<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN"> <html> <head> <title>Group Interface (H5G)</title> <!-- #BeginLibraryItem "/ed_libs/styles_UG.lbi" --> <!-- * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * Copyright by the Board of Trustees of the University of Illinois. * * All rights reserved. * * * * This file is part of HDF5. The full HDF5 copyright notice, including * * terms governing use, modification, and redistribution, is contained in * * the files COPYING and Copyright.html. COPYING can be found at the root * * of the source code distribution tree; Copyright.html can be found at the * * root level of an installed copy of the electronic HDF5 document set and * * is linked from the top-level documents page. It can also be found at * * http://hdf.ncsa.uiuc.edu/HDF5/doc/Copyright.html. If you do not have * * access to either file, you may request a copy from hdfhelp@ncsa.uiuc.edu. * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * --> <link href="ed_styles/UGelect.css" rel="stylesheet" type="text/css"> <!-- #EndLibraryItem --></head> <body bgcolor="#FFFFFF"> <!-- #BeginLibraryItem "/ed_libs/NavBar_UG.lbi" --><hr> <center> <table border=0 width=98%> <tr><td valign=top align=left> <a href="index.html">HDF5 documents and links</a> <br> <a href="H5.intro.html">Introduction to HDF5</a> <br> <a href="RM_H5Front.html">HDF5 Reference Manual</a> <br> <a href="http://hdf.ncsa.uiuc.edu/HDF5/doc/UG/index.html">HDF5 User's Guide for Release 1.6</a> <br> <!-- <a href="Glossary.html">Glossary</a><br> --> </td> <td valign=top align=right> And in this document, the <a href="H5.user.html"><strong>HDF5 User's Guide from Release 1.4.5:</strong></a> <br> <a href="Files.html">Files</a> <a href="Datasets.html">Datasets</a> <a href="Datatypes.html">Datatypes</a> <a href="Dataspaces.html">Dataspaces</a> <a href="Groups.html">Groups</a> <br> <a href="References.html">References</a> <a href="Attributes.html">Attributes</a> <a href="Properties.html">Property Lists</a> <a href="Errors.html">Error Handling</a> <br> <a href="Filters.html">Filters</a> <a href="Caching.html">Caching</a> <a href="Chunking.html">Chunking</a> <a href="MountingFiles.html">Mounting Files</a> <br> <a href="Performance.html">Performance</a> <a href="Debugging.html">Debugging</a> <a href="Environment.html">Environment</a> <a href="ddl.html">DDL</a> </td></tr> </table> </center> <hr> <!-- #EndLibraryItem --><h1>The Group Interface (H5G)</h1> <h2>1. Introduction</h2> <p>An object in HDF5 consists of an object header at a fixed file address that contains messages describing various properties of the object such as its storage location, layout, compression, etc. and some of these messages point to other data such as the raw data of a dataset. The address of the object header is also known as an <em>OID</em> and HDF5 has facilities for translating names to OIDs. <p>Every HDF5 object has at least one name and a set of names can be stored together in a group. Each group implements a name space where the names are any length and unique with respect to other names in the group. <p>Since a group is a type of HDF5 object it has an object header and a name which exists as a member of some other group. In this way, groups can be linked together to form a directed graph. One particular group is called the <em>Root Group</em> and is the group to which the HDF5 file super block points. Its name is "/" by convention. The <em>full name</em> of an object is created by joining component names with slashes much like Unix. <p> <center> <img alt="Group Graph Example" src="group_p1.gif"> </center> <p>However, unlike Unix which arranges directories hierarchically, HDF5 arranges groups in a directed graph. Therefore, there is no ".." entry in a group since a group can have more than one parent. There is no "." entry either but the library understands it internally. <h2>2. Names</h2> <p>HDF5 places few restrictions on names: component names may be any length except zero and may contain any character except slash ("/") and the null terminator. A full name may be composed of any number of component names separated by slashes, with any of the component names being the special name ".". A name which begins with a slash is an <em>absolute</em> name which is looked up beginning at the root group of the file while all other <em>relative</em> names are looked up beginning at the specified group. Multiple consecutive slashes in a full name are treated as single slashes and trailing slashes are not significant. A special case is the name "/" (or equivalent) which refers to the root group. <p>Functions which operate on names generally take a location identifier which is either a file ID or a group ID and perform the lookup with respect to that location. Some possibilities are: <p> <center> <table border cellpadding=4> <tr> <th>Location Type</th> <th>Object Name</th> <th>Description</th> </tr> <tr> <td>File ID</td> <td><code>/foo/bar</code></td> <td>The object <code>bar</code> in group <code>foo</code> in the root group.