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<html>
  <head>
    <title>
      HDF5 Disk-Format Specification
    </title>
  </head>
  <body bgcolor="#FFFFFF">
    <center><h1>HDF5: Disk Format Implementation</h1></center>

    <ol type=I>
      <li><a href="#BootBlock">
	  Disk Format Level 0 - File Signature and Boot Block</a>
      <li><a href="#ObjectDir">
	  Disk Format Level 1 - File Infrastructure</a>
	<ol type=A>
	  <li><a href="#Btrees">
	      Disk Format Level 1A - B-link Trees</a>
	  <li><a href="#SymbolTable">
	      Disk Format Level 1B - Symbol Table</a>
	  <li><a href="#SymbolTableEntry">
	      Disk Format Level 1C - Symbol Table Entry</a>
	  <li><a href="#LocalHeap">
	      Disk Format Level 1D - Local Heaps</a>
	  <li><a href="#GlobalHeap">
	      Disk Format Level 1E - Global Heap</a>
	  <li><a href="#FreeSpaceIndex">
	      Disk Format Level 1F - Free-Space Index</a>
	</ol>
      <li><a href="#DataObject">
	  Disk Format Level 2 - Data Objects</a>
	<ol type=A>
	  <li><a href="#ObjectHeader">
	      Disk Format Level 2a - Data Object Headers</a>
	    <ol type=1>
	      <li><a href="#NILMessage">                        <!-- 0x0000 -->
		  Name: NIL</a>
	      <li><a href="#SimpleDataSpace">                   <!-- 0x0001 -->
		  Name: Simple Data Space</a>
	      <li><a href="#DataSpaceMessage">                  <!-- 0x0002 -->
		  Name: Data-Space</a>
	      <li><a href="#DataTypeMessage">                   <!-- 0x0003 -->
		  Name: Data-Type</a>
	      <li><a href="#FillValueMessage">                  <!-- 0x0004 -->
		  Name: Data Storage - Fill Value</a>
	      <li><a href="#ReservedMessage_0005">              <!-- 0x0005 -->
		  Name: Reserved - not assigned yet</a>
	      <li><a href="#CompactDataStorageMessage">         <!-- 0x0006 -->
		  Name: Data Storage - Compact</a>
	      <li><a href="#ExternalFileListMessage">           <!-- 0x0007 -->
		  Name: Data Storage - External Data Files</a>
	      <li><a href="#LayoutMessage">                     <!-- 0x0008 -->
		  Name: Data Storage - Layout</a>
	      <li><a href="#ReservedMessage_0009">              <!-- 0x0009 -->
		  Name: Reserved - not assigned yet</a>
	      <li><a href="#ReservedMessage_000A">              <!-- 0x000a -->
		  Name: Reserved - not assigned yet</a>
	      <li><a href="#FilterMessage">                	<!-- 0x000b -->
		  Name: Data Storage - Filter Pipeline</a>
	      <li><a href="#AttributeMessage">                  <!-- 0x000c -->
		  Name: Attribute</a>
	      <li><a href="#NameMessage">                       <!-- 0x000d -->
		  Name: Object Name</a>
	      <li><a href="#ModifiedMessage">                   <!-- 0x000e -->
		  Name: Object Modification Date & Time</a>
	      <li><a href="#SharedMessage">                     <!-- 0x000f -->
		  Name: Shared Object Message</a>
	      <li><a href="#ContinuationMessage">               <!-- 0x0010 -->
		  Name: Object Header Continuation</a>
	      <li><a href="#SymbolTableMessage">                <!-- 0x0011 -->
		  Name: Symbol Table Message</a>
	    </ol>
	  <li><a href="#SharedObjectHeader">
	      Disk Format: Level 2b - Shared Data Object Headers</a>
	  <li><a href="#DataStorage">
	      Disk Format: Level 2c - Data Object Data Storage</a>
	</ol>
    </ol>


    <h2>Disk Format Implementation</h2>

    <P>The format of a HDF5 file on disk encompasses several
      key ideas of the current HDF4 & AIO file formats as well as
      addressing some short-comings therein.  The new format is
      more self-describing than the HDF4 format and is more
      uniformly applied to data objects in the file.
      
    <P>An HDF5 file can be thought of as a directed graph.  
      The nodes of this graph are the higher-level HDF5 objects, 
      including groups, datasets, datatypes, and dataspaces.  
      This document describes the lower-level data objects used by 
      the HDF5 library to represent those higher-level objects and 
      their properties.

    <P>At the lowest level, an HDF5 file is made up of the following 
      objects:
      <ul>
         <li>A bool block
         <li>B-tree nodes (containing either symbol nodes or raw data chunks)
         <li>Object headers
         <li>Collections
         <li>Local heaps
         <li>Free space
      </ul>
      As indicated above, the HDF5 library uses and interprets these 
      low-level objects to describe the high-level HDF5 objects that 
      are revealed to the user, and to higher-level applications, 
      through the HDF5 APIs.

<!--
<blockquote>
<pre>

---------- Edit from here... -------------


Once you know about all these low-level objects you can build bigger
and better things, which is what most people are interested in and
which are the objects that the hdf5 library exposes in the API.  They
are:

    * Groups
    * Datasets
    * Datatypes
    * Data spaces

For instance, a group is an object header that contains a message that
points to a local heap and to a B-tree which points to symbol nodes.
A dataset is an object header that contains messages that describe
data type, space, layout, filters, external files, fill value, etc
with the layout message pointing to either a raw data chunk or to a
B-tree that points to raw data chunks.

Elena>    Would it be more logical to discuss things in this order?

Elena>          Intro ( What is HDF5 file, etc.)
Elena>          File Header  
Elena>          File Body
Elena>               Objects
Elena>                   Object Header
Elena>                   Object Header Message Data 
Elena> NOTE: give reference to the detailed discussion of the B-trees
Elena>       when needed. Right now we do not have specification (only general one)
Elena>       for the Symbol Table B-trees and B-trees used to manage chunked datasets.
Elena>               B-trees
Elena>                   General Discussion
Elena>                   Object related discussions
Elena>               Symbol Tables
Elena>               Global heap
Elena>               "Free-space object"

That might be a good order for someone that's familiar with the API
but if you're trying to get all the way down to the file format level
it results in a lot of forward references in the documentation. It
might be better to do a bottom-up documentation similar to the order I 
used above:

    * General file layout
    * Boot block
    * Format-level objects (B-trees, symbol nodes, object headers, etc).
    * Object header messages
    * High-level objects (datasets, groups, named types and spaces, etc)

where "high-level objects" description mostly describes which object
header messages are required, optional, mutually exclusive, etc. for
each high-level object.

---------- ...to here. -------------

</pre>
</blockquote>
-->



    <P>Three levels of information compose the file format.  The level
      0 contains basic information for identifying and
      "boot-strapping" the file.  Level 1 information is composed of
      the object directory (stored as a B-tree) and is used as the
      index for all the objects in the file.  The rest of the file is
      composed of data-objects at level 2, with each object
      partitioned into header (or "meta") information and data
      information.

    <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 three exceptions: (1) The size may be
      overridden by specifying a size in parentheses, (2) the size of
      addresses is determined by the <em>Size of Addresses</em> field
      in the boot block, and (3) the size of size fields is determined
      by the <em>Size of Sizes</em> field in the boot block.

    <h3><a name="BootBlock">
	Disk Format: Level 0 - File Signature and Boot Block</a></h3>

    <P>The boot block 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 the
      file without requiring the modification of the actual file's
      information.  The boot-block is located by searching for the
      HDF5 file signature at byte offset 0, byte offset 512 and at
      successive locations in the file, each a multiple of two of
      the previous location, i.e.  0, 512, 1024, 2048, etc.

    <P>The boot-block is composed of a file signature, followed by
      boot block and object directory version numbers, information
      about the sizes of offset and length values used to describe
      items within the file, the size of each object directory page,
      and a symbol table entry for the root object in the file.

    <p>
    <center>
      <table border align=center cellpadding=4 width="80%">
	<caption align=top>
	  <B>HDF5 Boot Block 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><br>HDF5 File Signature (8 bytes)<br><br></td>
	</tr>

	<tr align=center>
	  <td>Version # of Boot Block</td>
	  <td>Version # of Global Free-Space Storage</td>
	  <td>Version # of Object Directory</td>
	  <td>Reserved</td>
	</tr>

	<tr align=center>
	  <td>Version # of Shared Header Message Format</td>
	  <td>Size of Addresses</td>
	  <td>Size of Sizes</td>
	  <td>Reserved (zero)</td>
	</tr>

	<tr align=center>
	  <td colspan=2>Symbol Table Leaf Node K</td>
	  <td colspan=2>Symbol Table Internal Node K</td>
	</tr>

	<tr align=center>
	  <td colspan=4>File Consistency Flags</td>
	</tr>

	<tr align=center>
	  <td colspan=4>Base Address</td>
	</tr>

	<tr align=center>
	  <td colspan=4>Address of Global Free-Space Heap</td>
	</tr>

	<tr align=center>
	  <td colspan=4>End of File Address</td>
	</tr>

	<tr align=center>
	  <td colspan=4>Driver Information Block Address</td>
	</tr>

	<tr align=center>
	  <td colspan=4><br>Root Group Symbol Table Entry<br><br></td>
	</tr>
      </table>
    </center>

    <p>
    <center>
      <table align=center width="80%">
	<tr>
	  <th width="30%">Field Name</th>
	  <th width="70%">Description</th>
	</tr>

	<tr valign=top>
	  <td>File Signature</td>
	  <td>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 a HDF5 file always
	    contain the following values:

	    <br><br><center>
	      <table border align=center cellpadding=4 width="80%">
		<tr align=center>
		  <td>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>hexadecimal</td>
		  <td width="8%">89</td>
		  <td width="8%">48</td>
		  <td width="8%">44</td>
		  <td width="8%">46</td>
		  <td width="8%">0d</td>
		  <td width="8%">0a</td>
		  <td width="8%">1a</td>
		  <td width="8%">0a</td>
		</tr>

		<tr align=center>
		  <td>ASCII C Notation</td>
		  <td width="8%">\211</td>
		  <td width="8%">H</td>
		  <td width="8%">D</td>
		  <td width="8%">F</td>
		  <td width="8%">\r</td>
		  <td width="8%">\n</td>
		  <td width="8%">\032</td>
		  <td width="8%">\n</td>
		</tr>
	      </table>
	    </center>
	    <br>

	    This signature both identifies the file as a 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 a 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 PNG file signature.)</td>
	</tr>

	<tr valign=top>
	  <td>Version # of the Boot Block</td>
	  <td>This value is used to determine the format of the
	    information in the boot block.  When the format of the
	    information in the boot block is changed, the version #
	    is incremented to the next integer and can be used to
	    determine how the information in the boot block is
	    formatted.</td>
	</tr>

	<tr valign=top>
	  <td>Version # of the Global Free-Space Storage</td>
	  <td>This value is used to determine the format of the
	    information in the Global Free-Space Heap.  Currently,
	    this is implemented as a B-tree of length/offset pairs
	    to locate free space in the file, but future advances in
	    the file-format could change the method of finding
	    global free-space.  When the format of the information
	    is changed, the version # is incremented to the next
	    integer and can be used to determine how the information
	    is formatted.</td>
	</tr>

