Size</p></td>
<td><p>The length of the datatype description in the <em>Datatype</em>
field below.</p></td>
</tr>
<tr>
<td><p>Dataspace Size</p></td>
<td><p>The length of the dataspace description in the <em>Dataspace</em>
field below.</p></td>
</tr>
<tr>
<td><p>Name Character Set Encoding</p></td>
<td><p>The character set encoding for the attribute’s name:
<table class="list">
<tr>
<th width="20%" align="center">Value</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>ASCII character set encoding
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>UTF-8 character set encoding
</td>
</tr>
</table>
</p>
</td>
</tr>
<tr>
<td><p>Name</p></td>
<td><p>The null-terminated attribute name. This field is <em>not</em>
padded with additional bytes.</p></td>
</tr>
<tr>
<td><p>Datatype</p></td>
<td><p>The datatype description follows the same format as
described for the datatype object header message.
</p>
<p>If the
<em>Flag</em> field indicates this attribute’s datatype is
shared, this field will contain a “shared message” encoding
instead of the datatype encoding.
</p>
<p>This field is <em>not</em> padded with additional bytes.
</p>
</td>
</tr>
<tr>
<td><p>Dataspace</p></td>
<td><p>The dataspace description follows the same format as
described for the dataspace object header message.
</p>
<p>If the
<em>Flag</em> field indicates this attribute’s dataspace is
shared, this field will contain a “shared message” encoding
instead of the dataspace encoding.
</p>
<p>This field is <em>not</em> padded with additional bytes.</p>
</td>
</tr>
<tr>
<td><p>Data</p></td>
<td><p>The raw data for the attribute. The size is determined
from the datatype and dataspace descriptions.
</p>
<p>This field is <em>not</em> padded with additional zero bytes.
</p>
</td>
</tr>
</table>
</div>
<h4><a name="CommentMessage">IV.A.2.n. The Object Comment
Message</a></h4>
<!-- start msgdesc table -->
<center>
<table class="msgdesc">
<tr><td colspan="2"><b>Header Message Name:</b> Object
Comment</td></tr>
<tr><td colspan="2"><b>Header Message Type:</b> 0x000D</td></tr>
<tr><td colspan="2"><b>Length:</b> Varies</td></tr>
<tr><td colspan="2"><b>Status:</b> Optional; may not be
repeated.</td></tr>
<tr><td><b>Description:</b></td>
<td>The object comment is designed to be a short description of
an object. An object comment is a sequence of non-zero
(<code>\0</code>) ASCII characters with no other formatting
included by the library.</td></tr>
<tr><td colspan="2"><b>Format of Data:</b> See the tables
below.</td></tr>
</table></center>
<!-- end msgdesc table -->
<div align="center">
<table class="format">
<caption>
Layout: Object Comment Message
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td colspan="4"><br />Comment <em>(variable size)</em><br /><br /></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Object Comment Message
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Name</p></td>
<td><p>A null terminated ASCII character string.</p></td>
</tr>
</table>
</div>
<h4><a name="OldModificationTimeMessage">IV.A.2.o. The Object
Modification Time (Old) Message</a></h4>
<!-- start msgdesc table -->
<center>
<table class="msgdesc">
<tr><td colspan="2"><b>Header Message Name:</b> Object
Modification Time (Old)</td></tr>
<tr><td colspan="2"><b>Header Message Type:</b> 0x000E</td></tr>
<tr><td colspan="2"><b>Length:</b> Fixed</td></tr>
<tr><td colspan="2"><b>Status:</b> Optional; may not be
repeated.</td></tr>
<tr><td><b>Description:</b></td>
<td><p>The object modification date and time is a timestamp
which indicates (using ISO-8601 date and time format) the last
modification of an object. The time is updated when any object
header message changes according to the system clock where the
change was posted. All fields of this message should be
interpreted as coordinated universal time (UTC).</p>
<p>This modification time message is deprecated in favor of
the “new” <a href="#ModificationTimeMessage">Object
Modification Time</a> message and is no longer written to the
file in versions of the HDF5 Library after the 1.6.0
version.</p></td></tr>
<tr><td colspan="2"><b>Format of Data:</b> See the tables
below.</td></tr>
</table></center>
<!-- end msgdesc table -->
<div align="center">
<table class="format">
<caption>
Layout: Modification Time Message (Old)
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td colspan="4">Year</td>
</tr>
<tr>
<td colspan="2">Month</td>
<td colspan="2">Day of Month</td>
</tr>
<tr>
<td colspan="2">Hour</td>
<td colspan="2">Minute</td>
</tr>
<tr>
<td colspan="2">Second</td>
<td colspan="2">Reserved</td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Modification Time Message (Old)
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Year</p></td>
<td><p>The four-digit year as an ASCII string. For example,
<code>1998</code>.
</p></td>
</tr>
<tr>
<td><p>Month</p></td>
<td><p>The month number as a two digit ASCII string where
January is <code>01</code> and December is <code>12</code>.</p></td>
</tr>
<tr>
<td><p>Day of Month</p></td>
<td><p>The day number within the month as a two digit ASCII
string. The first day of the month is <code>01</code>.</p></td>
</tr>
<tr>
<td><p>Hour</p></td>
<td><p>The hour of the day as a two digit ASCII string where
midnight is <code>00</code> and 11:00pm is <code>23</code>.</p></td>
</tr>
<tr>
<td><p>Minute</p></td>
<td><p>The minute of the hour as a two digit ASCII string where
the first minute of the hour is <code>00</code> and
the last is <code>59</code>.</p></td>
</tr>
<tr>
<td><p>Second</p></td>
<td><p>The second of the minute as a two digit ASCII string
where the first second of the minute is <code>00</code>
and the last is <code>59</code>.</p></td>
</tr>
<tr>
<td><p>Reserved</p></td>
<td><p>This field is reserved and should always be zero.</p></td>
</tr>
</table>
</div>
<h4><a name="SOHMTableMessage">IV.A.2.p. The Shared Message Table
Message</a></h4>
<!-- start msgdesc table -->
<center>
<table class="msgdesc">
<tr><td colspan="2"><b>Header Message Name:</b> Shared Message
Table</td></tr>
<tr><td colspan="2"><b>Header Message Type:</b> 0x000F</td></tr>
<tr><td colspan="2"><b>Length:</b> Fixed</td></tr>
<tr><td colspan="2"><b>Status:</b> Optional; may not be
repeated.</td></tr>
<tr><td><b>Description:</b></td>
<td>This message is used to locate the table of shared object
header message (SOHM) indexes. Each index consists of information
to find the shared messages from either the heap or object header.
This message is <em>only</em> found in the superblock
extension.</td></tr>
<tr><td colspan="2"><b>Format of Data:</b> See the tables
below.</td></tr>
</table></center>
<!-- end msgdesc table -->
<div align="center">
<table class="format">
<caption>
Layout: Shared Message Table Message
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td>Version</td>
<td colspan="3" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4"><br />Shared Object Header Message Table Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td>Number of Indices</td>
<td colspan="3" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
</table>
<table class="note">
<tr>
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Shared Message Table Message
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Version</p></td>
<td><p>The version number for this message. This document describes version 0.</p></td>
</tr>
<tr>
<td><p>Shared Object Header Message Table Address</p></td>
<td><p>This field is the address of the master table for shared
object header message indexes.</p>
</td>
</tr>
<tr>
<td><p>Number of Indices</p></td>
<td><p>This field is the number of indices in the master table.
</p></td>
</tr>
</table>
</div>
<h4><a name="ContinuationMessage">IV.A.2.q. The Object Header
Continuation Message</a></h4>
<!-- start msgdesc table -->
<center>
<table class="msgdesc">
<tr><td colspan="2"><b>Header Message Name:</b> Object Header
Continuation</td></tr>
<tr><td colspan="2"><b>Header Message Type:</b> 0x0010</td></tr>
<tr><td colspan="2"><b>Length:</b> Fixed</td></tr>
<tr><td colspan="2"><b>Status:</b> Optional; may be
repeated.</td></tr>
<tr><td><b>Description:</b></td>
<td>The object header continuation is the location in the file
of a block containing more header messages for the current data
object. This can be used when header blocks become too large or
are likely to change over time.</td></tr>
<tr><td colspan="2"><b>Format of Data:</b> See the tables
below.</td></tr>
</table></center>
<!-- end msgdesc table -->
<div align="center">
<table class="format">
<caption>
Layout: Object Header Continuation Message
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td colspan="4"><br />Offset<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Length<sup>L</sup><br /><br /></td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
<tr>
<td> </td>
<td>
(Items marked with an ‘L’ in the above table are
of the size specified in the <a href="#SizeOfLengthsV0">Size
of Lengths</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Object Header Continuation Message
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Offset</p></td>
<td><p>This value is the address in the file where the
header continuation block is located.</p></td>
</tr>
<tr>
<td><p>Length</p></td>
<td><p>This value is the length in bytes of the header continuation
block in the file.</p></td>
</tr>
</table>
</div>
<br />
<p>The format of the header continuation block that this message points
to depends on the version of the object header that the message is
contained within.
</p>
<p>
Continuation blocks for version 1 object headers have no special
formatting information; they are merely a list of object header
message info sequences (type, size, flags, reserved bytes and data
for each message sequence). See the description
of <a href="#V1ObjectHeaderPrefix">Version 1 Data Object Header Prefix.</a>
</p>
<p>Continuation blocks for version 2 object headers <em>do</em> have
special formatting information as described here
(see also the description of
<a href="#V2ObjectHeaderPrefix">Version 2 Data Object Header Prefix.</a>):
</p>
<div align="center">
<table class="format">
<caption>
Layout: Version 2 Object Header Continuation Block
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan="4">Signature</td>
</tr>
<tr>
<td>Header Message Type #1</td>
<td colspan="2">Size of Header Message Data #1</td>
<td>Header Message #1 Flags</td>
</tr>
<tr>
<td colspan="2">Header Message #1 Creation Order <em>(optional)</em></td>
<td colspan="2" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4"><br />Header Message Data #1<br /><br /></td>
</tr>
<tr>
<td colspan="4">.<br />.<br />.<br /></td>
</tr>
<tr>
<td>Header Message Type #n</td>
<td colspan="2">Size of Header Message Data #n</td>
<td>Header Message #n Flags</td>
</tr>
<tr>
<td colspan="2">Header Message #n Creation Order <em>(optional)</em></td>
<td colspan="2" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4"><br />Header Message Data #n<br /><br /></td>
</tr>
<tr>
<td colspan="4">Gap <em>(optional, variable size)</em></td>
</tr>
<tr>
<td colspan="4">Checksum</td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Version 2 Object Header Continuation Block
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Signature</p></td>
<td>
<p>The ASCII character string “<code>OCHK</code>”
is used to indicate the beginning of an object header
continuation block. This gives file consistency checking
utilities a better chance of reconstructing a damaged file.
</p>
</td>
</tr>
<tr>
<td><p>Header Message #n Type</p></td>
<td>
<p>Same format as version 1 of the object header, described above.
</p></td>
</tr>
<tr>
<td><p>Size of Header Message #n Data</p></td>
<td>
<p>Same format as version 1 of the object header, described above.
</p></td>
</tr>
<tr>
<td><p>Header Message #n Flags</p></td>
<td>
<p>Same format as version 1 of the object header, described above.
</p></td>
</tr>
<tr>
<td><p>Header Message #n Creation Order</p></td>
<td>
<p>This field stores the order that a message of a given type
was created in.</p>
<p>This field is present if bit 2 of <em>flags</em> is set.</p>
</td>
</tr>
<tr>
<td><p>Header Message #n Data</p></td>
<td>
<p>Same format as version 1 of the object header, described above.
</p></td>
</tr>
<tr>
<td><p>Gap</p></td>
<td>
<p>A gap in an object header chunk is inferred by the end of the
messages for the chunk before the beginning of the chunk’s
checksum. Gaps are always smaller than the size of an
object header message prefix (message type + message size +
message flags).</p>
<p>Gaps are formed when a message (typically an attribute message)
in an earlier chunk is deleted and a message from a later
chunk that does not quite fit into the free space is moved
into the earlier chunk.</p>
</td>
</tr>
<tr>
<td><p>Checksum</p></td>
<td>
<p>This is the checksum for the object header chunk.