</td> </tr> <tr> <td>Group ID</td> <td><code>/foo/bar</code></td> <td>The object <code>bar</code> in group <code>foo</code> in the root group of the file containing the specified group. In other words, the group ID's only purpose is to supply a file.</td> </tr> <tr> <td>File ID</td> <td><code>/</code></td> <td>The root group of the specified file.</td> </tr> <tr> <td>Group ID</td> <td><code>/</code></td> <td>The root group of the file containing the specified group.</td> </tr> <tr> <td>File ID</td> <td><code>foo/bar</code></td> <td>The object <code>bar</code> in group <code>foo</code> in the specified group.</td> </tr> <tr> <td>Group ID</td> <td><code>foo/bar</code></td> <td>The object <code>bar</code> in group <code>foo</code> in the specified group.</td> </tr> <tr> <td>File ID</td> <td><code>.</code></td> <td>The root group of the file.</td> </tr> <tr> <td>Group ID</td> <td><code>.</code></td> <td>The specified group.</td> </tr> <tr> <td>Other ID</td> <td><code>.</code></td> <td>The specified object.</td> </tr> </table> </center> <p>Note, however, that object names within a group must be unique. For example, <code>H5Dcreate</code> returns an error if a dataset with the dataset name specified in the parameter list already exists at the location specified in the parameter list. <h2>3. Creating, Opening, and Closing Groups</h2> <p>Groups are created with the <code>H5Gcreate()</code> function, and existing groups can be access with <code>H5Gopen()</code>. Both functions return an object ID which should be eventually released by calling <code>H5Gclose()</code>. <dl> <dt><code>hid_t H5Gcreate (hid_t <em>location_id</em>, const char *<em>name</em>, size_t <em>size_hint</em>)</code> <dd>This function creates a new group with the specified name at the specified location which is either a file ID or a group ID. The name must not already be taken by some other object and all parent groups must already exist. The <em>size_hint</em> is a hint for the number of bytes to reserve to store the names which will be eventually added to the new group. Passing a value of zero for <em>size_hint</em> is usually adequate since the library is able to dynamically resize the name heap, but a correct hint may result in better performance. The return value is a handle for the open group and it should be closed by calling <code>H5Gclose()</code> when it's no longer needed. A negative value is returned for failure. <br><br> <dt><code>hid_t H5Gopen (hid_t <em>location_id</em>, const char *<em>name</em>)</code> <dd>This function opens an existing group with the specified name at the specified location which is either a file ID or a group ID and returns an object ID. The object ID should be released by calling <code>H5Gclose()</code> when it is no longer needed. A negative value is returned for failure. <br><br> <dt><code>herr_t H5Gclose (hid_t <em>group_id</em>)</code> <dd>This function releases resources used by an group which was opened by <code>H5Gcreate()</code> or <code>H5Gopen()</code>. After closing a group the <em>group_id</em> should not be used again. This function returns zero for success or a negative value for failure. </dl> <h2>4. Objects with Multiple Names</h2> <p>An object (including a group) can have more than one name. Creating the object gives it the first name, and then functions described here can be used to give it additional names. The association between a name and the object is called a <em>link</em> and HDF5 supports two types of links: a <em>hard link</em> is a direct association between the name and the object where both exist in a single HDF5 address space, and a <em>soft link</em> is an indirect association. <p> <center> <img alt="Hard Link Example" src="group_p2.gif"> </center> <p> <center> <img alt="Soft Link Example" src="group_p3.gif"> </center> <dl> <dt>Object Creation</dt> <dd>The creation of an object creates a hard link which is indistinguishable from other hard links that might be added later. <br><br> <dt><code>herr_t H5Glink (hid_t <em>file_id</em>, H5G_link_t <em>link_type</em>, const char *<em>current_name</em>, const char *<em>new_name</em>)</code> <dd>Creates