	<tr valign=top>
	  <td>Version # of the Object Directory</td>
	  <td>This value is used to determine the format of the
	    information in the Object Directory.  When the format of
	    the information in the Object Directory is changed, the
	    version # is incremented to the next integer and can be
	    used to determine how the information in the Object
	    Directory is formatted.</td>
	</tr>

	<tr valign=top>
	  <td>Version # of the Shared Header Message Format</td>
	  <td>This value is used to determine the format of the
	    information in a shared object header message, which is
	    stored in the global small-data heap.  Since the format
	    of the shared header messages differ from the private
	    header messages, a version # is used to identify changes
	    in the format.</td>
	</tr>

	<tr valign=top>
	  <td>Size of Addresses</td>
	  <td>This value contains the number of bytes used for
	    addresses in the file.  The values for the addresses of
	    objects in the file are relative to a base address,
	    usually the address of the boot block signature.  This
	    allows a wrapper to be added after the file is created
	    without invalidating the internal offset locations.</td>
	</tr>

	<tr valign=top>
	  <td>Size of Sizes</td>
	  <td>This value contains the number of bytes used to store
	    the size of an object.</td>
	</tr>

	<tr valign=top>
	  <td>Symbol Table Leaf Node K</td>
	  <td>Each leaf node of a symbol table B-tree will have at
	    least this many entries but not more than twice this
	    many.  If a symbol table has a single leaf node then it
	    may have fewer entries.</td>
	</tr>

	<tr valign=top>
	  <td>Symbol Table Internal Node K</td>
	  <td>Each internal node of a symbol table B-tree will have
	    at least K pointers to other nodes but not more than 2K
	    pointers.  If the symbol table has only one internal
	    node then it might have fewer than K pointers.</td>
	</tr>

	<tr valign=top>
	  <td>Bytes per B-Tree Page</td>
	  <td>This value contains the # of bytes used for symbol
	    pairs per page of the B-Trees used in the file.  All
	    B-Tree pages will have the same size per page.  <br>(For
	    32-bit file offsets, 340 objects is the maximum per 4KB
	    page, and for 64-bit file offset, 254 objects will fit
	    per 4KB page.  In general, the equation is: <br> &lt;#
	    of objects&gt; = FLOOR((&lt;page size&gt;-&lt;offset
	    size&gt;)/(&lt;Symbol size&gt;+&lt;offset size&gt;))-1 )</td>
	</tr>

	<tr valign=top>
	  <td>File Consistency Flags</td>
	  <td>This value contains flags to indicate information
	    about the consistency of the information contained
	    within the file.  Currently, the following bit flags are
	    defined: bit 0 set indicates that the file is opened for
	    write-access and bit 1 set indicates that the file has
	    been verified for consistency and is guaranteed to be
	    consistent with the format defined in this document.
	    Bits 2-31 are reserved for future use.  Bit 0 should be
	    set as the first action when a file is opened for write
	    access and should be cleared only as the final action
	    when closing a file.  Bit 1 should be cleared during
	    normal access to a file and only set after the file's
	    consistency is guaranteed by the library or a
	    consistency utility.</td>
	</tr>

	<tr valign=top>
	  <td>Base Address</td>
	  <td>This is the absolute file address of the first byte of
	    the hdf5 data within the file.  Unless otherwise noted,
	    all other file addresses are relative to this base
	    address.</td>
	</tr>

	<tr valign=top>
	  <td>Address of Global Free-Space Heap</td>
	  <td>This value contains the relative address of the B-Tree
	    used to manage the blocks of data which are unused in the
	    file currently.  The free-space heap is used to manage the
	    blocks of bytes at the file-level which become unused with
	    objects are moved within the file.</td>
	</tr>

	<tr valign=top>
	  <td>End of File Address</td>
	  <td>This is the relative file address of the first byte past
	    the end of all HDF5 data.  It is used to determine if a
	    file has been accidently truncated and as an address where
	    file memory allocation can occur if the free list is not
	    used.</td>
	</tr>

        <tr valign=top>
          <td>Driver Information Block Address</td>
          <td>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
            undefined address (all bits set).</td>
        </tr>

	<tr valign=top>
	  <td>Root Group Symbol Table Entry</td>
	  <td>This symbol-table entry (described later in this
	    document) refers to the entry point into the group
	    graph.  If the file contains a single object, then that
	    object can be the root object and no groups are used.</td>
	</tr>
      </table>
    </center>

    <p>The file driver information block is an optional region of the
      file which contains information needed by the file driver in
      order to reopen a file.  The format of the driver information
      block is:

    <p>
    <center>
      <table border align=center cellpadding=4 width="80%">
	<caption align=top>
	  <B>Driver Information Block</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>Version</td>
          <td colspan=3>Reserved (zero)</td>
        </tr>

        <tr align=center>
          <td colspan=4>Driver Information Size (4 bytes)</td>
        </tr>

        <tr align=center>
          <td colspan=4><br>Driver Identification (8 bytes)<br><br></td>
        </tr>

        <tr align=center>
          <td colspan=4><br><br>Driver Information<br><br><br></td>
        </tr>
      </table>
    </center>

    <p>
    <center>
      <table align=center width="80%">
	<tr>
	  <th width="30%">Field Name</th>
	  <th width="70%">Description</th>
	</tr>

	<tr valign=top>
          <td>Version</td>
          <td>The version number of the driver information block. The
            file format documented here is version zero.</td>
        </tr>

        <tr valign=top>
          <td>Driver Information Size</td>
          <td>The size in bytes of the Driver Information part of this 
            structure.</td>
        </tr>

        <tr valign=top>
          <td>Driver Identification</td>
          <td>This is an eight-byte ASCII string without null
            termination which identifies the driver and version number 
            of the Driver Information block. The predefined drivers
            supplied with the HDF5 library are identified by the
            letters "NCSA" 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.  For example, the various versions of the "family"
            driver will be identified by "NCSAfami", "NCSAfam0",
            NCSAfam1", etc. Identification for user-defined drivers is 
            arbitrary but should be unique.</td>
        </tr>

        <tr valign=top>
          <td>Driver Information</td>
          <td>Driver information is stored in a format defined by the
            file driver and encoded/decoded by the driver callbacks
            invoked from the <code>H5FD_sb_encode</code> and
            <code>H5FD_sb_decode</code> functions.</td>
        </tr>
      </table>
    </center>


    <h3><a name="Btrees">Disk Format: Level 1A - B-link Trees</a></h3>

    <p>B-link 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.  The B-link tree, in which the sibling nodes at a
      particular level in the tree are stored in a doubly-linked list,
      is described in the "Efficient Locking for Concurrent Operations
      on B-trees" paper by Phillip Lehman and S. Bing Yao as published
      in the <em>ACM Transactions on Database Systems</em>, Vol. 6,
      No. 4, December 1981.

    <p>The B-link 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 other file data types.
      Notwithstanding that difference, internal nodes and leaf nodes
      are identical.

    <p>
    <center>
      <table border cellpadding=4 width="80%">
	<caption align=top>
	  <B>B-tree Nodes</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 align=center>
	  <td colspan=4>Node Signature</td>

	<tr align=center>
	  <td>Node Type</td>
	  <td>Node Level</td>
	  <td colspan=2>Entries Used</td>

	<tr align=center>
	  <td colspan=4>Address of Left Sibling</td>

	<tr align=center>
	  <td colspan=4>Address of Right Sibling</td>

	<tr align=center>
	  <td colspan=4>Key 0 (variable size)</td>

	<tr align=center>
	  <td colspan=4>Address of Child 0</td>

	<tr align=center>
	  <td colspan=4>Key 1 (variable size)</td>

	<tr align=center>
	  <td colspan=4>Address of Child 1</td>

	<tr align=center>
	  <td colspan=4>...</td>

	<tr align=center>
	  <td colspan=4>Key 2<em>K</em> (variable size)</td>

	<tr align=center>
	  <td colspan=4>Address of Child 2<em>K</em></td>

	<tr align=center>
	  <td colspan=4>Key 2<em>K</em>+1 (variable size)</td>
      </table>
    </center>

    <p>
    <center>
      <table align=center width="80%">
	<tr>
	  <th width="30%">Field Name</th>
	  <th width="70%">Description</th>
	</tr>

	<tr valign=top>
	  <td>Node Signature</td>
	  <td>The value ASCII 'TREE' is used to indicate the
	    beginning of a B-link tree node.  This gives file
	    consistency checking utilities a better chance of
	    reconstructing a damaged file.</td>
	</tr>

	<tr valign=top>
	  <td>Node Type</td>
	  <td>Each B-link 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.
	    <br>
	    <dl compact>
	      <dt>0
	      <dd>This tree points to symbol table nodes.
	      <dt>1
	      <dd>This tree points to a (partial) linear address space.
	    </dl>
	  </td>
	</tr>

	<tr valign=top>
	  <td>Node Level</td>
	  <td>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
	    damanged trees.</td>
	</tr>

	<tr valign=top>
	  <td>Entries Used</td>
	  <td>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.</td>
	</tr>

	<tr valign=top>
	  <td>Address of Left Sibling</td>
	  <td>This is the file address of the left sibling of the
	    current node relative to the boot block.  If the current
	    node is the left-most node at this level then this field
	    is the undefined address (all bits set).</td>
	</tr>

	<tr valign=top>
	  <td>Address of Right Sibling</td>
	  <td>This is the file address of the right sibling of the
	    current node relative to the boot block.  If the current
	    node is the right-most node at this level then this
	    field is the undefined address (all bits set).</td>
	</tr>

	<tr valign=top>
	  <td>Keys and Child Pointers</td>
	  <td>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 `Entries Used' field.  If
	    that field is <em>N</em> then the B-link tree contains
	    <em>N</em> child pointers and <em>N</em>+1 keys.</td>
	</tr>

	<tr valign=top>
	  <td>Key</td>
	  <td>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.</td>
	</tr>

	<tr valign=top>
	  <td>Address of Children</td>
	  <td>The tree node contains file addresses of subtrees or
	    data depending on the node level (0 implies data
	    addresses).</td>
	</tr>
      </table>
    </center>

    <h3><a name="SymbolTable">Disk Format: Level 1B - Symbol Table</a></h3>

    <p>A symbol table is a group internal to the file that allows
      arbitrary nesting of objects (including other symbol
      tables). A symbol table maps a set of names to a set of file
      address relative to the file boot block.  Certain meta data
      for an object to which the symbol table points can be cached
      in the symbol table in addition to (or in place of?) the
      object header.

    <p>An HDF5 object name space can be stored hierarchically by
      partitioning the name into components and storing each
      component in a symbol table.  The symbol table entry for a
      non-ultimate component points to the symbol table containing
      the next component.  The symbol table entry for the last
      component points to the object being named.