</p>
</td>
</tr>
</table>
</div>
<h4><a name="SymbolTableMessage">IV.A.2.r. The Symbol Table
Message</a></h4>
<!-- start msgdesc table -->
<center>
<table class="msgdesc">
<tr><td colspan="2"><b>Header Message Name:</b> Symbol Table
Message</td></tr>
<tr><td colspan="2"><b>Header Message Type:</b> 0x0011</td></tr>
<tr><td colspan="2"><b>Length:</b> Fixed</td></tr>
<tr><td colspan="2"><b>Status:</b> Required for
“old style” groups; may not be repeated.</td></tr>
<tr><td><b>Description:</b></td>
<td>Each “old style” group has a v1 B-tree and a
local heap for storing symbol table entries, which are located
with this message.</td></tr>
<tr><td colspan="2"><b>Format of data:</b> See the tables
below.</td></tr>
</table></center>
<!-- end msgdesc table -->
<div align="center">
<table class="format">
<caption>
Layout: Symbol Table Message
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td colspan="4"><br />v1 B-tree Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Local Heap Address<sup>O</sup><br /><br /></td>
</tr>
</table>
<table class="note">
<tr>
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Symbol Table Message
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>v1 B-tree Address</p></td>
<td><p>This value is the address of the v1 B-tree containing the
symbol table entries for the group.</p></td>
</tr>
<tr>
<td><p>Local Heap Address</p></td>
<td><p>This value is the address of the local heap containing
the link names for the symbol table entries for the group.</p></td>
</tr>
</table>
</div>
<h4><a name="ModificationTimeMessage">IV.A.2.s. The Object
Modification Time Message</a></h4>
<!-- start msgdesc table -->
<center>
<table class="msgdesc">
<tr><td colspan="2"><b>Header Message Name:</b> Object
Modification Time</td></tr>
<tr><td colspan="2"><b>Header Message Type:</b> 0x0012</td></tr>
<tr><td colspan="2"><b>Length:</b> Fixed</td></tr>
<tr><td colspan="2"><b>Status:</b> Optional; may not be
repeated.</td></tr>
<tr><td><b>Description:</b></td>
<td>The object modification time is a timestamp which indicates
the time of the last modification of an object. The time is
updated when any object header message changes according to
the system clock where the change was posted.</td></tr>
<tr><td colspan="2"><b>Format of Data:</b> See the tables
below.</td></tr>
</table></center>
<!-- end msgdesc table -->
<div align="center">
<table class="format">
<caption>
Layout: Modification Time Message
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td>Version</td>
<td colspan="3">Reserved <em>(zero)</em></td>
</tr>
<tr>
<td colspan="4">Seconds After UNIX Epoch</td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Modification Time Message
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Version</p></td>
<td><p>The version number is used for changes in the format of Object Modification Time
and is described here:
<table class="list">
<tr>
<th width="20%" align="center">Version</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>Never used.</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>Used by Version 1.6.1 and after of the library to encode time. In
this version, the time is the seconds after Epoch.</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Seconds After UNIX Epoch</p></td>
<td><p>A 32-bit unsigned integer value that stores the number of
seconds since 0 hours, 0 minutes, 0 seconds, January 1, 1970,
Coordinated Universal Time.</p></td>
</tr>
</table>
</div>
<h4><a name="BtreeKValuesMessage">IV.A.2.t. The B-tree
‘K’ Values Message</a></h4>
<!-- start msgdesc table -->
<center>
<table class="msgdesc">
<tr><td colspan="2"><b>Header Message Name:</b> B-tree
‘K’ Values</td></tr>
<tr><td colspan="2"><b>Header Message Type:</b> 0x0013</td></tr>
<tr><td colspan="2"><b>Length:</b> Fixed</td></tr>
<tr><td colspan="2"><b>Status:</b> Optional; may not be
repeated.</td></tr>
<tr><td><b>Description:</b></td>
<td>This message retrieves non-default ‘K’ values
for internal and leaf nodes of a group or indexed storage v1
B-trees. This message is <em>only</em> found in the superblock
extension.</td></tr>
<tr><td colspan="2"><b>Format of Data:</b> See the tables
below.</td></tr>
</table></center>
<!-- end msgdesc table -->
<div align="center">
<table class="format">
<caption>
Layout: B-tree ‘K’ Values Message
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td>Version</td>
<td colspan="2">Indexed Storage Internal Node K</td>
<td bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="2">Group Internal Node K</td>
<td colspan="2">Group Leaf Node K</td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: B-tree ‘K’ Values Message
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Version</p></td>
<td><p>The version number for this message. This document describes
version 0.</p>
</td>
</tr>
<tr>
<td><p>Indexed Storage Internal Node K</p></td>
<td><p>This is the node ‘K’ value for each internal node of an
indexed storage v1 B-tree. See the description of this field
in version 0 and 1 of the superblock as well the section on
v1 B-trees.
</p>
</td>
</tr>
<tr>
<td><p>Group Internal Node K</p></td>
<td><p>This is the node ‘K’ value for each internal node of a group
v1 B-tree. See the description of this field in version 0 and
1 of the superblock as well as the section on v1 B-trees.
</p>
</td>
</tr>
<tr>
<td><p>Group Leaf Node K</p></td>
<td><p>This is the node ‘K’ value for each leaf node of a group v1
B-tree. See the description of this field in version 0 and 1
of the superblock as well as the section on v1 B-trees.
</p>
</td>
</tr>
</table>
</div>
<h4><a name="DrvInfoMessage">IV.A.2.u. The Driver Info
Message</a></h4>
<!-- start msgdesc table -->
<center>
<table class="msgdesc">
<tr><td colspan="2"><b>Header Message Name:</b> Driver
Info</td></tr>
<tr><td colspan="2"><b>Header Message Type:</b> 0x0014</td></tr>
<tr><td colspan="2"><b>Length:</b> Varies</td></tr>
<tr><td colspan="2"><b>Status:</b> Optional; may not be
repeated.</td></tr>
<tr><td>
<b>Description:</b></td>
<td>This message contains information needed by the file driver
to reopen a file. This message is <em>only</em> found in the
superblock extension: see the <a href="#SuperblockExt">
“Disk Format: Level 0C - Superblock Extension”</a>
section for more information. For more information on the fields
in the driver info message, see the <a href="#DriverInfo">
“Disk Format: Level 0B - File Driver Info”</a>
section; those who use the multi and family file drivers will
find this section particularly helpful.</td></tr>
<tr><td colspan="2"><b>Format of Data:</b> See the tables
below.</td></tr>
</table></center>
<!-- end msgdesc table -->
<div align="center">
<table class="format">
<caption>
Layout: Driver Info Message
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td>Version</td>
<td colspan="3" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4"><br />Driver Identification</td>
</tr>
<tr>
<td colspan="2">Driver Information Size</td>
<td colspan="2" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4"><br /><br />Driver Information <em>(variable size)</em><br /><br /><br /></td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Driver Info Message
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Version</p></td>
<td><p>The version number for this message. This document describes
version 0.</p>
</td>
</tr>
<tr>
<td><p>Driver Identification</p></td>
<td><p>This is an eight-byte ASCII string without null termination which
identifies the driver.
</p>
</td>
</tr>
<tr>
<td><p>Driver Information Size</p></td>
<td><p>The size in bytes of the <em>Driver Information</em> field of this
message.</p>
</td>
</tr>
<tr>
<td><p>Driver Information</p></td>
<td><p>Driver information is stored in a format defined by the file driver.</p>
</td>
</tr>
</table>
</div>
<h4><a name="AinfoMessage">IV.A.2.v. The Attribute Info
Message</a></h4>
<!-- start msgdesc table -->
<center>
<table class="msgdesc">
<tr><td colspan="2"><b>Header Message Name:</b> Attribute
Info</td></tr>
<tr><td colspan="2"><b>Header Message Type:</b> 0x0015</td></tr>
<tr><td colspan="2"><b>Length:</b> Varies</td></tr>
<tr><td colspan="2"><b>Status:</b> Optional; may not be
repeated.</td></tr>
<tr><td><b>Description:</b></td>
<td>This message stores information about the attributes on an
object, such as the maximum creation index for the attributes
created and the location of the attribute storage when the
attributes are stored “densely”.</td></tr>
<tr><td colspan="2"><b>Format of Data:</b> See the tables
below.</td></tr>
</table></center>
<!-- end msgdesc table -->
<div align="center">
<table class="format">
<caption>
Layout: Attribute Info Message
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td>Version</td>
<td>Flags</td>
<td colspan="2">Maximum Creation Index <em>(optional)</em></td>
</tr>
<tr>
<td colspan="4"><br />Fractal Heap Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Attribute Name v2 B-tree Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Attribute Creation Order v2 B-tree Address<sup>O</sup> <em>(optional)</em><br /><br /></td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Attribute Info Message
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Version</p></td>
<td><p>The version number for this message. This document describes
version 0.</p>
</td>
</tr>
<tr>
<td><p>Flags</p></td>
<td><p>This is the attribute index information flag with the
following definition:
<table class="list">
<tr>
<th width="20%" align="center">Bit</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>If set, creation order for attributes is tracked.
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>If set, creation order for attributes is indexed.
</td>
</tr>
<tr>
<td align="center"><code>2-7</code></td>
<td>Reserved</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Maximum Creation Index</p></td>
<td><p>The is the maximum creation order index value for the
attributes on the object.</p>
<p>This field is present if bit 0 of <em>Flags</em> is set.</p>
</td>
</tr>
<tr>
<td><p>Fractal Heap Address</p></td>
<td><p>This is the address of the fractal heap to store dense
attributes.
Each attribute stored in the fractal heap is described by
the <a href="#AttributeMessage">Attribute Message.</a>
</p>
</td>
</tr>
<tr>
<td><p>Attribute Name v2 B-tree Address</p></td>
<td><p>This is the address of the version 2 B-tree to index the
names of densely stored attributes.</p>
</td>
</tr>
<tr>
<td><p>Attribute Creation Order v2 B-tree Address</p></td>
<td><p>This is the address of the version 2 B-tree to index the
creation order of densely stored attributes.</p>
<p>This field is present if bit 1 of <em>Flags</em> is set.</p>
</td>
</tr>
</table>
</div>
<h4><a name="RefCountMessage">IV.A.2.w. The Object Reference
Count Message</a></h4>
<!-- start msgdesc table -->
<center>
<table class="msgdesc">
<tr><td colspan="2"><b>Header Message Name:</b> Object Reference
Count</td></tr>
<tr><td colspan="2"><b>Header Message Type:</b> 0x0016</td></tr>
<tr><td colspan="2"><b>Length:</b> Fixed</td></tr>
<tr><td colspan="2"><b>Status:</b> Optional; may not be
repeated.</td></tr>
<tr><td><b>Description:</b></td>
<td>This message stores the number of hard links (in groups or
objects) pointing to an object: in other words, its
<em>reference count</em>.</td></tr>
<tr><td colspan="2"><b>Format of Data:</b> See the tables
below.</td></tr>
</table></center>
<!-- end msgdesc table -->
<div align="center">
<table class="format">
<caption>
Layout: Object Reference Count
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td>Version</td>
<td colspan="3" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4">Reference count</td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Object Reference Count
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Version</p></td>
<td><p>The version number for this message. This document describes
version 0.</p>
</td>
</tr>
<tr>
<td><p>Reference Count</p></td>
<td><p>The unsigned 32-bit integer is the reference count for the
object. This message is only present in “version 2”
(or later) object headers, and if not present those object
header versions, the reference count for the object is assumed
to be 1.</p>
</td>
</tr>
</table>
</div>
<br />
<h4><a name="FsinfoMessage">IV.A.2.x. The File Space Info
Message</a></h4>
<center>
<table class="msgdesc">
<tr><td colspan="2"><b>Header Message Name:</b> File Space
Info</td></tr>
<tr><td colspan="2"><b>Header Message Type:</b> 0x0017</td></tr>
<tr><td colspan="2"><b>Length:</b> Fixed</td></tr>
<tr><td colspan="2"><b>Status:</b> Optional; may not be
repeated.</td></tr>
<tr><td>
<b>Description:</b></td>
<td>This message stores the file space management information
that the library uses in handling file space
requests for the file. Version 0 of the message is used for release 1.10.0 only.
Version 1 of the message is used for release 1.10.1+.
There is no File Space Info message before release 1.10 as the library does
not track file space across multiple file opens.
<p>
Note that version 0 is deprecated starting release 1.10.1.