a new name for an object that has some current name (possibly one of many names it currently has). If the <em>link_type</em> is <code>H5G_LINK_HARD</code> then a new hard link is created. Otherwise if <em>link_type</em> is <code>H5T_LINK_SOFT</code> a soft link is created which is an alias for the <em>current_name</em>. When creating a soft link the object need not exist. This function returns zero for success or negative for failure. <br><br> <dt><code>herr_t H5Gunlink (hid_t <em>file_id</em>, const char *<em>name</em>)</code> <dd>This function removes an association between a name and an object. Object headers keep track of how many hard links refer to the object and when the hard link count reaches zero the object can be removed from the file (but objects which are open are not removed until all handles to the object are closed). </dl> <h2>5. Comments</h2> <p>Objects can have a comment associated with them. The comment is set and queried with these two functions: <dl> <dt><code>herr_t H5Gset_comment (hid_t <em>loc_id</em>, const char *<em>name</em>, const char *<em>comment</em>)</code> <dd>The previous comment (if any) for the specified object is replace with a new comment. If the <em>comment</em> argument is the empty string or a null pointer then the comment message is removed from the object. Comments should be relatively short, null-terminated, ASCII strings. <br><br> <dt><code>herr_t H5Gget_comment (hid_t <em>loc_id</em>, const char *<em>name</em>, size_t <em>bufsize</em>, char *<em>comment</em>)</code> <dd>The comment string for an object is returned through the <em>comment</em> buffer. At most <em>bufsize</em> characters including a null terminator are copied, and the result is not null terminated if the comment is longer than the supplied buffer. If an object doesn't have a comment then the empty string is returned. </dl> <a name="H5GUnlinkToCorrupt"> <h2>6. Unlinking Datasets with H5Gmove and H5Gunlink</h2> </a> <p>Exercise caution in the use of <code>H5Gmove</code> and <code>H5Gunlink</code>. <p>Note that <code>H5Gmove</code> and <code>H5Gunlink</code> each include a step that unlinks pointers to a set or group. If the link that is removed is on the only path leading to a dataset or group, that dataset or group will become inaccessible in the file. <p>Consider the following example. Assume that the group <code>group2</code> can only be accessed via the following path, where <code>top_group</code> is a member of the file's root group: <pre> <code>/top_group/group1/group2/</code> </pre> Using <code>H5Gmove</code>, <code>top_group</code> is renamed to be a member of <code>group2</code>. At this point, since <code>top_group</code> was the only route from the root group to <code>group1</code>, there is no longer a path by which one can access <code>group1</code>, <code>group2</code>, or any member datasets. <code>top_group</code> and any member datasets have also become inaccessible.</p> <!-- #BeginLibraryItem "/ed_libs/NavBar_UG.lbi" --><hr> <center> <table border=0 width=98%> <tr><td valign=top align=left> <a href="index.html">HDF5 documents and links</a> <br> <a href="H5.intro.html">Introduction to HDF5</a> <br> <a href="RM_H5Front.html">HDF5 Reference Manual</a> <br> <a href="http://hdf.ncsa.uiuc.edu/HDF5/doc/UG/index.html">HDF5 User's Guide for Release 1.6</a> <br> <!-- <a href="Glossary.html">Glossary</a><br> --> </td> <td valign=top align=right> And in this document, the <a href="H5.user.html"><strong>HDF5 User's Guide from Release 1.4.5:</strong></a> <br> <a href="Files.html">Files</a> <a href="Datasets.html">Datasets</a> <a href="Datatypes.html">Datatypes</a> <a href="Dataspaces.html">Dataspaces</a> <a href="Groups.html">Groups</a> <br> <a href="References.html">References</a> <a href="Attributes.html">Attributes</a> <a href="Properties.html">Property Lists</a> <a href="Errors.html">Error Handling</a> <br> <a href="Filters.html">Filters</a> <a href="Caching.html">Caching</a> <a href="Chunking.html">Chunking</a> <a href="MountingFiles.html">Mounting Files</a> <br> <a href="Performance.html">Performance</a> <a href="Debugging.html">Debugging</a> <a href="Environment.html">Environment</a> <a href="ddl.html">DDL</a> </td></tr> </table> </center> <hr> <!-- #EndLibraryItem --><!-- #BeginLibraryItem "/ed_libs/Footer.lbi" --><address> <a href="mailto:hdfhelp@ncsa.uiuc.edu">HDF Help Desk</a> <br> Describes HDF5 Release 1.4.5, February 2003 </address><!-- #EndLibraryItem --><!-- Created: Tue Jan 27 09:11:27 EST 1998 --> <!-- hhmts start --> Last modified: 1 November 2000 <!-- hhmts end --> </body> </html>