    <p>A symbol table is a collection of symbol table nodes pointed
      to by a B-link tree.  Each symbol table node contains entries
      for one or more symbols.  If an attempt is made to add a
      symbol 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>
    <center>
      <table border cellpadding=4 width="80%">
	<caption align=top>
	  <B>Symbol Table Node</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 align=center>
	  <td colspan=4>Node Signature</td>

	<tr align=center>
	  <td>Version Number</td>
	  <td>Reserved for Future Use</td>
	  <td colspan=2>Number of Symbols</td>

	<tr align=center>
	  <td colspan=4><br><br>Symbol Table Entries<br><br><br></td>
      </table>
    </center>

    <p>
    <center>
      <table align=center width="80%">
	<tr>
	  <th width="30%">Field Name</th>
	  <th width="70%">Description</th>
	</tr>

	<tr valign=top>
	  <td>Node Signature</td>
	  <td>The value ASCII 'SNOD' 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.</td>
	</tr>

	<tr valign=top>
	  <td>Version Number</td>
	  <td>The version number for the symbol table node.  This
	    document describes version 1.</td>
	</tr>

	<tr valign=top>
	  <td>Number of Symbols</td>
	  <td>Although all symbol table nodes have the same length,
	    most contain fewer than the maximum possible number of
	    symbol 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.</td>
	</tr>

	<tr valign=top>
	  <td>Symbol Table Entries</td>
	  <td>Each symbol has an entry in the symbol table node.
	    The format of the entry is described below.</td>
	</tr>
      </table>
    </center>

    <h3><a name="SymbolTableEntry">
	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 commonly stored scientific objects.
      Toward that design goal, the format of the symbol-table entries
      includes space for caching certain constant meta data from the
      object header.

    <p>
    <center>
      <table border cellpadding=4 width="80%">
	<caption align=top>
	  <B>Symbol Table Entry</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>Name Offset (&lt;size&gt; bytes)</td>
	</tr>

	<tr align=center>
	  <td colspan=4>Object Header Address</td>
	</tr>

	<tr align=center>
	  <td colspan=4>Symbol-Type</td>
	</tr>

	<tr align=center>
	  <td colspan=4>Reserved</td>
	</tr>

	<tr align=center>
	  <td colspan=4><br><br>Scratch-pad Space (16 bytes)<br><br><br></td>
	</tr>
      </table>
    </center>

    <p>
    <center>
      <table align=center width="80%">
	<tr>
	  <th width="30%">Field Name</th>
	  <th width="70%">Description</th>
	</tr>

	<tr valign=top>
	  <td>Name Offset</td>
	  <td>This is the byte offset into the symbol table local
	    heap for the name of the symbol. The name is null
	    terminated.</td>
	</tr>

	<tr valign=top>
	  <td>Object Header Address</td>
	  <td>Every object has an object header which serves as a
	    permanent home for the object's meta data.  In addition
	    to appearing in the object header, the meta data can be
	    cached in the scratch-pad space.</td>
	</tr>

	<tr valign=top>
	  <td>Symbol-Type</td>
	  <td>The symbol type is determined from the object header.
	    It also determines the format for the scratch-pad space.
	    The value zero indicates that no object header meta data
	    is cached in the symbol table entry.
	    <br>
	    <dl compact>
	      <dt>0
	      <dd>No data is cached by the symbol table entry.  This
		is guaranteed to be the case when an object header
		has a link count greater than one.

	      <dt>1
	      <dd>Symbol table meta data is cached in the symbol
		table entry.  This implies that the symbol table
		entry refers to another symbol table.

	      <dt>2
	      <dd>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.

	      <dt><em>N</em>
	      <dd>Other cache values can be defined later and
	      libraries that don't understand the new values will
	      still work properly.
	    </dl>
	  </td>
	</tr>

	<tr valign=top>
	  <td>Reserved</td>
	  <td>These for bytes are present so that the scratch pad
	    space is aligned on an eight-byte boundary.  They are
	    always set to zero.</td>
	</tr>

	<tr valign=top>
	  <td>Scratch-Pad Space</td>
	  <td>This space is used for different purposes, depending
	    on the value of the Symbol Type field. Any meta-data
	    about a dataset object represented in the scratch-pad
	    space is duplicated in the object header for that
	    dataset.  Furthermore, no data is cached in the symbol
	    table entry scratch-pad space if the object header for
	    the symbol table entry has a link count greater than
	    one.</td>
	</tr>
      </table>
    </center>

    <p>The symbol table entry scratch-pad space is formatted
      according to the value of the Symbol Type field.  If the
      Symbol Type field has the value zero then no information is
      stored in the scratch pad space.

    <p>If the Symbol Type field is one, then the scratch pad space
      contains cached meta data for another symbol table with the format:

    <p>
    <center>
      <table border cellpadding=4 width="80%">
	<caption align=top>
	  <B>Symbol Table Scratch-Pad Format</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 align=center>
	  <td colspan=4>Address of B-tree</td>

	<tr align=center>
	  <td colspan=4>Address of Name Heap</td>
      </table>
    </center>

    <p>
    <center>
      <table align=center width="80%">
	<tr>
	  <th width="30%">Field Name</th>
	  <th width="70%">Description</th>
	</tr>

	<tr valign=top>
	  <td>Address of B-tree</td>
	  <td>This is the file address for the symbol table's
	    B-tree.</td>
	</tr>

	<tr valign=top>
	  <td>Address of Name Heap</td>
	  <td>This is the file address for the symbol table's local
	    heap that stores the symbol names.</td>
	</tr>
      </table>
    </center>

    <p>
    <center>
      <table border cellpadding=4 width="80%">
	<caption align=top>
	  <B>Symbolic Link Scratch-Pad Format</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>Offset to Link Value</td>
	</tr>
      </table>
    </center>

    <p>
    <center>
      <table align=center width="80%">
	<tr>
	  <th width="30%">Field Name</th>
	  <th width="70%">Description</th>
	</tr>

	<tr valign=top>
	  <td>Offset to Link Value</td>
	  <td>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.</td>
	</tr>
      </table>
    </center>

    <h3><a name="LocalHeap">Disk Format: Level 1D - Local Heaps</a></h3>

    <p>A heap is a collection of small heap objects.  Objects can be
      inserted and removed from the heap at any time and the address
      of a heap doesn't change once the heap is created. Note: this
      is the "local" version of the heap mostly intended for the
      storage of names in a symbol table. The storage of small
      objects in a global heap is described below.

    <p>
    <center>
      <table border cellpadding=4 width="80%">
	<caption align=top>
	  <b>Local Heaps</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>Heap Signature</td>
	</tr>

	<tr align=center>
	  <td colspan=4>Reserved (zero)</td>
	</tr>

	<tr align=center>
	  <td colspan=4>Data Segment Size</td>
	</tr>

	<tr align=center>
	  <td colspan=4>Offset to Head of Free-list (&lt;size&gt; bytes)</td>
	</tr>

	<tr align=center>
	  <td colspan=4>Address of Data Segment</td>
	</tr>
      </table>
    </center>

    <p>
    <center>
      <table align=center width="80%">
	<tr>
	  <th width="30%">Field Name</th>
	  <th width="70%">Description</th>
	</tr>

	<tr valign=top>
	  <td>Heap Signature</td>
	  <td>The valid ASCII 'HEAP' is used to indicate the
	    beginning of a heap.  This gives file consistency
	    checking utilities a better chance of reconstructing a
	    damaged file.</td>
	</tr>

	<tr valign=top>
	  <td>Data Segment Size</td>
	  <td>The total amount of disk memory allocated for the heap
	    data.  This may be larger than the amount of space
	    required by the object stored in the heap.  The extra
	    unused space holds a linked list of free blocks.</td>
	</tr>

	<tr valign=top>
	  <td>Offset to Head of Free-list</td>
	  <td>This is the offset within the heap data segment of the
	    first free block (or all 0xff bytes if there is no free
	    block).  The free block contains &lt;size&gt; bytes that
	    are the offset of the next free chunk (or all 0xff bytes
	    if this is the last free chunk) followed by &lt;size&gt;
	    bytes that store the size of this free chunk.</td>
	</tr>

	<tr valign=top>
	  <td>Address of Data Segment</td>
	  <td>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 to another part of the file.</td>
	</tr>
      </table>
    </center>

    <p>Objects within the heap should be aligned on an 8-byte boundary.

    <h3><a name="GlobalHeap">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:

    <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's
	probable that the object will be accessed repeatedly.

	<br><br>
      <li>Collections of related global heap objects should result in
	fewer and larger I/O requests.  For instance, a dataset of
	void pointers will have a global heap object for each
	pointer.  Reading the entire set of void pointer objects
	should result in a few large I/O requests instead of one small
	I/O request for each object.

	<br><br>
      <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.
	<br><br>
    </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 top-level 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.

    <p>
    <center>
      <table border cellpadding=4 width="80%">
	<caption align=top>
	  <B>Global Heap Collection</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>Magic Number</td>
	</tr>
	  
	<tr align=center>
	  <td>Version</td>
	  <td colspan=3>Reserved</td>
	</td>
	  
	<tr align=center>
	  <td colspan=4>Collection Size</td>
	</tr>
	  
	<tr align=center>
	  <td colspan=4><br>Object 1<br><br></td>
	</tr>

	<tr align=center>
	  <td colspan=4><br>Object 2<br><br></td>
	</tr>

	<tr align=center>
	  <td colspan=4><br>...<br><br></td>
	</tr>

	<tr align=center>
	  <td colspan=4><br>Object <em>N</em><br><br></td>
	</tr>

	<tr align=center>
	  <td colspan=4><br>Object 0 (free space)<br><br></td>
	</tr>
      </table>
    </center>

    <p>
    <center>
      <table align=center width="80%">
	<tr>
	  <th width="30%">Field Name</th>
	  <th width="70%">Description</th>
	</tr>

	<tr valign=top>
	  <td>Magic Number</td>
	  <td>The magic number for global heap collections are the
	    four bytes `G', `C', `O', `L'.</td>
	</tr>
	  
	<tr valign=top>
	  <td>Version</td>
	  <td>Each collection has its own version number so that new
	    collections can be added to old files.  This document
	    describes version zero of the collections.
	</tr>

	<tr valign=top>
	  <td>Collection Data Size</td>
	  <td>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 and which allows for 170 16-byte heap
	    objects plus their overhead.</td>
	</tr>

	<tr valign=top>
	  <td>Object <em>i</em> for positive <em>i</em></td> <td>The
	    objects are stored in any order with no intervening unused
	    space.</td>
	</tr>

	<tr valign=top>
	  <td>Object 0</td>
	  <td>Object 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 zero (described below) then the
	    header is implied.
      </table>
    </center>
    
    <p>
    <center>
      <table border cellpadding=4 width="80%">
	<caption align=top>
	  <B>Global Heap Object</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=2>Object ID</td>
	  <td colspan=2>Reference Count</td>
	</tr>

	<tr align=center>
	  <td colspan=4>Reserved</td>
	</tr>

	<tr align=center>
	  <td colspan=4>Object Data Size</td>
	</tr>

	<tr align=center>
	  <td colspan=4><br>Object Data<br><br></td>
	</tr>
      </table>
    </center>

    <p>
    <center>
      <table align=center width="80%">
	<tr>
	  <th width="30%">Field Name</th>
	  <th width="70%">Description</th>
	</tr>

	<tr valign=top>
	  <td>Object ID</td>
	  <td>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 `0' always refers to the
	    object which represents all free space within the
	    collection.</td>
	</tr>

	<tr valign=top>
	  <td>Reference Count</td>
	  <td>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 zero is always zero.</td>
	</tr>

	<tr valign=top>
	  <td>Reserved</td>
	  <td>Zero padding to align next field on an 8-byte
	    boundary.</td>
	</tr>

	<tr valign=top>
	  <td>Object Size</td> <td>This is the size of the the fields
	    above plus the object data stored for the object.  The
	    actual storage size is rounded up to a multiple of
	    eight.</td>
	</tr>

	<tr valign=top>
	  <td>Object Data</td>
	  <td>The object data is treated as a one-dimensional array
	    of bytes to be interpreted by the caller.</td>
	</tr>
      </table>
    </center>

    <h3><a name="FreeSpaceIndex">Disk Format: Level 1F - Free-Space
	Index (NOT FULLY DEFINED)</a></h3>

    <p>The Free-Space Index is a collection of blocks of data,
      dispersed throughout the file, which are currently not used by
      any file objects.  The blocks of data are indexed by a B-tree of
      their length within the file.