That means when the 1.10.1+ library opens an HDF5 file with a version 0 message,
the library will decode and map the message to version 1.
On file close, it will encode the message as a version 1 message.
<p>
The library uses the following three mechanisms to manage file space in an HDF5 file:
<ul>
<li> Free-space managers
<br> They track free-space sections of various sizes in the file that are not currently
allocated. Each free-space manager corresponds to a file space type.
There are two main groups of file space types: metadata and raw data.
Metadata is further divided into five types: superblock, B-tree, global heap,
local heap, and object header.
See the description of <a href="#FreeSpaceManager">Free-space
Manager</a> as well the description of file space allocation types in
<a href="#AppendixB">Appendix B</a>
</li>
<li> Aggregators
<br> The library manages two aggregators, one for metadata and one for raw data.
Aggregator is a contiguous block of free-space in the file.
The size of each aggregator is tunable via public routines
<code>H5Pset_meta_block_size</code> and <code>H5Pset_small_data_block_size</code> respectively.
</li>
<li> Virtual file drivers
<br> The library's virtual file driver interface dispatches requests for additional
space to the allocation routine of the file driver associated with the file.
For example, if the sec2 file driver is being used, its allocation routine will
increase the size of the file to service the requests.
</li>
</ul>
<p>
For release 1.10.0, the library derives the following four file space strategies
based on the mechanisms:
<ul>
<li>H5F_FILE_SPACE_ALL
<ul>
<li>Mechanisms used: free-space managers, aggregators, and virtual file drivers</li>
<li>Does not persist free-space across file opens</li>
<li>This strategy is the library default</li>
</ul>
</li>
<li>H5F_FILE_SPACE_ALL_PERSIST</li>
<ul>
<li>Mechanisms used: free-space managers, aggregators, and virtual file drivers</li>
<li>Persist free-space across file opens</li>
</ul>
<li>H5F_FILE_SPACE_AGGR_VFD</li>
<ul>
<li>Mechanisms used: aggregators and virtual file drivers</li>
<li>Does not persist free-space across file opens</li>
</ul>
<li>H5F_FILE_SPACE_VFD</li>
<ul>
<li>Mechanisms used: virtual file drivers</li>
<li>Does not persist free-space across file opens</li>
</ul>
</ul>
For release 1.10.1+, the free-space manager mechanism is modified to handle paged aggregation
which aggregates small metadata and raw data allocations into constant-sized well-aligned pages
to allow efficient I/O accesses.
With the support of this feature, the library derives the following four file space strategies:
<ul>
<li>H5F_FSPACE_STRATEGY_FSM_AGGR </li>
<ul>
<li>Mechanisms used: free-space managers, aggregators, and virtual file drivers</li>
<li>This strategy is the library default</li>
</ul>
<li>H5F_FSPACE_STRATEGY_PAGE</li>
<ul>
<li>Mechanisms used: free-space managers with embedded paged aggregation and virtual file drivers</li>
</ul>
<li>H5F_FSPACE_STRATEGY_AGGR</li>
<ul>
<li>Mechanisms used: aggregators and virtual file drivers</li>
</ul>
<li>H5F_FSPACE_STRATEGY_NONE</li>
<ul>
<li>Mechanisms used: virtual file drivers</li>
</ul>
</ul>
The default is not persisting free-space across file opens for the above four strategies.
User can use the public routine <code>H5Pset_file_space_strategy</code> to request
persisting free-space.
</td></tr>
<p>
<tr><td colspan="2"><b>Format of Data:</b> See the tables
below.</td></tr>
</table></center>
<p>
<div align="center">
<table class="format">
<caption>
Layout: File Space Info - Version 0
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td>Version</td>
<td>Strategy</td>
<td colspan="2">Threshold<sup>L</sup></td>
</tr>
<tr>
<td colspan="4"><br />Free-space manager address<sup>O</sup> for H5FD_MEM_SUPER<br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Free-space manager address<sup>0</sup> for H5FD_MEM_BTREE<br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Free-space manager address<sup>0</sup> for H5FD_MEM_DRAW<br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Free-space manager address<sup>0</sup> for H5FD_MEM_GHEAP<br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Free-space manager address<sup>0</sup> for H5FD_MEM_LHEAP<br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Free-space manager address<sup>0</sup> for H5FD_MEM_OHDR<br /><br /></td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
<tr>
<td> </td>
<td>
(Items marked with an ‘L’ in the above table are
of the size specified in the <a href="#SizeOfLengthsV0">Size
of Lengths</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: File Space Info
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Version</p></td>
<td><p>This is version 0 of this message.</p>
</td>
</tr>
<tr>
<td><p>Strategy</p></td>
<td><p>This is the file space strategy used to manage file space.
There are four types:
<table class="list">
<tr>
<th width="20%" align="center">Value</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>H5F_FILE_SPACE_ALL_PERSIST</td>
</tr>
<tr>
<td align="center"><code>2</code></td>
<td>H5F_FILE_SPACE_ALL</td>
</tr>
<tr>
<td align="center"><code>3</code></td>
<td>H5F_FILE_SPACE_AGGR_VFD</td>
</tr>
<tr>
<td align="center"><code>4</code></td>
<td>H5F_FILE_SPACE_VFD</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Threshold</p></td>
<td><p>This is the smallest free-space section size that the
free-space manager will track.
</td>
</tr>
<tr>
<td><p>Free-space manager addresses</p></td>
<td><p>These are the six free-space manager addresses for the
six file space allocation types:
<ul>
<li>H5FD_MEM_SUPER</li>
<li>H5FD_MEM_BTREE</li>
<li>H5FD_MEM_DRAW</li>
<li>H5FD_MEM_GHEAP</li>
<li>H5FD_MEM_LHEAP</li>
<li>H5FD_MEM_OHDR</li>
</ul>
Note that these six fields exist only if the value for the field
“<em>Strategy</em>” is H5F_FILE_SPACE_ALL_PERSIST.
</p>
</td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="format">
<caption>
Layout: File Space Info - Version 1
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td>Version</td>
<td>Strategy</td>
<td>Persisting free-space</td>
<td colspan="1" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4">Free-space Section Threshold<sup>L</sup></td>
</tr>
<tr>
<td colspan="4">File Space Page Size</td>
</tr>
<tr>
<td colspan="2">Page-end Metadata threshold</td>
<td colspan="2" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4"><br />EOA<sup>0</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Address<sup>O</sup> of small-sized free-space manager for H5FD_MEM_SUPER<br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Address<sup>O</sup> of small-sized free-space manager for H5FD_MEM_BTREE<br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Address<sup>O</sup> of small-sized free-space manager for H5FM_MEM_DRAW<br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Address<sup>O</sup> of small-sized free-space manager for H5FD_MEM_GHEAP<br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Address<sup>O</sup> of small-sized free-space manager for H5FD_MEM_LHEAP<br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Address<sup>O</sup> of small-sized free-space manager for H5FD_MEM_OHDR<br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Address<sup>O</sup> of large-sized free-space manager for H5FD_MEM_SUPER<br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Address<sup>O</sup> of large-sized free-space manager for H5FD_MEM_BTREE<br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Address<sup>O</sup> of large-sized free-space manager for H5FM_MEM_DRAW<br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Address<sup>O</sup> of large-sized free-space manager for H5FD_MEM_GHEAP<br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Address<sup>O</sup> of large-sized free-space manager for H5FD_MEM_LHEAP<br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Address<sup>O</sup> of large-sized free-space manager for H5FD_MEM_OHDR<br /><br /></td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
<tr>
<td> </td>
<td>
(Items marked with an ‘L’ in the above table are
of the size specified in the <a href="#SizeOfLengthsV0">Size
of Lengths</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: File Space Info
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Version</p></td>
<td><p>This is version 1 of this message.</p>
</td>
</tr>
<tr>
<td><p>Strategy</p></td>
<td><p>This is the file space strategy used to manage file space.
There are four types:
<table class="list">
<tr>
<th width="20%" align="center">Value</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>H5F_FSPACE_STRATEGY_FSM_AGGR</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>H5F_FSPACE_STRATEGY_PAGE</td>
</tr>
<tr>
<td align="center"><code>2</code></td>
<td>H5F_FSPACE_STRATEGY_AGGR</td>
</tr>
<tr>
<td align="center"><code>3</code></td>
<td>H5F_FSPACE_STRATEGY_NONE</td>
</tr>
</table></p>
</td>
</tr>
<tr>
<td><p>Persisting free-space</p></td>
<td><p>True or false in persisting free-space.
</td>
</tr>
<tr>
<td><p>Free-space Section Threshold</p></td>
<td><p>This is the smallest free-space section size that the
free-space manager will track.
</td>
</tr>
<tr>
<td><p>File space page size</p></td>
<td><p>This is the file space page size, which is used when the paged aggregation feature
is enabled.
</td>
</tr>
<tr>
<td><p>Page-end metadata threshold</p></td>
<td><p>This is the smallest free-space section size at the end of a page that
the free-space manager will track. This is used when the paged aggregation feature
is enabled.
</td>
</tr>
<tr>
<td><p>EOA</p></td>
<td><p>The EOA before the allocation of free-space manager header and section info for the
self-referential free-space managers when persisting free-space.
<br>
Note that self-referential free-space managers are managers that involve file space
allocation for the managers' free-space header and section info.
</td>
</tr>
<tr>
<td><p>Addresses of small-sized free-space managers</p></td>
<td><p>These are the addresses of the six small-sized free-space managers for
the six file space allocation types:
</p>
<ul>
<li>H5FD_MEM_SUPER</li>
<li>H5FD_MEM_BTREE</li>
<li>H5FD_MEM_DRAW</li>
<li>H5FD_MEM_GHEAP</li>
<li>H5FD_MEM_LHEAP</li>
<li>H5FD_MEM_OHDR</li>
</ul>
Note that these six fields exist only if the value for the field
“<em>Persisting free-space</em>” is true.
</ul>
</td>
</tr>
<tr>
<td><p>Addresses of large-sized free-space managers</p></td>
<td><p>These are the addresses of the six large-sized free-space managers for
the six file space allocation types:
</p>
<ul>
<li>H5FD_MEM_SUPER</li>
<li>H5FD_MEM_BTREE</li>
<li>H5FD_MEM_DRAW</li>
<li>H5FD_MEM_GHEAP</li>
<li>H5FD_MEM_LHEAP</li>
<li>H5FD_MEM_OHDR</li>
</ul>
Note that these six fields exist only if the value for the field
“<em>Persisting free-space</em>” is true.
</ul>
</td>
</tr>
</table>
</div>
<h3><a name="DataStorage">
IV.B. Disk Format: Level 2B - Data Object Data Storage</a></h3>
<p>The data for an object is stored separately from its header
information in the file and may not actually be located in the HDF5 file
itself if the header indicates that the data is stored externally. The
information for each record in the object is stored according to the
dimensionality of the object (indicated in the dataspace header message).
Multi-dimensional array data is stored in C order; in other words, the
“last” dimension changes fastest.</p>
<p>Data whose elements are composed of atomic datatypes are stored in IEEE
format, unless they are specifically defined as being stored in a different
machine format with the architecture-type information from the datatype
header message. This means that each architecture will need to [potentially]
byte-swap data values into the internal representation for that particular
machine.</p>
<p> Data with a variable-length datatype is stored in the global heap
of the HDF5 file. Global heap identifiers are stored in the
data object storage.</p>
<p>Data whose elements are composed of reference datatypes are stored in
several different ways depending on the particular reference type involved.
Object pointers are just stored as the offset of the object header being
pointed to with the size of the pointer being the same number of bytes as
offsets in the file.</p>
<p>Dataset region references are stored as a heap-ID which points to
the following information within the file-heap: an offset of the object
pointed to, number-type information (same format as header message),
dimensionality information (same format as header message), sub-set start
and end information (in other words, a coordinate location for each),
and field start and end names (in other words, a [pointer to the] string
indicating the first field included and a [pointer to the] string name
for the last field). </p>
<p>Data of a compound datatype is stored as a contiguous stream of the items
in the structure, with each item formatted according to its datatype.
<p>
Description of datatypes for variable-length, references and compound classes can be found
in <a href="#DatatypeMessage">Datatype Message</a>.
<p>
Information about global heap and heap ID can be found in <a href="#GlobalHeap">Global Heap</a>.
<p>
For reference datatype,
see also the encoding description for <a href="#ReferenceEncodeRV">Reference Encoding (Revised) </a> and
<a href="#ReferenceEncodeDP">Reference Encoding (Backward Compatibility)</a> in Appendix D.
</p>
<h2><a name="AppendixA">
V. Appendix A: Definitions</a></h2>
<p>Definitions of various terms used in this document are included in
this section.</p>
<div align="center">
<table class="glossary">
<tr>
<th width="20%">Term</th>
<th>Definition</th>
</tr>
<tr>
<td>Undefined Address</td>
<td>The <a name="UndefinedAddress">undefined
address</a> for a file is a file address with all bits
set: in other words, <code>0xffff...ff</code>.</td>
</tr>
<tr>
<td>Unlimited Size</td>
<td>The <a name="UnlimitedDim">unlimited size</a>
for a size is a value with all bits set: in other words,
<code>0xffff...ff</code>.</td>
</tr>
</table>
</div>
<h2><a name="AppendixB">
VI. Appendix B: File Space Allocation Types</a></h2>
<p>There are six basic types of file space allocation as follows:
</p>
<div align="center">
<table class="desc">
<tr>
<th width="30%">Basic Allocation Type</th>
<th>Description</th>
</tr>
<tr>
<td>H5FD_MEM_SUPER</td>
<td>File space allocated for <em>Superblock.</em></td>
</tr>
<tr>
<td>H5FD_MEM_BTREE</td>
<td>File space allocated for <em>B-tree.</em></td>
</tr>
<tr>
<td>H5FD_MEM_DRAW</td>
<td>File space allocated for <em>raw data</em>.</td>
</tr>
<tr>
<td>H5FD_MEM_GHEAP</td>
<td>File space allocated for <em>Global Heap.</em></td>
</tr>
<tr>
<td>H5FD_MEM_LHEAP</td>
<td>File space allocated for <em>Local Heap.</em></td>
</tr>
<tr>
<td>H5FD_MEM_OHDR</td>
<td>File space allocated for <em>Object Header.</em></td>
</tr>
</table>
</div>
<br />
<p>There are other file space allocation types that are mapped to the
above six basic types because they are similar in nature.
The mapping and the corresponding description are listed in the following two tables:
</p>
<div align="center">
<table class="desc">
<tr>
<th width="30%">Basic Allocation Type</th>
<th>Mapping of Allocation Types to Basic Allocation Types</th>
</tr>
<tr>
<td>H5FD_MEM_SUPER</td>
<td><em>none</em></td>
</tr>
<tr>
<td>H5FD_MEM_BTREE</td>
<td>H5FD_MEM_SOHM_INDEX</td>
</tr>
<tr>
<td>H5FD_MEM_DRAW</td>
<td>H5FD_MEM_FHEAP_HUGE_OBJ</td>
</tr>
<tr>
<td>H5FD_MEM_GHEAP</td>
<td><em>none</em></td>
</tr>
<tr>
<td>H5FD_MEM_LHEAP</td>
<td>H5FD_MEM_FHEAP_DBLOCK, H5FD_MEM_FSPACE_SINFO</td>
</tr>
<tr>
<td>H5FD_MEM_OHDR</td>
<td>H5FD_MEM_FHEAP_HDR, H5FD_MEM_FHEAP_IBLOCK, H5FD_MEM_FSPACE_HDR, H5FD_MEM_SOHM_TABLE</td>
</tr>
</table>
</div>
<br />
</p>
<div align="center">
<table class="desc">
<tr>
<th width="30%">Allocation Type</th>
<th>Description</th>
</tr>
<tr>
<td>H5FD_MEM_FHEAP_HDR</td>
<td>File space allocated for <em>Fractal Heap Header.</em></td>
</tr>
<tr>
<td>H5FD_MEM_FHEAP_DBLOCK</td>
<td>File space allocated for <em>Fractal Heap Direct Blocks.</em></td>
</tr>
<tr>
<td>H5FD_MEM_FHEAP_IBLOCK</td>
<td>File space allocated for <em>Fractal Heap Indirect Blocks.</em></td>
</tr>
<tr>
<td>H5FD_MEM_FHEAP_HUGE_OBJ</td>
<td>File space allocated for huge objects in the fractal heap.</td>
</tr>
<tr>
<td>H5FD_MEM_FSPACE_HDR</td>
<td>File space allocated for <em>Free-space Manager Header.</em></td>
</tr>
<tr>
<td>H5FD_MEM_FSPACE_SINFO</td>
<td>File space allocated for <em>Free-space Section List</em> of the free-space manager.</td>
</tr>
<tr>
<td>H5FD_MEM_SOHM_TABLE</td>
<td>File space allocated for <em>Shared Object Header Message Table.</em></td>
</tr>
<tr>
<td>H5FD_MEM_SOHM_INDEX</td>
<td>File space allocated for <em>Shared Message Record List.</em></td>
</tr>
</table>
</div>
<h2><a name="AppendixC"> VII. Appendix C:
Types of Indexes for Dataset Chunks</a></h2>
<p>For an HDF5 file without the latest format enabled, the library
uses the <a href="#V1Btrees">Version 1 B-tree</a> to index dataset
chunks.</p>
<p>For an HDF5 file with the latest format enabled, the library uses
one of the following five indexing types depending on a chunked
dataset’s dimension specification and the way it is extended.
</p>
<a name="SingleChunk">
<h3>VII.A. The Single Chunk Index</h3></a>
<p>The <i>Single Chunk</i> index can be used when the dataset fulfills
the following condition:</p>
<ul>
<li>the current, maximum, and chunk dimension sizes are all the same</li>
</ul>
<p>The dataset has only one chunk, and the address of the single
chunk is stored in the version 4 <i>Data Layout</i> message.
See the <a href="#ChunkedStorage">Chunked Storage Property
Description</i></a> layout and field description tables.</p>
<a name="Implicit">
<h3>VII.B. The Implicit Index</h3></a>
<p>The <i>Implicit</i> index can be used when the dataset fulfills
the following conditions:</p>
<ul>
<li>fixed maximum dimension sizes</li>
<li>no filter applied to the dataset</li>
<li>the timing for the space allocation of the dataset chunks is
<code>H5P_ALLOC_TIME_EARLY</code></li>
</ul>
<p>Since the dataset’s dimension sizes are known and storage space
is to be allocated early, an array of dataset chunks are allocated
based on the maximum dimension sizes when the dataset is created.
The base address of the array is stored in the version 4
<i>Data Layout</i> message. See the
<a href="#ChunkedStorage">Chunked Storage Property
Description</i></a> layout and field description tables.
</p>
<p>When accessing a dataset chunk with a specified offset, the
address of the chunk in the array is computed as below:</p>
<dir><p><code>base address + (size of a chunk in bytes * chunk index
associated with the offset)</code></p></dir>
<p>A chunk index starts at 0 and increases according to the
fastest changing dimension, then the next fastest, and so on.
<a name="ChunkIndex"></a>
The chunk index for a dataset chunk offset is computed as below:
<ol>
<li>Calculate the scaled offset for each dimension in
<code>scaled_offset</code>:
<br />
<pre>
scaled_offset = chunk_offset/chunk_dims
</pre></li>
<li>Calculate the # of chunks for each dimension in
<code>nchunks</code>:
<br />
<pre>
nchunks = (curr_dims + chunk_dims - 1)/chunk_dims
</pre></li>
<li>Calculate the down chunks for each dimension in
<code>down_chunks</code>:
<br />
<pre>
/* n is the # of dimensions */
for(i = (int)(n-1), acc = 1; i >= 0; i--) {
down_chunks[i] = acc;
acc *= nchunks[i];
}
</pre>
</li>
<li>Calculate the chunk index in <code>chunk_index</code>:
<br />
<pre>
/* n is the # of dimensions */
for(u = 0, chunk_index = 0; u < n; u++)
chunk_index += down_chunks[u] * scaled_offset[u];
</pre>
</li>
</ol>
<p>
For example, for a 2-dimensional dataset with
<code>curr_dims[4,5]</code> and <code>chunk_dims[3,2]</code>,
there will be a total of 6 chunks, with 3 chunks in the fastest
changing dimension and 2 chunks in the slowest changing dimension.
See the figure below.
The chunk index for the chunk offset <code>[3,4]</code>
is computed as below:
<ol>
<code>
<li>scaled_offset[0] = 1, scaled_offset[1] = 2</li>
<li>nchunks[0] = 2, nchunks[1] = 3</li>
<li>down_chunks[0] = 3, down_chunks[1] = 1</li>
<li>chunk_index = 5</li>
</code>
</ol>
<table align="center" width="400" border="0">
<tr valign="center" align="center">
<td>
<hr size="2"/>
<img height="250" src="FileFormatSpecChunkDiagram.jpg"
alt="Chunk Diagram"></td>
</tr>
<tr valign="top" align="center">
<td>
<hr size="1" />
<b>Figure 3. Implicit index chunk diagram </b>
<hr size="2"/></td>
</tr>
</table>
<a name="FixedArray">
<h3>VII.C. The Fixed Array Index</h3></a>
<p>The <i>Fixed Array</i> index can be used when the dataset fulfills
the following condition:</p>
<ul>
<li>fixed maximum dimension sizes</li>
</ul>
<p>Since the maximum number of chunks is known, an array of
in-file-on-disk addresses based on the maximum number of chunks is
allocated when data is written to the dataset. To access a dataset
chunk with a specified offset, the
<a href="#ChunkIndex">chunk index</i></a> associated with the offset
is calculated. The index is mapped into the array to locate the
disk address for the chunk.</p>
<p>The Fixed Array (FA) index structure provides space and speed
improvements in locating chunks over index structures that handle
more dynamic data accesses like a
<a href="#AppendV2Btrees">Version 2 B-tree</a> index.
The entry into the Fixed Array is the Fixed Array header which
contains metadata about the entries stored in the array. The
header contains a pointer to a data block which stores the array
of entries that describe the dataset chunks. For greater efficiency,
the array will be divided into multiple pages if the number of
entries exceeds a threshold value. The space for the data block
and possibly data block pages are allocated as a single contiguous
block of space.</p>
<p>The content of the data block depends on whether paging is
activated or not. When paging is not used, elements that describe
the chunks are stored in the data block. If paging is turned on,
the data block contains a bitmap indicating which pages are
initialized. Then subsequent data block pages will contain the
entries that describe the chunks.</p>
<p>An entry describes either a filtered or non-filtered dataset
chunk. The formats for both element types are described below.
</p>
<br />
<div align="center">
<table class="format">
<caption>
Layout: Fixed Array Header
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan="4">Signature</td>
</tr>
<tr>
<td>Version</td>
<td>Client ID</td>
<td>Entry Size</td>
<td>Page Bits</td>
</tr>
<tr>
<td colspan="4"><br />Max Num
Entries<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Data Block
Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4">Checksum</td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘L’ in the above table are
of the size specified in the <a href="#SizeOfLengthsV0">Size
of Lengths</a> field in the superblock.)
</td></tr>
<tr>
<td> </td>
<td>
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Fixed Array Header
</caption>
<tr>
<th width="40%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Signature</p></td>
<td>
<p>The ASCII character string “<code>FAHD</code>”
is used to indicate the beginning of a Fixed Array header.
This gives file consistency checking utilities a better
chance of reconstructing a damaged file.
</p>
</td>
</tr>
<tr>
<td><p>Version</p></td>
<td>
<p>This document describes version 0.</p>
</td>
</tr>
<tr>
<td><p>Client ID</p></td>
<td>
<p>The ID for identifying the client of the
Fixed Array:
<table class="list">
<tr>
<th width="20%" align="center">ID</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>Non-filtered dataset chunks
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>Filtered dataset chunks
</td>
</tr>
<tr>
<td align="center"><code>2+</code></td>
<td>Reserved
</td>
</tr>
</table>
</p>
</td>
</tr>
<tr>
<td><p>Entry Size</p></td>
<td>
<p>The size in bytes of an entry in the Fixed Array.
</p>
</td>
</tr>
<tr>
<td><p>Page Bits</p></td>
<td>
<p>The number of bits needed to store the maximum
number of entries in a
<a href="#FADataBlockPage">data block page.</a></p>
</td>
</tr>
<tr>
<td><p>Max Num Entries</p></td>
<td>
<p>The maximum number of entries in the Fixed
Array.</p>
</td>
</tr>
<tr>
<td><p>Data Block Address</p></td>
<td>
<p>The address of the data block in the Fixed Array.