    <p>Each B-Tree page is composed of the following entries and
      B-tree management information, organized as follows:

    <p>
    <center>
      <table border cellpadding=4 width="80%">
	<caption align=bottom>
	  <B>HDF5 Free-Space Heap Page</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 align=center>
	  <td colspan=4>Free-Space Heap Signature</td>
	<tr align=center>
	  <td colspan=4>B-Tree Left-Link Offset</td>
	<tr align=center>
	  <td colspan=4><br>Length of Free-Block #1<br> <br></td>
	<tr align=center>
	  <td colspan=4><br>Offset of Free-Block #1<br> <br></td>
	<tr align=center>
	  <td colspan=4>.<br>.<br>.<br></td>
	<tr align=center>
	  <td colspan=4><br>Length of Free-Block #n<br> <br></td>
	<tr align=center>
	  <td colspan=4><br>Offset of Free-Block #n<br> <br></td>
	<tr align=center>
	  <td colspan=4>"High" Offset</td>
	<tr align=center>
	  <td colspan=4>Right-Link Offset</td>
      </table>
    </center>

    <p>
    <dl>
      <dt> The elements of the free-space heap page are described below:
      <dd>
	<dl>
	  <dt>Free-Space Heap Signature: (4 bytes)
	  <dd>The value ASCII: 'FREE' is used to indicate the
	    beginning of a free-space heap B-Tree page.  This gives
	    file consistency checking utilities a better chance of
	    reconstructing a damaged file.

	  <dt>B-Tree Left-Link Offset: (&lt;offset&gt; bytes)
	  <dd>This value is used to indicate the offset of all offsets
	    in the B-link-tree which are smaller than the value of the
	    offset in entry #1.  This value is also used to indicate a
	    leaf node in the B-link-tree by being set to all ones.

	  <dt>Length of Free-Block #n: (&lt;length&gt; bytes)
	  <dd>This value indicates the length of an un-used block in
	    the file.

	  <dt>Offset of Free-Block #n: (&lt;offset&gt; bytes)
	  <dd>This value indicates the offset in the file of an
	    un-used block in the file.

	  <dt>"High" Offset: (4-bytes)
	  <dd>This offset is used as the upper bound on offsets
	    contained within a page when the page has been split.

	  <dt>Right-link Offset: (&lt;offset&gt; bytes)
	  <dd>This value is used to indicate the offset of the next
	    child to the right of the parent of this object directory
	    page.  When there is no node to the right, this value is
	    all zeros.
	</dl>
    </dl>

    <p>The algorithms for searching and inserting objects in the
      B-tree pages are described fully in the Lehman & Yao paper,
      which should be read to provide a full description of the
      B-Tree's usage.

    <h3><a name="DataObject">Disk Format: Level 2 - Data Objects </a></h3>

    <p>Data objects contain the real 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
      these data objects.

    <p>A data object is composed of header information and data
      information.  The header information contains the information
      needed to interpret the data information for the data object as
      well as additional "meta-data" or pointers to additional
      "meta-data" used to describe or annotate each data object.

    <h3><a name="ObjectHeader">
	Disk Format: Level 2a - Data Object Headers</a></h3>

    <p>The header information of an object is designed to encompass
      all the information about an object which would be desired to be
      known, except for the data itself.  This information includes
      the dimensionality, number-type, information about how the data
      is stored on disk (in external files, compressed, broken up in
      blocks, etc.), as well as other information used by the library
      to speed up access to the data objects or maintain a file's
      integrity.  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.

    <p>
    <center>
      <table border cellpadding=4 width="80%">
	<caption align=top>
	  <B>Object Headers</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 width="25%">Version # of Object Header</td>
	  <td colspan=1 width="25%">Reserved</td>
	  <td colspan=2 width="50%">Number of Header Messages</td>
	</tr>
	<tr align=center>
	  <td colspan=4>Object Reference Count</td>
	</tr>
	<tr align=center>
	  <td colspan=4><br>Total Object-Header Size<br><br></td>
	</tr>
	<tr align=center>
	  <td colspan=2>Header Message Type #1</td>
	  <td colspan=2>Size of Header Message Data #1</td>
	</tr>
	<tr align=center>
	  <td>Flags</td>
	  <td colspan=3>Reserved</td>
	</tr>
	<tr align=center>
	  <td colspan=4><br>Header Message Data #1<br><br></td>
	</tr>
	<tr align=center>
	  <td colspan=4>.<br>.<br>.<br></td>
	</tr>
	<tr align=center>
	  <td colspan=2>Header Message Type #n</td>
	  <td colspan=2>Size of Header Message Data #n</td>
	</tr>
	<tr align=center>
	  <td>Flags</td>
	  <td colspan=3>Reserved</td>
	</tr>
	<tr align=center>
	  <td colspan=4><br>Header Message Data #n<br><br></td>
	</tr>
      </table>
    </center>

    <p>
    <center>
      <table align=center width="80%">
	<tr>
	  <th width="30%">Field Name</th>
	  <th width="70%">Description</th>
	</tr>

	<tr valign=top>
	  <td>Version # of the object header</td>
	  <td>This value is used to determine the format of the
	    information in the object header.  When the format of the
	    information in the object header is changed, the version #
	    is incremented and can be used to determine how the
	    information in the object header is formatted.</td>
	</tr>

	<tr valign=top>
	  <td>Reserved</td>
	  <td>Always set to zero.</td>
	</tr>

	<tr valign=top>
	  <td>Number of header messages</td>
	  <td>This value determines the number of messages listed in
	    this object header.  This provides a fast way for software
	    to prepare storage for the messages in the header.</td>
	</tr>

	<tr valign=top>
	  <td>Object Reference Count</td>
	  <td>This value specifies the number of references to this
	    object within the current file.  References to the
	    data-object from external files are not tracked.</td>
	</tr>

	<tr valign=top>
	  <td>Total Object-Header Size</td>
	  <td>This value specifies the total number of bytes of header
	    message data following this length field for the current
	    message as well as any continuation data located elsewhere
	    in the file.</td>
	</tr>

	<tr valign=top>
	  <td>Header Message Type</td>
	  <td>The header message type specifies the type of
	    information included in the header message data following
	    the type along with a small amount of other information.
	    Bit 15 of the message type is set if the message is
	    constant (constant messages cannot be changed since they
	    may be cached in symbol table entries throughout the
	    file).  The header message types for the pre-defined
	    header messages will be included in further discussion
	    below.</td>
	</tr>

	<tr valign=top>
	  <td>Size of Header Message Data</td>
	  <td>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.</td>
	</tr>

	<tr valign=top>
	  <td>Flags</td>
	  <td>This is a bit field with the following definition:
	    <dl>
	      <dt><code>0</code>
	      <dd>If set, the message data is constant.  This is used
		for messages like the data type message of a dataset.
	      <dt><code>1</code>
	      <dd>If set, the message is stored in the global heap and
		the Header Message Data field contains a Shared Object
		message.  and the Size of Header Message Data field
		contains the size of that Shared Object message.
	      <dt><code>2-7</code>
	      <dd>Reserved
	    </dl>
	  </td>

	<tr valign=top>
	  <td>Header Message Data</td>
	  <td>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 zeros to make the
	    size a multiple of eight.</td>
	</tr>
      </table>
    </center>

    <p>The header message types and the message data associated with
      them compose the critical "meta-data" 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>The following is a list of currently defined header messages:

    <hr>
    <h3><a name="NILMessage">Name: NIL</a></h3>
    <b>Type: </b>0x0000<br>
    <b>Length:</b> varies<br>
    <b>Status:</b> Optional, may be repeated.<br>
    <b>Purpose and Description:</b> The NIL message is used to
    indicate a message 
    which is to be ignored when reading the header messages for a data object.
    [Probably one which has been deleted for some reason.]<br>
    <b>Format of Data:</b> Unspecified.<br>
    <b>Examples:</b> None.


    <hr>
    <h3><a name="SimpleDataSpace">Name: Simple Data Space</a></h3>

    <b>Type: </b>0x0001<br>
    <b>Length:</b> varies<br>
    <b>Status:</b> One of the <em>Simple Data Space</em> or
    <em>Data-Space</em> messages is required (but not both) and may
    not be repeated.<br>

    <p>The <em>Simple Dimensionality</em> message describes the number
      of dimensions and size of each dimension that the data object
      has.  This message is only used for datasets which have a
      simple, rectilinear grid layout, datasets requiring a more
      complex layout (irregularly or unstructured grids, etc) must use
      the <em>Data-Space</em> message for expressing the space the
      dataset inhabits.

    <p>
    <center>
      <table border cellpadding=4 width="80%">
	<caption align=top>
	  <b>Simple Data Space Message</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>Version</td>
	  <td>Dimensionality</td>
	  <td>Flags</td>
	  <td>Reserved</td>
	</tr>

	<tr align=center>
	  <td colspan=4>Reserved</td>
	</tr>

	<tr align=center>
	  <td colspan=4>Dimension Size #1 (&lt;size&gt; bytes)</td>
	<tr align=center>
	  <td colspan=4>.<br>.<br>.<br></td>
	<tr align=center>
	  <td colspan=4>Dimension Size #n (&lt;size&gt; bytes)</td>
	<tr align=center>
	  <td colspan=4>Dimension Maximum #1 (&lt;size&gt; bytes)</td>
	<tr align=center>
	  <td colspan=4>.<br>.<br>.<br></td>
	<tr align=center>
	  <td colspan=4>Dimension Maximum #n (&lt;size&gt; bytes)</td>
	<tr align=center>
	  <td colspan=4>Permutation Index #1</td>
	<tr align=center>
	  <td colspan=4>.<br>.<br>.<br></td>
	<tr align=center>
	  <td colspan=4>Permutation Index #n</td>
      </table>
    </center>

    <p>
    <center>
      <table align=center width="80%">
	<tr>
	  <th width="30%">Field Name</th>
	  <th width="70%">Description</th>
	</tr>

	<tr valign=top>
	  <td>Dimensionality</td>
	  <td>This value is the number of dimensions that the data
	    object has.</td>
	</tr>

	<tr valign=top>
	  <td>Flags</td>
	  <td>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 for each dimension.</td>
	</tr>

	<tr valign=top>
	  <td>Dimension Size #n (&lt;size&gt; bytes)</td>
	  <td>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.</td>
	</tr>

	<tr valign=top>
	  <td>Dimension Maximum #n (&lt;size&gt; bytes)</td>
	  <td>This value is the maximum size of the dimension of the
	    data as stored in the file.  This value may be the special
	    value &lt;UNLIMITED&gt; (all bits set) which indicates
	    that the data may expand along this dimension
	    indefinitely.  If these values are not stored, the maximum
	    value of each dimension is assumed to be the same as the
	    current size value.</td>
	</tr>

	<tr valign=top>
	  <td>Permutation Index #n (4 bytes)</td>
	  <td>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.</td>
	</tr>
      </table>
    </center>

    <h4>Examples</h4>
    <dl>
      <dt> Example #1 
      <dd>A sample 640 horizontally by 480 vertically raster image
	dimension header.  The number of dimensions would be set to 2
	and the first dimension's size and maximum would both be set
	to 480.  The second dimension's size and maximum would both be
	set to 640
.
      <dt>Example #2 
      <dd>A sample 4 dimensional scientific dataset which is composed
	of 30x24x3 slabs of data being written out in an unlimited
	series every several minutes as timestep data (currently there
	are five slabs).  The number of dimensions is 4.  The first
	dimension size is 5 and it's maximum is &lt;UNLIMITED&gt;. The
	second through fourth dimensions' size and maximum value are
	set to 3, 24, and 30 respectively.

      <dt>Example #3 
      <dd>A sample unlimited length text string, currently of length
	83. The number of dimensions is 1, the size of the first
	dimension is 83 and the maximum of the first dimension is set
	to &lt;UNLIMITED&gt;, allowing further text data to be
	appended to the string or possibly the string to be replaced
	with another string of a different size.  (This could also be
	stored as a scalar dataset with number-type set to "string")
    </dl>

    <hr>
    <h3><a name="DataSpaceMessage">Name: Data-Space (Fiber Bundle?)</a></h3>
    <b>Type: </b>0x0002<br>
    <b>Length:</b> varies<br>

    <b>Status:</b> One of the <em>Simple Dimensionality</em> or
    <em>Data-Space</em> messages is required (but not both) and may
    not be repeated.<br> <b>Purpose and Description:</b> The
    <em>Data-Space</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
    data-space 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 data-space 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>
    <b>Format of Data:</b>

    <center>
      <table border cellpadding=4 width="80%">
	<caption align=bottom>
	  <B>HDF5 Data-Space 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 align=center>
	  <td colspan=4>Mesh Type</td>
	<tr align=center>
	  <td colspan=4>Logical Dimensionality</td>
      </table>
    </center>

    <p>
    <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>

	    <P>
	    <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 align=center>
		  <td colspan=1>Mesh Embedding</td>
		  <td colspan=1>Coordinate System</td>
		  <td colspan=1>Structure</td>
		  <td colspan=1>Regularity</td>
	      </table>
	    </center>
	    The following are the definitions of mesh-type bytes:
	    <dl>
	      <dt>Mesh Embedding
	      <dd>This value indicates whether the dataset data-space
		is located within 
		another dataspace or not:
		<dl> <dl>
		    <dt>&lt;STANDALONE&gt;
		    <dd>The dataset mesh is self-contained and is not
		      embedded in another mesh. 
		    <dt>&lt;EMBEDDED&gt;
		    <dd>The dataset's data-space is located within
		      another data-space, 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>&lt;POLAR&gt;
		    <dd>The last two dimensions are in polar
		      coordinates, higher dimensions are 
		      cartesian.
		    <dt>&lt;SPHERICAL&gt;
		    <dd>The last three dimensions are in spherical
		      coordinates, higher dimensions 
		      are cartesian.
		    <dt>&lt;CARTESIAN&gt;
		    <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>&lt;STRUCTURED&gt;
		    <dd>All grid-points are on integral, sequential
		      locations, starting from 0. 
		    <dt>&lt;UNSTRUCTURED&gt;
		    <dd>Grid-points locations in each dimension are
		      explicitly defined and 
		      may be of any numeric data-type.
		  </dl> </dl>
	      <dt>Regularity
	      <dd>This value defines the locations of the dataset
		points on the grid: 
		<dl> <dl>
		    <dt>&lt;REGULAR&gt;
		    <dd>All dataset elements are located at the
		      grid-points defined. 
		    <dt>&lt;IRREGULAR&gt;
		    <dd>Each dataset element has a particular
		      grid-location defined. 
		  </dl> </dl>
	    </dl>
	    <p>The following grid combinations are currently allowed:
	    <dl> <dl>
		<dt>&lt;POLAR-STRUCTURED-REGULAR&gt;
		<dt>&lt;SPHERICAL-STRUCTURED-REGULAR&gt;
		<dt>&lt;CARTESIAN-STRUCTURED-REGULAR&gt;
		<dt>&lt;POLAR-UNSTRUCTURED-REGULAR&gt;
		<dt>&lt;SPHERICAL-UNSTRUCTURED-REGULAR&gt;
		<dt>&lt;CARTESIAN-UNSTRUCTURED-REGULAR&gt;
		<dt>&lt;CARTESIAN-UNSTRUCTURED-IRREGULAR&gt;
	      </dl> </dl>
	    All of the above grid types can be embedded within another
	    data-space.
	    <br> <br>
	  <dt>Logical Dimensionality: (unsigned 32-bit integer)
	  <dd>This value is the number of dimensions that the dataset occupies.

	    <P>
	    <center>
	      <table border cellpadding=4 width="80%">
		<caption align=bottom>
		  <B>HDF5 Data-Space 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 align=center>
		  <td colspan=4>Embedded Dimensionality</td>
		<tr align=center>
		  <td colspan=4>Embedded Dimension Size #1</td>
		<tr align=center>
		  <td colspan=4>.<br>.<br>.<br></td>
		<tr align=center>
		  <td colspan=4>Embedded Dimension Size #n</td>
		<tr align=center>
		  <td colspan=4>Embedded Origin Location #1</td>
		<tr align=center>
		  <td colspan=4>.<br>.<br>.<br></td>
		<tr align=center>
		  <td colspan=4>Embedded Origin Location #n</td>
	      </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.  i.e. a planar dataset located within a 3-D space,
	    or a 3-D dataset 
	    which is a subset of another 3-D space, etc.
	  <dt>Embedded Dimension Size: (unsigned 32-bit integer)
	  <dd>These values are the sizes of the dimensions of the
	    embedded data-space 
	    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 data-space. 
	</dl>
    </dl>
    [Comment: need some way to handle different orientations of the
    dataset data-space 
    within the embedded data-space]<br>

    <P>
    <center>
      <table border cellpadding=4 width="80%">
	<caption align=bottom>
	  <B>HDF5 Data-Space 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 align=center>
	  <td colspan=4>Logical Dimension Size #1</td>
	<tr align=center>
	  <td colspan=4>Logical Dimension Maximum #1</td>
	<tr align=center>
	  <td colspan=4>.<br>.<br>.<br></td>
	<tr align=center>
	  <td colspan=4>Logical Dimension Size #n</td>
	<tr align=center>
	  <td colspan=4>Logical Dimension Maximum #n</td>
      </table>
    </center>

    <p>
    <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
	    &lt;UNLIMITED&gt; which 
	    indicates that the data may expand along this dimension
	    indefinitely. 
	</dl>
    </dl>
    <P>
    <center>
      <table border cellpadding=4 width="80%">
	<caption align=bottom>
	  <B>HDF5 Data-Space 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 align=center>
	  <td colspan=4># of Grid Points in Dimension #1</td>
	<tr align=center>
	  <td colspan=4>.<br>.<br>.<br></td>
	<tr align=center>
	  <td colspan=4># of Grid Points in Dimension #n</td>
	<tr align=center>
	  <td colspan=4>Data-Type of Grid Point Locations</td>
	<tr align=center>
	  <td colspan=4>Location of Grid Points in Dimension #1</td>
	<tr align=center>
	  <td colspan=4>.<br>.<br>.<br></td>
	<tr align=center>
	  <td colspan=4>Location of Grid Points in Dimension #n</td>
      </table>
    </center>

    <P>
    <center>
      <table border cellpadding=4 width="80%">
	<caption align=bottom>
	  <B>HDF5 Data-Space 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 align=center>
	  <td colspan=4># of Grid Points</td>
	<tr align=center>
	  <td colspan=4>Data-Type of Grid Point Locations</td>
	<tr align=center>
	  <td colspan=4>Grid Point Locations<br>.<br>.<br></td>
      </table>
    </center>

    <h4><a name="DataSpaceExample">Examples:</a></h4>
    Need some good examples, this is complex!


    <hr>
    <h3><a name="DataTypeMessage">Name: Data Type</a></h3>

    <b>Type:</b> 0x0003<br>
    <b>Length:</b> variable<br>
    <b>Status:</b> One required per dataset<br>

    <p>The data type message defines the data type for each data point
      of a dataset.  A data type can describe an atomic type like a
      fixed- or floating-point type or a compound type like a C
      struct.  A data type does not, however, describe how data points
      are combined to produce a dataset. Data types are stored on disk
      as a data type message, which is a list of data type classes and
      their associated properties.

    <p>
    <center>
      <table border cellpadding=4 width="80%">
	<caption align=top>
	  <b>Data Type Message</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>Type Class and Version</td>
	  <td colspan=3>Class Bit Field</td>
	</tr>

	<tr align=center>
	  <td colspan=4>Size in Bytes (4 bytes)</td>
	</tr>

	<tr align=center>
	  <td colspan=4><br><br>Properties<br><br><br></td>
	</tr>
      </table>
    </center>

    <p>The Class Bit Field and Properties fields vary depending
      on the Type Class, which is the low-order four bits of the Type
      Class and Version field (the high-order four byte are the
      version which should be set to the value one).  The type class
      is one of: 0 (fixed-point number), 1 (floating-point number), 2
      (date and time), 3 (text string), 4 (bit field), 5 (opaque), 6
      (compound).  The Class Bit Field is zero and the size of the
      Properties field is zero except for the cases noted here.

    <p>
    <center>
      <table border cellpadding=4 width="80%">
	<caption align=top>
	  <b>Bit Field for Fixed-Point Numbers (Class 0)</b>
	</caption>

	<tr align=center>
	  <th width="10%">Bits</th>
	  <th width="90%">Meaning</th>
	</tr>

	<tr valign=top>
	  <td>0</td>
	  <td><b>Byte Order.</b> If zero, byte order is little-endian;
	    otherwise, byte order is big endian.</td>
	</tr>

	<tr valign=top>
	  <td>1, 2</td>
	  <td><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.</td>
	</tr>

	<tr valign=top>
	  <td>3</td>
	  <td><b>Signed.</b> If this bit is set then the fixed-point
	    number is in 2's complement form.</td>
	</tr>

	<tr valign=top>
	  <td>4-23</td>
	  <td>Reserved (zero).</td>
	</tr>
      </table>
    </center>

    <p>
    <center>
      <table border cellpadding=4 width="80%">
	<caption align=top>
	  <b>Properties for Fixed-Point Numbers (Class 0)</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=2>Bit Offset</td>
	  <td colspan=2>Bit Precision</td>
	</tr>
      </table>
    </center>

    <p>
    <center>
      <table border cellpadding=4 width="80%">
	<caption align=top>
	  <b>Bit Field for Floating-Point Numbers (Class 1)</b>
	</caption>

	<tr align=center>
	  <th width="10%">Bits</th>
	  <th width="90%">Meaning</th>
	</tr>

	<tr valign=top>
	  <td>0</td>
	  <td><b>Byte Order.</b> If zero, byte order is little-endian;
	    otherwise, byte order is big endian.</td>
	</tr>

	<tr valign=top>
	  <td>1, 2, 3</td>
	  <td><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 or between
	    the sign bit, exponent, or mantissa, then the value of bit
	    1, 2, or 3 is copied to those locations.</td>
	</tr>

	<tr valign=top>
	  <td>4-5</td>
	  <td><b>Normalization.</b> The value can be 0 if there is no
	    normalization, 1 if the most significant bit of the
	    mantissa is always set (except for 0.0), and 2 if the most
	    signficant bit of the mantissa is not stored but is
	    implied to be set. The value 3 is reserved and will not
	    appear in this field.</td>
	</tr>

	<tr valign=top>
	  <td>6-7</td>
	  <td>Reserved (zero).</td>
	</tr>

	<tr valign=top>
	  <td>8-15</td>
	  <td><b>Sign.</b> This is the bit position of the sign
	    bit.</td>
	</tr>

	<tr valign=top>
	  <td>16-23</td>
	  <td>Reserved (zero).</td>
	</tr>

      </table>
    </center>

    <p>
    <center>
      <table border cellpadding=4 width="80%">
	<caption align=top>
	  <b>Properties for Floating-Point Numbers (Class 1)</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=2>Bit Offset</td>
	  <td colspan=2>Bit Precision</td>
	</tr>

	<tr align=center>
	  <td>Exponent Location</td>
	  <td>Exponent Size in Bits</td>
	  <td>Mantissa Location</td>
	  <td>Mantissa Size in Bits</td>
	</tr>

	<tr align=center>
	  <td colspan=4>Exponent Bias</td>
	</tr>
      </table>
    </center>

   <p>
    <center>
      <table border cellpadding=4 width="80%">
	<caption align=top>
	  <b>Bit Field for Strings (Class 3)</b>
	</caption>

	<tr align=center>
	  <th width="10%">Bits</th>
	  <th width="90%">Meaning</th>
	</tr>

	<tr valign=top>
	  <td>0-3</td>
	  <td><b>Padding type.</b>  This four-bit value determines the
	    type of padding to use for the string.  The values are:

	    <dl>
	      <dt><code>0</code> Null terminate.
	      <dd>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.
		