</p>
</td>
<tr>
<td><p>Checksum</p></td>
<td>
<p>The checksum for the header.</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Fixed Array Data Block
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan="4">Signature</td>
</tr>
<tr>
<td>Version</td>
<td>Client ID</td>
<td colspan="2" bgcolor="#DDDDDD"><em>This space inserted
only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4"><br />Header Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Page Bitmap <em>(variable size and
optional)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Elements <em>(variable size and
optional)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4">Checksum</td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Fixed Array Data Block
</caption>
<tr>
<th width="40%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Signature</p></td>
<td>
<p>The ASCII character string “<code>FADB</code>”
is used to indicate the beginning of a Fixed Array data
block. This gives file consistency checking utilities a
better chance of reconstructing a damaged file.
</p>
</td>
</tr>
<tr>
<td><p>Version</p></td>
<td>
<p>This document describes version 0.</p>
</td>
</tr>
<tr>
<td><p>Client ID</p></td>
<td>
<p>The ID for identifying the client of the
Fixed Array:
<table class="list">
<tr>
<th width="20%" align="center">ID</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>Non-filtered dataset chunks
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>Filtered dataset chunks
</td>
</tr>
<tr>
<td align="center"><code>2+</code></td>
<td>Reserved.
</td>
</tr>
</table>
</p>
</td>
</tr>
<tr>
<td><p>Header Address</p></td>
<td>
<p>The address of the Fixed Array header. Principally used
for file integrity checking.
</p>
</td>
</tr>
<tr>
<td><p>Page Bitmap</p></td>
<td><p>A bitmap indicating which data block pages are initialized.</p>
<p>Exists only if the data block is paged.</p></td>
</tr>
<tr>
<td><p>Elements</p></td>
<td>
<p>Contains the elements stored in the data block
and exists only if the data block is not paged.
There are two element types:
<table class="list">
<tr>
<th width="20%" align="center">ID</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td><a href="#FaNonFilterChunk">Non-filtered
dataset chunks</i></a>
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td><a href="#FaFilterChunk">Filtered dataset
chunks</i></a>
</td>
</tr>
</table>
</p>
</td>
</tr>
<tr>
<td><p>Checksum</p></td>
<td>
<p>The checksum for the Fixed Array data block.</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption><a name="FADataBlockPage">
Layout: Fixed Array Data Block Page</a>
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan="4"><br />Elements <em>(variable
size)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4">Checksum</td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Fixed Array Data Block Page
</caption>
<tr>
<th width="40%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Elements</p></td>
<td>
<p>Contains the elements stored in the data block page.
There are two element types:
<table class="list">
<tr>
<th width="20%" align="center">ID</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td><a href="#FaNonFilterChunk">Non-filtered dataset chunks</i></a>
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td><a href="#FaFilterChunk">Filtered dataset chunks</i></a>
</td>
</tr>
</table>
</p>
</td>
</tr>
<tr>
<td><p>Checksum</p></td>
<td>
<p>The checksum for a Fixed Array data block page.</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<a name="FaNonFilterChunk"></a>
<div align="center">
<table class="format">
<caption>
Layout: Data Block Element for Non-filtered Dataset Chunk
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td colspan="4"><br />Address<sup>O</sup><br /><br /></td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Data Block Element for Non-filtered Dataset Chunk
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Address</p></td>
<td><p>The address of the dataset chunk in the file.
</p>
</td>
</tr>
</table>
</div>
<!-- </p> -->
<br />
<br />
<br />
<a name="FaFilterChunk"></a>
<div align="center">
<table class="format">
<caption>
Layout: Data Block Element for Filtered Dataset Chunk
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td colspan="4"><br />Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Chunk Size <em>(variable size; at most
8 bytes)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4">Filter Mask</td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Data Block Element for Filtered Dataset Chunk
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Address</p></td>
<td><p>The address of the dataset chunk in the file.
</p>
</td>
</tr>
<tr>
<td><p>Chunk Size</p></td>
<td><p>The size of the dataset chunk in bytes.
</p>
</td>
</tr>
<tr>
<td><p>Filter Mask</p></td>
<td><p>Indicates the filter to skip for the dataset chunk. Each
filter has an index number in the pipeline; if that filter is
skipped, the bit corresponding to its index is set.
</p>
</td>
</tr>
</table>
</div>
<a name="ExtensibleArray">
<h3>VII.D. The Extensible Array Index</h3></a>
<p>The <i>Extensible Array</i> index can be used when the dataset
fulfills the following condition:</p>
<ul>
<li>only one dimension of unlimited extent</li>
</ul>
<p>The Extensible Array (EA) is a data structure that is used as a
chunk index in datasets where the dataspace has a single
unlimited dimension. In other words, one dimension is set to
<code>H5S_UNLIMITED</code>, and the other dimensions are any number
of fixed-size dimensions. The idea behind the extensible array is
that a particular data object can be located via a lightweight
indexing structure of fixed depth for a given address space. This
indexing structure requires only a few (2-3) file operations per
element lookup and gives good cache performance. Unlike the B-tree
structure, the extensible array is optimized for appends. Where a
B-tree would always add at the rightmost node under these
circumstances, either creating a deep tree (version 1) or requiring
expensive rebalances to correct (version 2), the extensible array
has already mapped out a pre-balanced internal structure. This
optimized internal structure is instantiated as needed when chunk
records are inserted into the structure.</p>
<!--
<p>A description of the rationale that leads to the present
implementation of the extensible array can be found at
<a href="https://svn.hdfgroup.org/hdf5doc/trunk/projects/1_10_alpha/ReviseChunks/skip_lists">
https://svn.hdfgroup.org/hdf5doc/trunk/projects/1_10_alpha/ReviseChunks/skip_lists</a>.
</p>
<p>The current implementation differs from the data structure
described in that reference in some ways, but the basic idea is the
same.</p>
-->
<p>An Extensible Array consists of a header, an index block,
secondary blocks, data blocks, and (optional) data block pages. The
general scheme is that the index block is used to reference a
secondary block, which is, in turn, used to reference the data block
page where the chunk information is stored. The data blocks will
be paged for efficiency when their size passes a threshold value.
These pages are laid out contiguously on the disk after the data
block, are initialized as needed, and are tracked via bitmaps
stored in the secondary block. The number of secondary and data
blocks/pages in a chunk index varies as they are allocated as
needed and the first few are (conceptually) stored in parent
elements as an optimization.</p>
<div align="center">
<table class="format">
<caption>
Layout: Extensible Array Header
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan="4">Signature</td>
</tr>
<tr>
<td>Version</td>
<td>Client ID</td>
<td>Element Size</td>
<td>Max Nelmts Bits</td>
</tr>
<tr>
<td>Index Blk Elmts</td>
<td>Data Blk Min Elmts</td>
<td>Secondary Blk Min Data Ptrs</td>
<td>Max Data Blk Page Nelmts Bits</td>
</tr>
<tr>
<td colspan="4"><br />Num Secondary Blks<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Secondary Blk Size<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Num Data Blks<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Data Blk Size<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Max Index Set<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Num Elements<sup>L</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Index Block Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4">Checksum</td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘L’ in the above table are
of the size specified in the <a href="#SizeOfLengthsV0">Size
of Lengths</a> field in the superblock.)
</td></tr>
<tr>
<td> </td>
<td>
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Extensible Array Header
</caption>
<tr>
<th width="40%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Signature</p></td>
<td>
<p>The ASCII character string “<code>EAHD</code>”
is used to indicate the beginning of an Extensible Array
header. This gives file consistency checking utilities a
better chance of reconstructing a damaged file.
</p>
</td>
</tr>
<tr>
<td><p>Version</p></td>
<td>
<p>This document describes version 0.</p>
</td>
</tr>
<tr>
<td><p>Client ID</p></td>
<td>
<p>The ID for identifying the client of the
Fixed Array:
<table class="list">
<tr>
<th width="20%" align="center">ID</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>Non-filtered dataset chunks
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>Filtered dataset chunks
</td>
</tr>
<tr>
<td align="center"><code>2+</code></td>
<td>Reserved.
</td>
</tr>
</table>
</p>
</td>
</tr>
<tr>
<td><p>Element Size</p></td>
<td>
<p>The size in bytes of an element in the Extensible Array.
</p>
</td>
</tr>
<tr>
<td><p>Max Nelmts Bits</p></td>
<td>
<p>The number of bits needed to store the
maximum number of elements in the Extensible Array.</p>
</td>
</tr>
<tr>
<td><p>Index Blk Elmts</p></td>
<td>
<p>The number of elements to store in the index block.
</p>
</td>
</tr>
<tr>
<td><p>Data Blk Min Elmts</p></td>
<td>
<p>The minimum number of elements per data block.
</p>
</td>
</tr>
<tr>
<td><p>Secondary Blk Min Data Ptrs</p></td>
<td>
<p>The minimum number of data block pointers for a
secondary block.
</p>
</td>
</tr>
<tr>
<td><p>Max Dblk Page Nelmts Bits</p></td>
<td>
<p>The number of bits needed to store the maximum number
of elements in a data block page.
</p>
</td>
</tr>
<tr>
<td><p>Num Secondary Blks</p></td>
<td>
<p>The number of secondary blocks created.
</p>
</td>
</tr>
<tr>
<td><p>Secondary Blk Size</p></td>
<td>
<p>The size of the secondary blocks created.
</p>
</td>
</tr>
<tr>
<td><p>Num Data Blks</p></td>
<td>
<p>The number of data blocks created.
</p>
</td>
</tr>
<tr>
<td><p>Data Blk Size</p></td>
<td>
<p>The size of the data blocks created.
</p>
</td>
</tr>
<tr>
<td><p>Max Index Set</p></td>
<td>
<p>The maximum index set.
</p>
</td>
</tr>
<tr>
<td><p>Num Elmts</p></td>
<td>
<p>The number of elements realized.
</p>
</td>
</tr>
<tr>
<td><p>Index Block Address</p></td>
<td>
<p>The address of the index block.
</p>
</td>
</tr>
<tr>
<td><p>Checksum</p></td>
<td>
<p>The checksum for the header.</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Extensible Array Index Block
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan="4">Signature</td>
</tr>
<tr>
<td>Version</td>
<td>Client ID</td>
<td colspan="2" bgcolor="#DDDDDD"><em>This space inserted
only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4"><br />Header Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Elements <em>(variable size and
optional)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Data Block Addresses <em>(variable
size and optional)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Secondary Block Addresses <em>(variable
size and optional)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4">Checksum</td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Extensible Array Index Block
</caption>
<tr>
<th width="40%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Signature</p></td>
<td>
<p>The ASCII character string “<code>EAIB</code>”
is used to indicate the beginning of an Extensible Array
Index Block. This gives file consistency checking utilities
a better chance of reconstructing a damaged file.
</p>
</td>
</tr>
<tr>
<td><p>Version</p></td>
<td>
<p>This document describes version 0.</p>
</td>
</tr>
<tr>
<td><p>Client ID</p></td>
<td>
<p>The client ID for identifying the user of the
Extensible Array:
<table class="list">
<tr>
<th width="20%" align="center">ID</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>Non-filtered dataset chunks
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>Filtered dataset chunks
</td>
</tr>
<tr>
<td align="center"><code>2+</code></td>
<td>Reserved.
</td>
</tr>
</table>
</p>
</td>
</tr>
<tr>
<td><p>Header Address</p></td>
<td>
<p>The address of the Extensible Array header. Principally
used for file integrity checking.</p>
</td>
</tr>
<tr>
<td><p>Elements</p></td>
<td>
<p>Contains the elements that are stored directly in
the index block. An optimization to avoid unnecessary
secondary blocks.