		<br><br>
	      <dt><code>1</code> Null pad.
	      <dd>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.

		<br><br>
	      <dt><code>2</code> Space pad.
	      <dd>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.

		<br><br>
	      <dt><code>3-15</code> Reserved.
	      <dd>These values are reserved for future use.
	    </dl>
	</tr>

	<tr valign=top>
	  <td>4-7</td>
	  <td><b>Character Set.</b>  The character set to use for
	    encoding the string.  The only character set supported is
	    the 8-bit ASCII (zero) so no translations have been defined
	    yet.</td>
	</tr>

	<tr valign=top>
	  <td>8-23</td>
	  <td>Reserved (zero).</td>
	</tr>
      </table>
    </center>

    <p>
    <center>
      <table border cellpadding=4 width="80%">
	<caption align=top>
	  <b>Bit Field for Bitfield types (Class 4)</b>
	</caption>

	<tr align=center>
	  <th width="10%">Bits</th>
	  <th width="90%">Meaning</th>
	</tr>

	<tr valign=top>
	  <td>0</td>
	  <td><b>Byte Order.</b> If zero, byte order is little-endian;
	    otherwise, byte order is big endian.</td>
	</tr>

	<tr valign=top>
	  <td>1, 2</td>
	  <td><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.</td>
	</tr>

	<tr valign=top>
	  <td>3-23</td>
	  <td>Reserved (zero).</td>
	</tr>
      </table>
    </center>

    <p>
    <center>
      <table border cellpadding=4 width="80%">
	<caption align=top>
	  <b>Properties for Bitfield types (Class 4)</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=2>Bit Offset</td>
	  <td colspan=2>Bit Precision</td>
	</tr>
      </table>
    </center>

    <p>
    <center>
      <table border cellpadding=4 width="80%">
	<caption align=top>
	  <b>Bit Field for Opaque types (Class 5)</b>
	</caption>

	<tr align=center>
	  <th width="10%">Bits</th>
	  <th width="90%">Meaning</th>
	</tr>

	<tr valign=top>
	  <td>0-23</td>
	  <td>Reserved (zero).</td>
	</tr>
      </table>
    </center>

    <p>
    <center>
      <table border cellpadding=4 width="80%">
	<caption align=top>
	  <b>Properties for Opaque types (Class 5)</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><br>Null-terminated ASCII Tag<br>
	    (multiple of 8 bytes)<br><br></td>
	</tr>
      </table>
    </center>

    <p>
    <center>
      <table border cellpadding=4 width="80%">
	<caption align=top>
	  <b>Bit Field for Compound Types (Class 6)</b>
	</caption>

	<tr align=center>
	  <th width="10%">Bits</th>
	  <th width="90%">Meaning</th>
	</tr>

	<tr valign=top>
	  <td>0-15</td>
	  <td><b>Number of Members.</b> This field contains the number
	    of members defined for the compound data type.  The member
	    definitions are listed in the Properties field of the data
	    type message.
	</tr>

	<tr valign=top>
	  <td>15-23</td>
	  <td>Reserved (zero).</td>
	</tr>
      </table>
    </center>

    <p>The Properties field of a compound data type is a list of the
      member definitions of the compound data type.  The member
      definitions appear one after another with no intervening bytes.
      The member types are described with a recursive data type
      message.

    <p>
    <center>
      <table border cellpadding=4 width="80%">
	<caption align=top>
	  <b>Properties for Compound Types (Class 6)</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><br><br>Name (null terminated, multiple of
	    eight bytes)<br><br><br></td>
	</tr>

	<tr align=center>
	  <td colspan=4>Byte Offset of Member in Compound Instance</td>
	</tr>

	<tr align=center>
	  <td>Dimensionality</td>
	  <td colspan=3>reserved</td>
	</tr>

	<tr align=center>
	  <td colspan=4>Dimension Permutation</td>
	</tr>

	<tr align=center>
	  <td colspan=4>Reserved</td>
	</tr>

	<tr align=center>
	  <td colspan=4>Size of Dimension 0 (required)</td>
	</tr>

	<tr align=center>
	  <td colspan=4>Size of Dimension 1 (required)</td>
	</tr>

	<tr align=center>
	  <td colspan=4>Size of Dimension 2 (required)</td>
	</tr>

	<tr align=center>
	  <td colspan=4>Size of Dimension 3 (required)</td>
	</tr>

	<tr align=center>
	  <td colspan=4><br><br>Member Type Message<br><br><br></td>
	</tr>

      </table>
    </center>

    <p>Data type examples are <a href="Datatypes.html">here</a>.
      

    <hr>
    <h3><a name="FillValueMessage">Name: Data Storage - Fill Value</a></h3>
    <b>Type:</b> 0x0004<br>
    <b>Length:</b> varies<br>
    <b>Status:</b> Optional, may not be repeated.<br>

    <p>The fill value message stores a single data point value which
      is returned to the application when an uninitialized data point
      is read from the dataset.  The fill value is interpretted with
      the same data type as the dataset.  If no fill value message is
      present then a fill value of all zero is assumed.

    <p>
    <center>
      <table border cellpadding=4 width="80%">
	<caption align=top>
	  <b>Fill Value Message</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>Size (4 bytes)</td>
	</tr>

	<tr align=center>
	  <td colspan=4><br>Fill Value<br><br></td>
	</tr>
      </table>
    </center>

    <p>
    <center>
      <table align=center width="80%">
	<tr>
	  <th width="30%">Field Name</th>
	  <th width="70%">Description</th>
	</tr>

	<tr valign=top>
	  <td>Size (4 bytes)</td>
	  <td>This is the size of the Fill Value field in bytes.</td>
	</tr>

	<tr valign=top>
	  <td>Fill Value</td>
	  <td>The fill value.  The bytes of the fill value are
	    interpreted using the same data type as for the dataset.</td>
	</tr>
      </table>
    </center>

    <hr>
    <h3><a name="ReservedMessage_0005">Name: Reserved - Not Assigned
	Yet</a></h3>
    <b>Type:</b> 0x0005<br>
    <b>Length:</b> N/A<br>
    <b>Status:</b> N/A<br>



    <hr>
    <h3><a name="CompactDataStorageMessage">Name: Data Storage - Compact</a></h3>

    <b>Type:</b> 0x0006<br>
    <b>Length:</b> varies<br>
    <b>Status:</b> Optional, may not be repeated.<br>

    <p>This message indicates that the data for the data object is
      stored within the current HDF file by including the actual 
      data within the header data for this message.  The data is
      stored internally in 
      the "normal" format, i.e. in one chunk, un-compressed, etc.

    <P>Note that one and only one of the "Data Storage" headers can be
      stored for each data object.

    <P><b>Format of Data:</b>  The message data is actually composed
      of dataset data, so the format will be determined by the dataset
      format.

    <h4><a name="CompactDataStorageExample">Examples:</a></h4>
    [very straightforward]

    <hr>
    <h3><a name="ExternalFileListMessage">Name: Data Storage -
	External Data Files</a></h3>
    <b>Type:</b> 0x0007<BR>
    <b>Length:</b> varies<BR>
    <b>Status:</b> Optional, may not be repeated.<BR>

    <p><b>Purpose and Description:</b> The external object 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.

    <p>
    <center>
      <table border cellpadding=4 width="80%">
	<caption align=top>
	  <b>External File List Message</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>Version</td>
	  <td colspan=3>Reserved</td>
	</tr>

	<tr align=center>
	  <td colspan=2>Allocated Slots</td>
	  <td colspan=2>Used Slots</td>
	</tr>

	<tr align=center>
	  <td colspan=4><br>Heap Address<br><br></td>
	</tr>

	<tr align=center>
	  <td colspan=4><br>Slot Definitions...<br><br></td>
	</tr>
      </table>
    </center>

    <p>
    <center>
      <table align=center width="80%">
	<tr>
	  <th width="30%">Field Name</th>
	  <th width="70%">Description</th>
	</tr>

	<tr valign=top>
	  <td>Heap Address</td>
	  <td>This is the address of a local name heap which contains
	    the names for the external files. The name at offset zero
	    in the heap is always the empty string.</td>
	</tr>

	<tr valign=top>
	  <td>Allocated Slots</td>
	  <td>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.</td>
	</tr>

	<tr valign=top>
	  <td>Used Slots</td>
	  <td>The number of initial slots which contain valid
	    information.  The remaining slots are zero filled.</td>
	</tr>

	<tr valign=top>
	  <td>Reserved</td>
	  <td>This field is reserved for future use.</td>
	</tr>

	<tr valign=top>
	  <td>Slot Definitions</td>
	  <td>The slot definitions are stored in order according to
	    the array addresses they represent. If more slots have
	    been allocated than what has been used then the defined
	    slots are all at the beginning of the list.</td>
	</tr>
      </table>
    </center>

    <p>
    <center>
      <table border cellpadding=4 width="80%">
	<caption align=top>
	  <b>External File List Slot</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><br>Name Offset (&lt;size&gt; bytes)<br><br></td>
	</tr>

	<tr align=center>
	  <td colspan=4><br>File Offset (&lt;size&gt; bytes)<br><br></td>
	</tr>

	<tr align=center>
	  <td colspan=4><br>Size<br><br></td>
	</tr>
      </table>
    </center>

    <p>
    <center>
      <table align=center width="80%">
	<tr>
	  <th width="30%">Field Name</th>
	  <th width="70%">Description</th>
	</tr>

	<tr valign=top>
	  <td>Name Offset (&lt;size&gt; bytes)</td>
	  <td>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).</td>
	</tr>

	<tr valign=top>
	  <td>File Offset (&lt;size&gt; bytes)</td>
	  <td>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.</td>
	</tr>

	<tr valign=top>
	  <td>Size</td>
	  <td>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 zeros
	    past the end of the file without failing.</td>
	</tr>
      </table>
    </center>


    <hr>
    <h3><a name="LayoutMessage">Name: Data Storage - Layout</a></h3>

    <b>Type:</b> 0x0008<BR>
    <b>Length:</b> varies<BR>
    <b>Status:</b> Required for datasets, may not be repeated.