<br />
<br />
There are two element types:
<table class="list">
<tr>
<th width="20%" align="center">ID</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td><a href="#EaNonFilterChunk">Non-filtered dataset chunks</i></a>
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td><a href="#EaFilterChunk">Filtered dataset chunks</i></a>
</td>
</tr>
</table>
</p>
</td>
</tr>
<tr>
<td><p>Data Block Addresses</p></td>
<td>
<p>Contains the addresses of the data blocks
that are stored directly in the Index Block. An
optimization to avoid unnecessary secondary blocks.</p>
</td>
</tr>
<tr>
<td><p>Secondary Block Addresses</p></td>
<td>
<p>Contains the addresses of the secondary
blocks.</p>
</td>
</tr>
<tr>
<td><p>Checksum</p></td>
<td>
<p>The checksum for the Extensible Array Index Block.</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Extensible Array Secondary Block
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan="4">Signature</td>
</tr>
<tr>
<td>Version</td>
<td>Client ID</td>
<td colspan="2" bgcolor="#DDDDDD"><em>This space inserted
only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4"><br />Header Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Block Offset <em>(variable
size)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Page Bitmap <em>(variable size and
optional)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Data Block Addresses <em>(variable
size and optional)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4">Checksum</td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Extensible Array Secondary Block
</caption>
<tr>
<th width="40%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Signature</p></td>
<td>
<p>The ASCII character string “<code>EASB</code>”
is used to indicate the beginning of an Extensible Array
Secondary Block. This gives file consistency checking utilities
a better chance of reconstructing a damaged file.
</p>
</td>
</tr>
<tr>
<td><p>Version</p></td>
<td>
<p>This document describes version 0.</p>
</td>
</tr>
<tr>
<td><p>Client ID</p></td>
<td>
<p>The ID for identifying the client of the
Extensible Array:
<table class="list">
<tr>
<th width="20%" align="center">ID</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>Non-filtered dataset chunks
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>Filtered dataset chunks
</td>
</tr>
<tr>
<td align="center"><code>2+</code></td>
<td>Reserved.
</td>
</tr>
</table>
</p>
</td>
</tr>
<tr>
<td><p>Header Address</p></td>
<td>
<p>The address of the Extensible Array header. Principally
used for file integrity checking.</p>
</td>
</tr>
<tr>
<td><p>Block Offset</p></td>
<td>
<p>Stores the offset of the block in the array.
</p>
</td>
</tr>
<tr>
<td><p>Page Bitmap</p></td>
<td>
<p>A bitmap indicating which
data block pages are initialized.
<p>
Exists only if the data block is paged.
</td>
</tr>
<tr>
<td><p>Data Block Addresses</p></td>
<td>
<p>Contains the addresses of the data blocks
referenced by this secondary block.</p>
</td>
</tr>
<tr>
<td><p>Checksum</p></td>
<td>
<p>The checksum for the Extensible Array
Secondary Block.</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Extensible Array Data Block
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan="4">Signature</td>
</tr>
<tr>
<td>Version</td>
<td>Client ID</td>
<td colspan="2" bgcolor="#DDDDDD"><em>This space inserted
only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4"><br />Header Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Block Offset <em>(variable
size)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Elements <em>(variable size and
optional)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4">Checksum</td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Extensible Array Data Block
</caption>
<tr>
<th width="40%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Signature</p></td>
<td>
<p>The ASCII character string “<code>EADB</code>”
is used to indicate the beginning of an Extensible Array
data block. This gives file consistency checking utilities
a better chance of reconstructing a damaged file.
</p>
</td>
</tr>
<tr>
<td><p>Version</p></td>
<td>
<p>This document describes version 0.</p>
</td>
</tr>
<tr>
<td><p>Client ID</p></td>
<td>
<p>The ID for identifying the client of the
Extensible Array:
<table class="list">
<tr>
<th width="20%" align="center">ID</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>Non-filtered dataset chunks
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>Filtered dataset chunks
</td>
</tr>
<tr>
<td align="center"><code>2+</code></td>
<td>Reserved.
</td>
</tr>
</table>
</p>
</td>
</tr>
<tr>
<td><p>Header Address</p></td>
<td>
<p>The address of the Extensible Array header. Principally
used for file integrity checking.
</p>
</td>
</tr>
<tr>
<td><p>Block Offset</p></td>
<td>
<p>The offset of the block in the array.
</td>
</tr>
<tr>
<td><p>Elements</p></td>
<td>
<p>Contains the elements stored in the data block and
exists only if the data block is not paged.
<br />
<br />
There are two element types:
<table class="list">
<tr>
<th width="20%" align="center">ID</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td><a href="#EaNonFilterChunk">Non-filtered dataset chunks</i></a>
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td><a href="#EaFilterChunk">Filtered dataset chunks</i></a>
</td>
</tr>
</table>
</p>
</td>
</tr>
<tr>
<td><p>Checksum</p></td>
<td>
<p>The checksum for the Extensible Array data block.</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Extensible Array Data Block Page
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td colspan="4"><br />Elements <em>(variable
size)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4">Checksum</td>
</tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Extensible Array Data Block Page
</caption>
<tr>
<th width="40%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Elements</p></td>
<td>
<p>Contains the elements stored in the data block
page.</p>
<p>
There are two element types:
<table class="list">
<tr>
<th width="20%" align="center">ID</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td><a href="#EaNonFilterChunk">Non-filtered dataset chunks</i></a>
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td><a href="#EaFilterChunk">Filtered dataset chunks</i></a>
</td>
</tr>
</table>
</p>
</td>
</tr>
<tr>
<td><p>Checksum</p></td>
<td>
<p>The checksum for an Extensible Array data block
page.</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<a name="EaNonFilterChunk"></a>
<div align="center">
<table class="format">
<caption>
Layout: Data Block Element for Non-filtered Dataset Chunk
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td colspan="4"><br />Address<sup>O</sup><br /><br /></td>
</tr>
</table>
<table class="note">
<tr><td>
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Data Block Element for Non-filtered Dataset Chunk
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Address</p></td>
<td><p>The address of the dataset chunk in the file.
</p>
</td>
</tr>
</table>
</div>
</p>
<br />
<br />
<br />
<a name="EaFilterChunk"></a>
<div align="center">
<table class="format">
<caption>
Layout: Data Block Element for Filtered Dataset Chunk
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td colspan="4"><br />Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan="4"><br />Chunk Size<em> (variable size; at
most 8 bytes)</em><br /><br /></td>
</tr>
<tr>
<td colspan="4">Filter Mask</td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Data Block Element for Filtered Dataset Chunk
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Address</p></td>
<td><p>The address of the dataset chunk in the file.
</p>
</td>
</tr>
<tr>
<td><p>Chunk Size</p></td>
<td><p>The size of the dataset chunk in bytes.
</p>
</td>
</tr>
<tr>
<td><p>Filter Mask</p></td>
<td><p>Indicates the filter to skip for the dataset chunk.
Each filter has an index number in the pipeline; if that
filter is skipped, the bit corresponding to its index is set.
</p>
</td>
</tr>
</table>
</div>
<a name="AppendV2Btrees">
<h3>VII.E. The Version 2 B-trees Index</h3></a>
<p>The <i>Version 2 B-trees</i> index can be used when the dataset
fulfills the following condition:</p>
<ul>
<li>more than one dimension of unlimited extent</li>
</ul>
<p>Version 2 B-trees can be used to index various objects in the
library. See <a href="#V2Btrees">“Version 2 B-trees”</a>
for more information. The B-tree types <a href="#V2BtType10">10</a>
and <a href="#V2BtreesType11">11</a> record layouts are for
indexing dataset chunks.</p>
<h2><a name="AppendixD"> VIII. Appendix D:
Encoding for dataspace and reference</a></h2>
<a name="DataspaceEncode">
<h3>VIII.A. Dataspace Encoding </h3></a>
<i>H5Sencode</i> is a public routine that encodes a dataspace description into a buffer while
<i>H5Sdecode</i> is the corresponding routine that decodes the description encoded in the buffer.
<p>
See the reference manual description for these two public routines.
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Dataspace Description for H5Sencode/H5Sdecode
</caption>
<tr>
<th>byte</th>
<th>byte</th>
<th>byte</th>
<th>byte</th>
</tr>
<tr>
<td>Dataspace ID</td>
<td>Encode Version</td>
<td>Size of Size</td>
<td colspan="2" bgcolor="#DDDDDD"><em>This space inserted
only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4"><br />Size of Extent
<br /><br /></td>
</tr>
<tr>
<td colspan="4"><br /><br />Dataspace Message
<em>(variable size)</em>
<br /><br /></td>
</tr>
<tr>
<td colspan="4"><br /><br />Dataspace Selection
<em>(variable size)</em>
<br /><br /></td>
</tr>
</table>
</div>
<br />
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Dataspace Description for H5Sencode/H5Sdecode
</caption>
<tr>
<th width="40%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Dataspace ID</p></td>
<td>
<p>The datspace message ID which is 1.</p>
</td>
</tr>
<tr>
<td><p>Encode Version</p></td>
<td>
<p>H5S_ENCODE_VERSION which is 0.
</p>
</td>
</tr>
<tr>
<td><p>Size of Size</p></td>
<td>
<p>The number of bytes used to store the size of an object.
</p>
</td>
</tr>
<tr>
<td><p>Size of Extent</p></td>
<td>
<p>Size of the dataspace message.
</p>
</td>
</tr>
<tr>
<td><p>Dataspace Message</p></td>
<td>
<p>The dataspace message information. See
<a href="#DataspaceMessage">Dataspace Message.</a></p>
</p>
</td>
</tr>
<tr>
<td><p>Dataspace Selection</p></td>
<td>
<p>The dataspace selection information. See
<a href="#DataspaceSEL">Dataspace Selection.</a></p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<a name="DataspaceSEL"></a>
<div align="center">
<table class="format">
<caption>
Layout: Dataspace Selection
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td colspan="4">Selection Type</td>
</tr>
<tr>
<td colspan="4"><br />Selection Info (<em>variable
size</em>)<br /><br /></td>
</tr>
</table>
</div>
<br />
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Dataspace Selection
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Selection Type</p></td>
<td>
<p>There are 4 types of selection:
<table class="list">
<tr>
<th width="20%" align="center">Value</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>H5S_SEL_NONE: Nothing selected
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>H5S_SEL_POINTS: Sequence of points selected
</td>
</tr>
<tr>
<td align="center"><code>2</code></td>
<td>H5S_SEL_HYPER: Hyperslab selected
</td>
</tr>
<tr>
<td align="center"><code>3</code></td>
<td>H5S_SEL_ALL: Entire extent selected
</td>
</tr>
</table>
</td>
</tr>
<tr>
<td><p>Selection Info</p></td>
<td>
<p>There are 4 types of selection info:
<table class="list">
<tr>
<th width="20%" align="center">Value</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>Selection info for <a href="#SelNONE">H5S_SEL_NONE</a>
</td>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>Selection info for <a href="#SelPOINTS">H5S_SEL_POINTS</a>
</td>
</tr>
<tr>
<td align="center"><code>2</code></td>
<td>Selection info for <a href="#SelHYPER">H5S_SEL_HYPER</a>
</td>
</tr>
<tr>
<td align="center"><code>3</code></td>
<td>Selection for <a href="#SelALL">H5S_SEL_ALL</a>
</td>
</tr>
</table>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<a name="SelNONE"/></a>
<div align="center">
<table class="format">
<caption>
Layout: Selection Info for H5S_SEL_NONE
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td colspan="4">Version</td>
</tr>
<tr>
<td colspan="4"><br />Reserved <em>(zero, 8 bytes)</em><br /><br /></td>
</tr>
</table>
</div>
<br />
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Selection Info for H5S_SEL_NONE
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Version</p></td>
<td><p>The version number for the H5S_SEL_NONE Selection Info.
The value is 1.</p></td>
</tr>
</table>
</div>
<br />
<br />
<br />
<a name="SelPOINTS"></a>
<div align="center">
<table class="format">
<caption>
Layout: Selection Info for H5S_SEL_POINTS
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td colspan="4">Version</td>
</tr>
<tr>
<td colspan="4"><br /><br />Points Selection Info <em>(variable size)</em>
<br /><br /><br /></td>
</tr>
</table>
</div>
<br />
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Selection Info for H5S_SEL_POINTS
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Version</p></td>
<td><p>The version number for the H5S_SEL_POINTS Selection Info.
The value is either 1 or 2.</p></td>
</tr>
<tr>
<td><p>Points Selection Info</p></td>
<td><p>Depending on <em>version</em>:
<table class="list">
<tr>
<th width="20%" align="center">Version</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>See <a href="#SelPOINTSV1">Version 1 Points Selection Info</a>
</td>
</tr>
<tr>
<td align="center"><code>2</code></td>
<td>See <a href="#SelPOINTSV2">Version 2 Points Selection Info</a>
</td>
</tr>
</table>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<a name="SelPOINTSV1"></a>
<div align="center">
<table class="format">
<caption>
Layout: Version 1 Points Selection Info
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td colspan="4">Reserved <em>(zero)</em></td>
</tr>
<tr>
<td colspan="4">Length</td>
</tr>
<tr>
<td colspan="4">Rank</td>
</tr>
<tr>
<td colspan="4">Num Points</td>
</tr>
<tr>
<td colspan="4">Point #1: coordinate #1</td>
</tr>
<tr>
<td colspan="4">.<br />.<br />.<br /></td>
</tr>
<tr>
<td colspan="4">Point #1: coordinate #u</td>
</tr>
<tr>
<td colspan="4">.<br />.<br />.<br /></td>
</tr>
<tr>
<td colspan="4">Point #n: coordinate #1</td>
</tr>
<tr>
<td colspan="4">.<br />.<br />.<br /></td>
</tr>
<tr>
<td colspan="4">Point #n: coordinate #u</td>
</tr>
</table>
</div>
<br />
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Version 1 Points Selection Info
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Length</p></td>
<td><p>The size in bytes from <em>Length</em> to the end of the
selection info.</td>
</tr>
<tr>
<td><p>Rank</p></td>
<td><p>The number of dimensions.</p></td>
</tr>
<tr>
<td><p>Num Points</p></td>
<td><p>The number of points in the selection.</p></td>
</tr>
<tr>
<td><p>Point #n: coordinate #u</p></td>
<td><p>The array of points in the selection.
<p>The points selected are #1 to #n where n is <em>Num Points</em>.
<p>The list of coordinates for each point are #1 to #u where u is
<em>Rank</em>.</p></td>
</tr>
</table>
</div>
<br />
<br />
<br />
<a name="SelPOINTSV2"></a>
<div align="center">
<table class="format">
<caption>
Layout: Version 2 Points Selection Info
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td colspan="1">Encode Size</td>
<td colspan="3" bgcolor="#DDDDDD"><em>This space inserted only to align table nicely</em>
</td>
</tr>
<tr>
<td colspan="4">Rank</td>
</tr>
<tr>
<td colspan="4">Num Points<p>(2, 4 or 8 bytes)<br /></td>
</tr>
<tr>
<td colspan="4">Point #1: coordinate #1<p>(2, 4 or 8 bytes)<br /></td>
</tr>
<tr>
<td colspan="4">.<br />.<br />.<br /></td>
</tr>
<tr>
<td colspan="4">Point #1: coordinate #u<p>(2, 4 or 8 bytes)<br /></td>
</tr>
<tr>
<td colspan="4">.<br />.<br />.<br /></td>
</tr>
<tr>
<td colspan="4">Point #n: coordinate #1 <p>(2, 4 or 8 bytes)<br /></td>
</tr>
<tr>
<td colspan="4">.<br />.<br />.<br /></td>
</tr>
<tr>
<td colspan="4">Point #n: coordinate #u<p>(2, 4 or 8 bytes)<br /></td>
</tr>
</table>
</div>
<br />
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Version 2 Points Selection Info
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Encode Size</td>
<td><p>The size for encoding the points selection info which can be 2, 4 or 8 bytes.
</td>
</tr>
<tr>
<td><p>Rank</p></td>
<td><p>The number of dimensions.</p></td>
</tr>
<tr>
<td><p>Num Points</p></td>
<td><p>The number of points in the selection.
<p>The field <em>Encode Size</em> indicates the size of this field</p></td>
</tr>
<tr>
<td><p>Point #n: coordinate #u</p></td>
<td><p>The array of points in the selection.
<p>The points selected are #1 to #n where n is <em>Num Points</em>.
<p>The list of coordinates for each point are #1 to #u where u is
<em>Rank</em>.
<p>The field <em>Encode Size</em> indicates the size of this field</p></td>
</tr>
</table>
</div>
<br />
<br />
<br />
<a name="SelHYPER"></a>
<div align="center">
<table class="format">
<caption>
Layout: Selection Info for H5S_SEL_HYPER
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td colspan="4">Version</td>
</tr>
<tr>
<td colspan="4"><br />Hyperslab Selection Info
(<em>variable size</em>)<br /><br /></td>
</tr>
</table>
</div>
<br />
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Selection Info for H5S_SEL_HYPER
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Version</p></td>
<td><p>The version number for the H5S_SEL_HYPER selection info.
The value is 1, 2 or 3.</p></td>
</tr>
<tr>
<td><p>Hyperslab Selection Info</p></td>
<td><p>Depending on <em>version</em>:
<table class="list">
<tr>
<th width="20%" align="center">Version</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>1</code></td>
<td>See <a href="#SelHYPERV1">Version 1 Hyperslab Selection Info</a>.
</td>
</tr>
<tr>
<td align="center"><code>2</code></td>
<td>See <a href="#SelHYPERV2">Version 2 Hyperslab Selection Info</a>
</td>
</tr>
<tr>
<td align="center"><code>3</code></td>
<td>See <a href="#SelHYPERV3">Version 3 Hyperslab Selection Info</a>
</td>
</tr>
</table>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<a name="SelHYPERV1"></a>
<div align="center">
<table class="format">
<caption>
Layout: Version 1 Hyperslab Selection Info
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td colspan="4">Reserved</td>
</tr>
<tr>
<td colspan="4">Length</td>
</tr>
<tr>
<td colspan="4">Rank</td>
</tr>
<tr>
<td colspan="4">Num Blocks</td>
</tr>
<tr>
<td colspan="4">Starting Offset #1 for Block #1</td>
</tr>
<tr>
<td colspan="4">.<br />.<br />.<br /></td>
</tr>
<tr>
<td colspan="4">Starting Offset #n for Block #1</td>
</tr>
<tr>
<td colspan="4">Ending Offset #1 for Block #1</td>
</tr>
<tr>
<td colspan="4">.<br />.<br />.<br /></td>
</tr>
<tr>
<td colspan="4">Ending Offset #n for Block #1</td>
</tr>
<tr>
<td colspan="4">.<br />.<br />.<br /></td>
</tr>
<tr>
<td colspan="4">.<br />.<br />.<br /></td>
</tr>
<tr>
<td colspan="4">.<br />.<br />.<br /></td>
</tr>
<tr>
<td colspan="4">Starting Offset #1 for Block #u</td>
</tr>
<tr>
<td colspan="4">.<br />.<br />.<br /></td>
</tr>
<tr>
<td colspan="4">Starting Offset #n for Block #u</td>
</tr>
<tr>
<td colspan="4">Ending Offset #1 for Block #u</em></td>
</tr>
<tr>
<td colspan="4">.<br />.<br />.<br /></td>
</tr>
<tr>
<td colspan="4">Ending Offset #n for Block #u</td>
</tr>
</table>
</div>
<br />
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Version 1 Hyperslab Selection Info
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Length</p></td>
<td><p>The size in bytes from the field <em>Rank</em> to the
end of the Selection Info.</td>
</tr>
<tr>
<td><p>Rank</p></td>
<td><p>The number of dimensions in the dataspace.</p></td>
</tr>
<tr>
<td><p>Num Blocks</p></td>
<td><p>The number of blocks in the selection.</p></td>
</tr>
<tr>
<td><p>Starting Offset #n for Block #u</p></td>
<td><p>The offset #n of the starting element in block #u.
<p>#n is from 1 to <em>Rank</em>.
<p>#u is from 1 to <em>Num Blocks</em> moving from the fastest
changing dimension to the slowest changing dimension.
</p></td>
</tr>
<tr>
<td><p>Ending Offset #n for Block #u</p></td>
<td><p>The offset #n of the ending element in block #u.
<p>#n is from 1 to <em>Rank</em>.
<p>#u is from 1 to <em>Num Blocks</em> moving from the fastest
changing dimension to the slowest changing dimension.
</p></td>
</tr>
</table>
</div>
<br />
<br />
<br />
<a name="SelHYPERV2"></a>
<div align="center">
<table class="format">
<caption>
Layout: Version 2 Hyperslab Selection Info
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td>Flags</td>
<td colspan="3" bgcolor="#DDDDDD"><em>This space inserted
only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4">Length</td>
</tr>
<tr>
<td colspan="4">Rank</td>
</tr>
<tr>
<td colspan="4">Start #1 <em>(8 bytes)</em><p></td>
</tr>
<tr>
<td colspan="4">Stride #1 <em>(8 bytes)</em><p></td>
</tr>
<tr>
<td colspan="4">Count #1 <em>(8 bytes)</em><p></td>
</tr>
<tr>
<td colspan="4">Block #1 <em>(8 bytes)</em><p></td>
</tr>
<tr>
<td colspan="4">.<br />.<br />.<br /></td>
</tr>
<tr>
<td colspan="4">Start #n <em>(8 bytes)</em><p></td>
</tr>
<tr>
<td colspan="4">Stride #n <em>(8 bytes)</em><p></td>
</tr>
<tr>
<td colspan="4">Count #n <em>(8 bytes)</em><p></td>
</tr>
<tr>
<td colspan="4">Block #n <em>(8 bytes)</em><p></td>
</tr>
</table>
</div>
<br />
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Version 2 Hyperslab Selection Info
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Flags</p></td>
<td><p>This is a bit field with the following definition.
Currently, this is always set to 0x1.
<p>
<table class="list">
<tr>
<th width="20%" align="center">Bit</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>If set, it is a regular hyperslab, otherwise, irregular.
</td>
</tr>
</table>
</td>
</tr>
<tr>
<td><p>Length</p></td>
<td><p>The size in bytes from the field <em>Rank</em> to the
end of the Selection Info.</td>
</tr>
<tr>
<td><p>Rank</p></td>
<td><p>The number of dimensions in the dataspace.</td>
</tr>
<tr>
<td><p>Start #n</p></td>
<td><p>The offset of the starting element in the block.
<p>#n is from 1 to <em>Rank</em>.
</p></td>
</tr>
<tr>
<td><p>Stride #n</p></td>
<td><p>The number of elements to move in each dimension.
<p>#n is from 1 to <em>Rank</em>.
</p></td>
</tr>
<tr>
<td><p>Count #n</p></td>
<td><p>The number of blocks to select in each dimension.
<p>#n is from 1 to <em>Rank</em>.
</p></td>
</tr>
<tr>
<td><p>Block #n</p></td>
<td><p>The size (in elements) of each block in each dimension.
<p>#n is from 1 to <em>Rank</em>.
</p></td>
</tr>
</table>
</div>
<br />
<br />
<br />
<a name="SelHYPERV3"></a>
<div align="center">
<table class="format">
<caption>
Layout: Version 3 Hyperslab Selection Info
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td>Flags</td>
<td>Encode Size</td>
<td colspan="2" bgcolor="#DDDDDD"><em>This space inserted
only to align table nicely</em></td>
</tr>
<tr>
<td colspan="4">Rank</td>
</tr>
<tr>
<td colspan="4"><br />Regular/Irregular Hyperslab Selection Info
<p><em>(variable size)</em><br /><br/></td>
</tr>
</table>
</div>
<br />
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Version 3 Hyperslab Selection Info
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Flags</p></td>
<td><p>This is a bit field with the following definition:
<p>
<table class="list">
<tr>
<th width="20%" align="center">Bit</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>If set, it is a regular hyperslab, otherwise, irregular.
</td>
</tr>
</table>
</td>
</tr>
<tr>
<td><p>Encode Size</p></td>
<td><p>The size for encoding hyperslab selection info, which can 2, 4 or 8 bytes.</td>
</tr>
<tr>
<td><p>Rank</p></td>
<td><p>The number of dimensions in the dataspace.</td>
</tr>
<tr>
<td><p>Regular/Irregular Hyperslab Selection Info</p></td>
<td><p>This is the selection info for version 3 hyperslab which can be regular or irregular.