    <p><b>Purpose and Description:</b> Data layout describes how the
      elements of a multi-dimensional array are arranged in the linear
      address space of the file. Two types of data layout are
      supported:

    <ol>
      <li>The array can be stored in one contiguous area of the file.
	The layout requires that the size of the array be constant and
	does not permit chunking, compression, checksums, encryption,
	etc.  The message stores the total size of the array and the
	offset of an element from the beginning of the storage area is
	computed as in C.

      <li>The array domain can be regularly decomposed into chunks and
	each chunk is allocated separately.  This layout supports
	arbitrary element traversals, compression, encryption, and
	checksums, and the chunks can be distributed across external
	raw data files (these features are described in other
	messages).  The message stores the size of a chunk instead of
	the size of the entire array; the size of the entire array can
	be calculated by traversing the B-tree that stores the chunk
	addresses.
    </ol>

    <p>
    <center>
      <table border cellpadding=4 width="80%">
	<caption align=top>
	  <B>Data Layout Message</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>Version</td>
	  <td>Dimensionality</td>
	  <td>Layout Class</td>
	  <td>Reserved</td>
	</tr>

	<tr align=center>
	  <td colspan=4>Reserved</td>
	</tr>

	<tr align=center>
	  <td colspan=4><br>Address<br><br></td>
	</tr>

	<tr align=center>
	  <td colspan=4>Dimension 0 (4-bytes)</td>
	</tr>

	<tr align=center>
	  <td colspan=4>Dimension 1 (4-bytes)</td>
	</tr>
	
	<tr align=center>
	  <td colspan=4>...</td>
	</tr>
      </table>
    </center>

    <p>
    <center>
      <table align=center width="80%">
	<tr>
	  <th width="30%">Field Name</th>
	  <th width="70%">Description</th>
	</tr>

	<tr valign=top>
	  <td>Version</td>
	  <td>A version number for the layout message. This
	    documentation describes version one.</td>
	</tr>

	<tr valign=top>
	  <td>Dimensionality</td>
	  <td>An array has a fixed dimensionality.  This field
	    specifies the number of dimension size fields later in the
	    message.</td>
	</tr>

	<tr valign=top>
	  <td>Layout Class</td>
	  <td>The layout class specifies how the other fields of the
	    layout message are to be interpreted.  A value of one
	    indicates contiguous storage while a value of two
	    indicates chunked storage.  Other values will be defined
	    in the future.</td>
	</tr>

	<tr valign=top>
	  <td>Address</td>
	  <td>For contiguous storage, this is the address of the first
	    byte of storage.  For chunked storage this is the address
	    of the B-tree that is used to look up the addresses of the
	    chunks.</td>
	</tr>

	<tr valign=top>
	  <td>Dimensions</td>
	  <td>For contiguous storage the dimensions define the entire
	    size of the array while for chunked storage they define
	    the size of a single chunk.</td>
	</tr>
      </table>
    </center>


    <hr>
    <h3><a name="ReservedMessage_0009">Name: Reserved - Not Assigned Yet</a></h3>
    <b>Type:</b> 0x0009<BR>
    <b>Length:</b> N/A<BR>
    <b>Status:</b> N/A<BR>
    <b>Purpose and Description:</b> N/A<BR>
    <b>Format of Data:</b> N/A

    <hr>
    <h3><a name="ReservedMessage_000A">Name: Reserved - Not Assigned Yet</a></h3>
    <b>Type:</b> 0x000A<BR>
    <b>Length:</b> N/A<BR>
    <b>Status:</b> N/A<BR>
    <b>Purpose and Description:</b> N/A<BR>
    <b>Format of Data:</b> N/A

    <hr>
    <h3><a name="FilterMessage">Name: Data Storage - Filter Pipeline</a></h3>
    <b>Type:</b> 0x000B<BR>
    <b>Length:</b> varies<BR>
    <b>Status:</b> Optional, may not be repeated.

    <p><b>Purpose and Description:</b> This message describes the
      filter pipeline which should be applied to the data stream by
      providing filter identification numbers, flags, a name, an
      client data.

    <p>
    <center>
      <table border align=center cellpadding=4 witdh="80%">
	<caption align=top>
	  <b>Filter Pipeline Message</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>Version</td>
	  <td>Number of Filters</td>
	  <td colspan=2>Reserved</td>
	</tr>

	<tr align=center>
	  <td colspan=4>Reserved</td>
	</tr>

	<tr align=center>
	  <td colspan=4><br>Filter List<br><br></td>
	</tr>
      </table>
    </center>

    <p>
    <center>
      <table align=center width="80%">
	<tr>
	  <th width="30%">Field Name</th>
	  <th width="70%">Description</th>
	</tr>

	<tr valign=top>
	  <td>Version</td>
	  <td>The version number for this message.  This document
	    describes version one.</td>
	</tr>

	<tr valign=top>
	  <td>Number of Filters</td>
	  <td>The total number of filters described by this
	    message. The maximum possible number of filters in a
	    message is 32.</td>
	</tr>

	<tr valign=top>
	  <td>Filter List</td>
	  <td>A description of each filter.  A filter description
	    appears in the next table.</td>
	</tr>
      </table>
    </center>

    <p>
    <center>
      <table border align=center cellpadding=4 witdh="80%">
	<caption align=top>
	  <b>Filter Pipeline Message</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=2>Filter Identification</td>
	  <td colspan=2>Name Length</td>
	</tr>

	<tr align=center>
	  <td colspan=2>Flags</td>
	  <td colspan=2>Client Data Number of Values</td>
	</tr>

	<tr align=center>
	  <td colspan=4><br>Name<br><br></td>
	</tr>

	<tr align=center>
	  <td colspan=4><br>Client Data<br><br></td>
	</tr>

	<tr align=center>
	  <td colspan=4>Padding</td>
	</tr>
      </table>
    </center>

    <p>
    <center>
      <table align=center width="80%">
	<tr>
	  <th width="30%">Field Name</th>
	  <th width="70%">Description</th>
	</tr>

	<tr valign=top>
	  <td>Filter Identification</td>
	  <td>This is a unique (except in the case of testing)
	    identifier for the filter.  Values from zero through 255
	    are reserved for filters defined by the NCSA HDF5
	    library. Values 256 through 511 have been set aside for
	    use when developing/testing new filters.  The remaining
	    values are allocated to specific filters by contacting the 
	    <a href="mailto:hdf5dev@ncsa.uiuc.edu">HDF5 Development
	    Team</a>.</td>
	</tr>

	<tr valign=top>
	  <td>Name Length</td>
	  <td>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.</td>
	</tr>

	<tr valign=top>
	  <td>Flags</td>
	  <td>The flags indicate certain properties for a filter.  The 
	    bit values defined so far are:

	    <dl>
	      <dt><code>bit 1</code>
	      <dd>If set then the filter is an optional filter.
		During output, if an optional filter fails it will be
		silently removed from the pipeline.
	    </dl>
	</tr>

	<tr valign=top>
	  <td>Client Data Number of Values</td>
	  <td>Each filter can store a few integer values to control
	    how the filter operates.  The number of entries in the
	    Client Data array is stored in this field.</td>
	</tr>

	<tr valign=top>
	  <td>Name</td>
	  <td>If the Name Length field is non-zero then it will
	    contain the size of this field, a multiple of eight.  This 
	    field contains a null-terminated, ASCII character
	    string to serve as a comment/name for the filter.</td>
	</tr>

	<tr valign=top>
	  <td>Client Data</td>
	  <td>This is an array of four-byte integers which will be
	    passed to the filter function.  The Client Data Number of
	    Values determines the number of elements in the
	    array.</td>
	</tr>

	<tr valign=top>
	  <td>Padding</td>
	  <td>Four bytes of zeros are added to the message at this
	    point if the Client Data Number of Values field contains
	    an odd number.</td>
	</tr>
      </table>
    </center>

    <hr>
    <h3><a name="AttributeMessage">Name: Attribute</a></h3>
    <b>Type:</b> 0x000C<BR>
    <b>Length:</b> varies<BR>
    <b>Status:</b> Optional, may be repeated.<BR>
    
    <p><b>Purpose and Description:</b>  The <em>Attribute</em>
      message is used to list objects in the HDF file which are used
      as attributes, or "meta-data" about the current object.  An
      attribute is a small dataset; it has a name, a data type, a data
      space, and raw data.  Since attributes are stored in the object
      header they must be relatively small (<64kb) and can be
      associated with any type of object which has an object header
      (groups, datasets, named types and spaces, etc.).

    <p>
    <center>
      <table border align=center cellpadding=4 width="80%">
	<caption align=top>
	  <b>Attribute Message</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>Version</td>
	  <td>Reserved</td>
	  <td colspan=2>Name Size</td>
	</tr>

	<tr align=center>
	  <td colspan=2>Type Size</td>
	  <td colspan=2>Space Size</td>
	</tr>

	<tr align=center>
	  <td colspan=4><br>Name<br><br></td>
	</tr>

	<tr align=center>
	  <td colspan=4><br>Type<br><br></td>
	</tr>

	<tr align=center>
	  <td colspan=4><br>Space<br><br></td>
	</tr>

	<tr align=center>
	  <td colspan=4><br>Data<br><br></td>
	</tr>
      </table>
    </center>

    <p>
    <center>
      <table align=center width="80%">
	<tr>
	  <th width="30%">Field Name</th>
	  <th width="70%">Description</th>
	</tr>

	<tr valign=top>
	  <td>Version</td>
	  <td>Version number for the message.  This document describes 
	    version 1 of attribute messages.</td>
	</tr>

	<tr valign=top>
	  <td>Name Size</td>
	  <td>The length of the attribute name in bytes including the
	    null terminator.  Note that the Name field below may
	    contain additional padding not represented by this
	    field.</td>
	</tr>

	<tr valign=top>
	  <td>Type Size</td>
	  <td>The length of the data type description in the Type
	    field below.  Note that the Type field may contain
	    additional padding not represented by this field.</td>
	</tr>

	<tr valign=top>
	  <td>Space Size</td>
	  <td>The length of the data space description in the Space
	    field below.  Note that the Space field may contain
	    additional padding not represented by this field.</td>
	</tr>

	<tr valign=top>
	  <td>Reserved</td>
	  <td>This field is reserved for later use and is set to
	    zero.</td>
	</tr>

	<tr valign=top>
	  <td>Name</td>
	  <td>The null-terminated attribute name.  This field is
	    padded with additional null characters to make it a
	    multiple of eight bytes.</td>
	</tr>

	<tr valign=top>
	  <td>Type</td>
	  <td>The data type description follows the same format as
	    described for the data type object header message.  This
	    field is padded with additional zero bytes to make it a
	    multiple of eight bytes.</td>
	</tr>

	<tr valign=top>
	  <td>Space</td>
	  <td>The data space description follows the same format as
	    described for the data space object header message.  This
	    field is padded with additional zero bytes to make it a
	    multiple of eight bytes.</td>
	</tr>

	<tr valign=top>
	  <td>Data</td>
	  <td>The raw data for the attribute.  The size is determined
	    from the data type and data space descriptions.  This
	    field is <em>not</em> padded with additional zero
	    bytes.</td>
	</tr>
      </table>
    </center>
    
    <hr>
    <h3><a name="NameMessage">Name: Object Name</a></h3>

    <p><b>Type:</b> 0x000D<br>
      <b>Length:</b> varies<br>
      <b>Status:</b> Optional, may not be repeated.

    <p><b>Purpose and Description:</b> The object name or comment is
      designed to be a short description of an object.  An object name
      is a sequence of non-zero ('\0') ASCII characters with no other
      formatting included by the library.

    <p>
    <center>
      <table border align=center cellpadding=4 width="80%">
	<caption align=top>
	  <b>Name Message</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><br>Name<br><br></td>
	</tr>
      </table>
    </center>

    <p>
    <center>
      <table align=center width="80%">
	<tr>
	  <th width="30%">Field Name</th>
	  <th width="70%">Description</th>
	</tr>

	<tr valign=top>
	  <td>Name</td>
	  <td>A null terminated ASCII character string.</td>
	</tr>
      </table>
    </center>
    
    <hr>
    <h3><a name="ModifiedMessage">Name: Object Modification Date &amp;
	Time</a></h3>

    <p><b>Type:</b> 0x000E<br>
      <b>Length:</b> fixed<br>
      <b>Status:</b> Optional, may not be repeated.

    <p><b>Purpose and Description:</b>  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.

    <p>
    <center>
      <table border align=center cellpadding=4 width="80%">
	<caption align=top>
	  <b>Modification Time Message</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>Year</td>
	</tr>

	<tr align=center>
	  <td colspan=2>Month</td>
	  <td colspan=2>Day of Month</td>
	</tr>

	<tr align=center>
	  <td colspan=2>Hour</td>
	  <td colspan=2>Minute</td>
	</tr>

	<tr align=center>
	  <td colspan=2>Second</td>
	  <td colspan=2>Reserved</td>
	</tr>
      </table>
    </center>

    <p>
    <center>
      <table align=center width="80%">
	<tr>
	  <th width="30%">Field Name</th>
	  <th width="70%">Description</th>
	</tr>

	<tr valign=top>
	  <td>Year</td>
	  <td>The four-digit year as an ASCII string. For example,
	    "1998".  All fields of this message should be interpreted
	    as coordinated universal time (UTC)</td>
	</tr>

	<tr valign=top>
	  <td>Month</td>
	  <td>The month number as a two digit ASCII string where
	    January is "01" and December is "12".</td>
	</tr>

	<tr valign=top>
	  <td>Day of Month</td>
	  <td>The day number within the month as a two digit ASCII
	    string. The first day of the month is "01".</td>
	</tr>

	<tr valign=top>
	  <td>Hour</td>
	  <td>The hour of the day as a two digit ASCII string where
	    midnight is "00" and 11:00pm is "23".</td>
	</tr>

	<tr valign=top>
	  <td>Minute</td>
	  <td>The minute of the hour as a two digit ASCII string where
	    the first minute of the hour is "00" and the last is
	    "59".</td>
	</tr>

	<tr valign=top>
	  <td>Second</td>
	  <td>The second of the minute as a two digit ASCII string
	    where the first second of the minute is "00" and the last
	    is "59".</td>
	</tr>

	<tr valign=top>
	  <td>Reserved</td>
	  <td>This field is reserved and should always be zero.</td>
	</tr>
      </table>
    </center>
    
    <hr>
    <h3><a name="SharedMessage">Name: Shared Object Message</a></h3>
    <b>Type:</b> 0x000F<br>
    <b>Length:</b> 4 Bytes<br>
    <b>Status:</b> Optional, may be repeated.

    <p>A constant message can be shared among several object headers
      by writing that message in the global heap and having the object
      headers all point to it.  The pointing is accomplished with a
      Shared Object message which is understood directly by the object
      header layer of the library. It is also possible to have a
      message of one object header point to a message in some other
      object header, but care must be exercised to prevent cycles.

    <p>If a message is shared, then the message appears in the global
      heap and its message ID appears in the Header Message Type
      field of the object header.  Also, the Flags field in the object
      header for that message will have bit two set (the
      <code>H5O_FLAG_SHARED</code> bit).  The message body in the
      object header will be that of a Shared Object message defined
      here and not that of the pointed-to message.

    <p>
    <center>
      <table border cellpadding=4 width="80%">
	<caption align=top>
	  <b>Shared Message Message</b>
	</caption>

	<tr align=center>
	  <th width="25%">byte</td>
	  <th width="25%">byte</td>
	  <th width="25%">byte</td>
	  <th width="25%">byte</td>
	</tr>

	<tr align=center>
	  <td>Version</td>
	  <td>Flags</td>
	  <td colspan=2>Reserved</td>
	</tr>

	<tr align=center>
	  <td colspan=4>Reserved</td>
	</tr>

	<tr align=center>
	  <td colspan=4><br>Pointer<br><br></td>
	</tr>
      </table>
    </center>

    <p>
    <center>
      <table align=center width="80%">
	<tr>
	  <th width="30%">Field Name</th>
	  <th width="70%">Description</th>
	</tr>

	<tr valign=top>
	  <td>Version</td>
	  <td>The version number for the message.  This document
	    describes version one of shared messages.</td>
	</tr>

	<tr valign=top>
	  <td>Flags</td>
	  <td>The Shared Message message points to a message which is
	    shared among multiple object headers.  The Flags field
	    describes the type of sharing:

	    <dl>
	      <dt><code>Bit 0</code>
	      <dd>If this bit is clear then the actual message is the
		first message in some other object header; otherwise
		the actual message is stored in the global heap.

	      <dt><code>Bits 2-7</code>
	      <dd>Reserved (always zero)
	    </dl>
	</tr>

	<tr valign=top>
	  <td>Pointer</td>
	  <td>This field points to the actual message.  The format of
	    the pointer depends on the value of the Flags field.  If
	    the actual message is in the global heap then the pointer
	    is the file address of the global heap collection that
	    holds the message, and a four-byte index into that
	    collection.  Otherwise the pointer is a symbol table entry 
	    that points to some other object header.</td>
	</tr>
      </table>
    </center>


<hr>
<h3><a name="ContinuationMessage">Name: Object Header Continuation</a></h3>
<b>Type:</b> 0x0010<BR>
<b>Length:</b> fixed<BR>
<b>Status:</b> Optional, may be repeated.<BR>
<b>Purpose and Description:</b>  The object header continuation is the location
in the file of more header messages for the current data object.  This can be
used when header blocks are large, or likely to change over time.<BR>
<b>Format of Data:</b><p>
    The object header continuation is formatted as follows (assuming a 4-byte
length &amp; offset are being used in the current file):

<P>
<center>
<table border cellpadding=4 width=60%>
<caption align=bottom>
<B>HDF5 Object Header Continuation 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 align=center>
<td colspan=4>Header Continuation Offset</td>
<tr align=center>
<td colspan=4>Header Continuation Length</td>
</table>
</center>

<P>
<dl>
<dt>The elements of the Header Continuation Message are described below:
<dd>
<dl>
<dt>Header Continuation Offset: (&lt;offset&gt; bytes)
<dd>This value is the offset in bytes from the beginning of the file where the
header continuation information is located.
<dt>Header Continuation Length: (&lt;length&gt; bytes)
<dd>This value is the length in bytes of the header continuation information in
the file.
</dl>
</dl>

<h4><a name="ContinuationExample">Examples:</a></h4>
    [straightforward]

<hr>
<h3><a name="SymbolTableMessage">Name: Symbol Table Message</a></h3>
<b>Type:</b> 0x0011<BR>
<b>Length:</b> fixed<BR>
<b>Status:</b> Required for symbol tables, may not be repeated.<BR>
<b>Purpose and Description:</b> Each symbol table has a B-tree and a
name heap which are pointed to by this message.<BR>
<b>Format of data:</b>
<p>The symbol table message is formatted as follows:

<p>
<center>
<table border cellpadding=4 width="80%">
<caption align=bottom>
<b>HDF5 Object Header Symbol Table 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 align=center>
<td colspan=4>B-Tree Address</td>

<tr align=center>
<td colspan=4>Heap Address</td>
</table>
</center>

<P>
<dl>
<dt>The elements of the Symbol Table Message are described below:
<dd>
<dl>
<dt>B-tree Address (&lt;offset&gt; bytes)
<dd>This value is the offset in bytes from the beginning of the file
where the B-tree is located.
<dt>Heap Address (&lt;offset&gt; bytes)
<dd>This value is the offset in bytes from the beginning of the file
where the symbol table name heap is located.
</dl>
</dl>

<h3><a name="SharedObjectHeader">Disk Format: Level 2b - Shared Data Object Headers</a></h3>
<P>In order to share header messages between several dataset objects, object
header messages may be placed into the global small-data heap.  Since these
messages require additional information beyond the basic object header message
information, the format of the shared message is detailed below.

<BR> <BR>
<center>
<table border cellpadding=4 width=60%>
<caption align=bottom>
<B>HDF5 Shared Object Header Message</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 align=center>
<td colspan=4>Reference Count of Shared Header Message</td>
<tr align=center>
<td colspan=4><br> Shared Object Header Message<br> <br></td>
</table>
</center>

<p>
<dl>
<dt> The elements of the shared object header message are described below:
<dd>
<dl>
<dt>Reference Count of Shared Header Message: (32-bit unsigned integer)
<dd>This value is used to keep a count of the number of dataset objects which
refer to this message from their dataset headers.  When this count reaches zero,
the shared message header may be removed from the global small-data heap.
<dt>Shared Object Header Message: (various lengths)
<dd>The data stored for the shared object header message is formatted in the
same way as the private object header messages described in the object header
description earlier in this document and begins with the header message Type.
</dl>
</dl>


<h3><a name="DataStorage">Disk Format: Level 2c - Data Object Data Storage</a></h3>
<P>The data information for an object is stored separately from the 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 dimensionality header message).
Multi-dimensional data is stored in C order [same as current scheme], i.e. the
"last" dimension changes fastest.
<P>Data whose elements are composed of simple number-types are stored in
native-endian IEEE format, unless they are specifically defined as being stored
in a different machine format with the architecture-type information from the
number-type header message.  This means that each architecture will need to
[potentially] byte-swap data values into the internal representation for that
particular machine.
<P> Data with a "variable" sized number-type is stored in an data heap
internal to the HDF file [which should not be user-modifiable].
<P>Data whose elements are composed of pointer number-types are stored in several
different ways depending on the particular pointer type involved. Simple
pointers are just stored as the dataset offset of the object being pointed to with the
size of the pointer being the same number of bytes as offsets in the file.
Partial-object pointers 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 (i.e. a coordinate
location for each), and field start and end names (i.e.  a [pointer to the]
string indicating the first field included and a [pointer to the] string name
for the last field).  
Browse pointers are stored as an heap-ID (for the name in the file-heap)
followed by a offset of the data object being referenced.
<P>Data of a compound data-type is stored as a contiguous stream of the items
in the structure, with each item formatted according to it's
data-type.

<hr>
<address><a href="mailto:koziol@ncsa.uiuc.edu">Quincey Koziol</a></address>
<address><a href="mailto:matzke@llnl.gov">Robb Matzke</a></address>
<!-- hhmts start -->
Last modified: Tue Aug 17 10:57:50 EDT 1999
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