<p>If bit 0 of the field <em>Flags</em> is set,
See <a href="#SelHYPERV3REG">Version 3 Regular Hyperslab Selection Info</a>
<p>Otherwise, see <a href="#SelHYPERV3IRREG">Version 3 Irregular Hyperslab Selection Info</a>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<a name="SelHYPERV3REG"></a>
<div align="center">
<table class="format">
<caption>
Layout: Version 3 Regular Hyperslab Selection Info
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td colspan="4">Start #1 <p><em>(2, 4 or 8 bytes)</em><p></td>
</tr>
<tr>
<td colspan="4">Stride #1 <p><em>(2, 4 or 8 bytes)</em><p></td>
</tr>
<tr>
<td colspan="4">Count #1 <p><em>(2, 4 or 8 bytes)</em><p></td>
</tr>
<tr>
<td colspan="4">Block #1 <p><em>(2, 4 or 8 bytes)</em><p></td>
</tr>
<tr>
<td colspan="4">.<br />.<br />.<br /></td>
</tr>
<tr>
<td colspan="4">Start #n <p><em>(2, 4 or 8 bytes)</em><p></td>
</tr>
<tr>
<td colspan="4">Stride #n <p><em>(2, 4 or 8 bytes)</em><p></td>
</tr>
<tr>
<td colspan="4">Count #n <p><em>(2, 4 or 8 bytes)</em><p></td>
</tr>
<tr>
<td colspan="4">Block #n <p><em>(2, 4 or 8 bytes)</em><p></td>
</tr>
</table>
</div>
<br />
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Version 3 Regular Hyperslab Selection Info
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Start #n</p></td>
<td><p>The offset of the starting element in the block.
<p>#n is from 1 to <em>Rank</em>.
<p>The field <em>Encode Size</em> indicates the size of this field.
</p></td>
</tr>
<tr>
<td><p>Stride #n</p></td>
<td><p>The number of elements to move in each dimension.
<p>#n is from 1 to <em>Rank</em>.
<p>The field <em>Encode Size</em> indicates the size of this field.
</p></td>
</tr>
<tr>
<td><p>Count #n</p></td>
<td><p>The number of blocks to select in each dimension.
<p>#n is from 1 to <em>Rank</em>.
<p>The field <em>Encode Size</em> indicates the size of this field.
</p></td>
</tr>
<tr>
<td><p>Block #n</p></td>
<td><p>The size (in elements) of each block in each dimension.
<p>#n is from 1 to <em>Rank</em>.
<p>The field <em>Encode Size</em> indicates the size of this field.
</p></td>
</tr>
</table>
</div>
<br />
<br />
<br />
<a name="SelHYPERV3IRREG"></a>
<div align="center">
<table class="format">
<caption>
Layout: Version 3 Irregular Hyperslab Selection Info
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td colspan="4">Num Blocks<p><em>(2, 4 or 8 bytes)</em><p></td>
</tr>
<tr>
<td colspan="4">Starting Offset #1 for Block #1<p><em>(2, 4 or 8 bytes)</em><p></td>
</tr>
<tr>
<td colspan="4">.<br />.<br />.<br /></td>
</tr>
<tr>
<td colspan="4">Starting Offset #n for Block #1<p><em>(2, 4 or 8 bytes)</em><p></td>
</tr>
<tr>
<td colspan="4">Ending Offset #1 for Block #1<p><em>(2, 4 or 8 bytes)</em><p></td>
</tr>
<tr>
<td colspan="4">.<br />.<br />.<br /></td>
</tr>
<tr>
<td colspan="4">Ending Offset #n for Block #1<p><em>(2, 4 or 8 bytes)</em><p></td>
</tr>
<tr>
<td colspan="4">.<br />.<br />.<br /></td>
</tr>
<tr>
<td colspan="4">.<br />.<br />.<br /></td>
</tr>
<tr>
<td colspan="4">.<br />.<br />.<br /></td>
</tr>
<tr>
<td colspan="4">Starting Offset #1 for Block #u<p><em>(2, 4 or 8 bytes)</em><p></td>
</tr>
<tr>
<td colspan="4">.<br />.<br />.<br /></td>
</tr>
<tr>
<td colspan="4">Starting Offset #n for Block #u<p><em>(2, 4 or 8 bytes)</em><p></td>
</tr>
<tr>
<td colspan="4">Ending Offset #1 for Block #u<p><em>(2, 4 or 8 bytes)</em><p></td>
</tr>
<tr>
<td colspan="4">.<br />.<br />.<br /></td>
</tr>
<tr>
<td colspan="4">Ending Offset #n for Block #u<p><em>(2, 4 or 8 bytes)</em><p></td>
</tr>
</table>
</div>
<br />
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Version 3 Irregular Hyperslab Selection Info
</caption>
<tr>
<td><p>Num Blocks</p></td>
<td><p>The number of blocks in the selection.
<p>The field <em>Encode Size</em> indicates the size of this field</p></td>
</tr>
<tr>
<td><p>Starting Offset #n for Block #u</p></td>
<td><p>The offset #n of the starting element in block #u.
<p>#n is from 1 to <em>Rank</em>.
<p>#u is from 1 to <em>Num Blocks</em> moving from the fastest
changing dimension to the slowest changing dimension.
<p>The field <em>Encode Size</em> indicates the size of this field
</p></td>
</tr>
<tr>
<td><p>Ending Offset #n for Block #u</p></td>
<td><p>The offset #n of the ending element in block #u.
<p>#n is from 1 to <em>Rank</em>.
<p>#u is from 1 to <em>Num Blocks</em> moving from the fastest
changing dimension to the slowest changing dimension.
<p>The field <em>Encode Size</em> indicates the size of this field
</p></td>
</tr>
</table>
</div>
<br />
<br />
<br />
<a name="SelALL"></a>
<div align="center">
<table class="format">
<caption>
Layout: Selection Info for H5S_SEL_ALL
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td colspan="4">Version</td>
</tr>
<tr>
<td colspan="4"><br />Reserved <em>(zero,
8 bytes)</em><br /><br /></td>
</tr>
</table>
</div>
<br />
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Selection Info for H5S_SEL_ALL
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Version</p></td>
<td><p>The version number for the H5S_SEL_ALL Selection Info;
the value is 1.</p></td>
</tr>
</table>
</div>
<a name="ReferenceEncodeRV">
<h3>VIII.B. Reference Encoding (Revised)</h3></a>
<p>
<br />
For the following reference type,
the Reference Header and Reference Block are stored together as the dataset's raw data:
<ul>
<li>Object Reference (H5R_OBJECT2) (without reference to an external file)</li>
</ul>
<p>
For the following reference types,
the Reference Header plus the <a href="#GlobalHeapID">Global Heap ID</a> are stored
as the dataset's raw data in the file.
The global heap ID is used to locate the Reference Block stored in the global heap:
<ul>
<li>Object Reference (H5R_OBJECT2) (with reference to an external file)</li>
<li>Dataset Region Reference (H5R_DATASET_REGION2) (with/without reference to an external file)</li>
<li>Attribute Reference (H5R_ATTR) (with/without reference to an external file)</li>
</ul>
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Reference Header
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td>Reference Type</td>
<td>Flags</td>
<td colspan="2" bgcolor="#DDDDDD"><em>This space inserted
only to align table nicely</em></td>
</tr>
</table>
</div>
<br />
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Reference Header
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Reference Type</p></td>
<td>
<p>There are 3 types of references:
<table class="list">
<tr>
<th width="20%" align="center">Value</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>2</code></td>
<td>H5R_OBJECT2: Object Reference
</td>
</tr>
<tr>
<td align="center"><code>3</code></td>
<td>H5R_DATASET_REGION2: Dataset Region Reference
</td>
</tr>
<tr>
<td align="center"><code>4</code></td>
<td>H5R_ATTR: Attribute Reference
</td>
</tr>
</table>
</td>
</tr>
<tr>
<td><p>Flags</p></td>
<td><p>This field describes the reference:
<table class="list">
<tr>
<th width="20%" align="center">Bit</th>
<th width="80%" align="left">Description</th>
</tr>
<tr>
<td align="center"><code>0</code></td>
<td>If set, the reference is to an external file.
</td>
</tr>
<tr>
<td align="center"><code>1-7</code></td>
<td>Reserved</td>
</tr>
</table></p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Reference Block
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td>Token Size</td>
<td colspan="3" bgcolor="#DDDDDD"><em>This space inserted
only to align table nicely</em></td>
</tr>
<tr>
<td colspan=4><br /><br />Token
<em>(variable size)</em><br /> <br /><br /></td>
</tr>
<tr>
<td colspan=2>Length of External File Name</td>
<td colspan="2" bgcolor="#DDDDDD"><em>This space inserted
only to align table nicely</em></td>
</tr>
<tr>
<td colspan=4><br /><br />External File Name
<em>(variable size)</em><br /><br /><br /></td>
</tr>
<tr>
<td colspan=4>Size of Dataspace Selection</td>
</tr>
<tr>
<td colspan=4>Rank of Dataspace Selection</td>
</tr>
<tr>
<td colspan=4><br /><br />Dataspace Selection Information
<em>(variable size)</em><br /><br /> <br /></td>
</td>
</tr>
<tr>
<td colspan=2>Length of Attribute Name </td>
<td colspan="2" bgcolor="#DDDDDD"><em>This space inserted
only to align table nicely</em></td>
</tr>
<tr>
<td colspan=4><br /><br />Attribute Name
<em>(variable size)</em><br /><br /><br /></td>
</tr>
</table>
</div>
<br />
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Reference Block
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Token size</p></td>
<td><p>This is the size of the token for the object.
</td>
</tr>
<tr>
<td><p>Token</p></td>
<td>
<p>
This is the token for the object.
</p>
</td>
</tr>
<tr>
<td><p>Length of External File Name</p></td>
<td><p>This is the length for the external file name.
<p>This field exists if bit 0 of <em>flags</em> is set.</p>
</p>
</td>
</tr>
<tr>
<td><p>External File Name</p></td>
<td><p>This is the name of the external file being referenced.</p>
</p>
<p>This field exists if bit 0 of <em>flags</em> is set.</p>
</td>
</tr>
<tr>
<td><p>Dataspace Selection Information</p></td>
<td><p>See <a href="#DataspaceSEL">Dataspace Selection.</a></p>
</p>
<p>This field exists if the <em>Reference Type</em> is H5R_DATASET_REGION2.</p>
</td>
</tr>
<tr>
<td><p>Length of Attribute Name</p></td>
<td><p>This is the length of the attribute name.
<p>This field exists if the <em>Reference Type</em> is H5R_ATTRIBUTE.</p>
</td>
</tr>
<tr>
<td><p>Attribute Name</p></td>
<td><p>This is the name of the attribute being referenced.
<p>This field exists if the <em>Reference Type</em> is H5R_ATTRIBUTE.</p>
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<a name="ReferenceEncodeDP">
<h3>VIII.C. Reference Encoding (Backward Compatibility)</h3></a>
<p>
<br />
The two references described below are maintained to preserve compatibility with previous versions of the library.
<p>
For the following reference type,
the reference encoding is stored as the dataset's raw data in the file:
<ul>
<li>Object Reference (H5R_OBJECT1)</li>
</ul>
<p>
For the following reference type,
the <a href="#GlobalHeapID">Global Heap ID</a> is stored as the dataset's raw data in the file.
The global heap ID is used to locate the reference encoding
stored in the global heap:
<ul>
<li>Dataset Region Reference (H5R_DATASET_REGION1)</li>
</ul>
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Reference for H5R_OBJECT1
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td colspan="4"><br />Object Address<sup>O</sup><br /><br /></td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Reference for H5R_OBJECT1
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Object Address</p></td>
<td>
<p>Address of the object being referenced
</td>
</tr>
</table>
</div>
<br />
<br />
<br />
<div align="center">
<table class="format">
<caption>
Layout: Reference for H5R_DATASET_REGION1
</caption>
<tr>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
<th width="25%">byte</th>
</tr>
<tr>
<td colspan="4"><br />Object Address<sup>O</sup><br /><br /></td>
</tr>
<tr>
<td colspan=4><br /><br />Dataspace Selection Information
<em>(variable size)</em><br /><br /> <br /></td>
</td>
</tr>
</table>
<table class="note">
<tr>
<td width="60%"> </td>
<td width="40%">
(Items marked with an ‘O’ in the above table are
of the size specified in the <a href="#SizeOfOffsetsV0">Size
of Offsets</a> field in the superblock.)
</td></tr>
</table>
</div>
<br />
<br />
<div align="center">
<table class="desc">
<caption>
Fields: Reference for H5R_DATASET_REGION1
</caption>
<tr>
<th width="30%">Field Name</th>
<th>Description</th>
</tr>
<tr>
<td><p>Object Address</p></td>
<td><p>This is the address of the object being referenced.
</td>
</tr>
<tr>
<td><p>Dataspace Selection Information</p></td>
<td><p>This is the dataspace selection for the object being referenced.
See <a href="#DataspaceSEL">Dataspace Selection.</a></p>
</p>
</td>
</tr>
</table>
</div>
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