cd3b105997
Intro
======
H5.intro.html
Major rewrite to Groups section. New Example 7 (groups).
Added TOC and requisite links.
Numbered sections.
Labelled figures and centered those that were not.
Fixed table formatting.
===========
User Guide
===========
H5.user.html
Linked in Chunking.html.
Linked in References.html.
Linked in DDL.html.
Chunking.html
Minor edits.
DDL.html
References.html
New documents.
Datatypes.html
Added "R Reference" to base name description and
"H5T_STD_ROBJ -- Reference to an entire object
in a file" to list of datatype names.
Files.html
H5Fflush
Added scope parameter.
Groups.html
Removed references to "current working group."
Removed H5Gpush, H5Gpop, and H5Gset functions.
Removed note that H5Glink and H5Gunlink were not implemented.
=================
Reference Manual
=================
RM_*.html and Tools.html
Updated Reference Manual internal cross-linking (the link
banner at the top and bottom of each page).
Changed
Returns SUCCEED (0) if successful;
otherwise FAIL (-1).
to read
Returns a non-negative value if successful;
otherwise returns a negative value.
and several derived changes where circumstances differred
only slightly.
Minor copy edits throughout.
RM_H5.html
Corrected H5open "Purpose" statement.
RM_H5A.html
Changed H5Aget_name return type to hssize_t.
RM_H5F.html
H5Fflush
Added scope parameter.
Added H5Freopen.
RM_H5Front.html
Reordered listing of interfaces to alphabetical order (H5,
H5A, H5D, ...)
Added H5I, H5R, and H5RA.
RM_H5G.html
H5Gopen
Edited "Description."
H5Gget_objinfo
Added named datatype to list of valid values for loc_id.
RM_H5I.html Identifier Interface
New section.
RM_H5P.html
Added H5Pset_fill_value and H5Pget_fill_value.
Several minor copy edits.
RM_H5R.html Reference Interface
New section.
H5RA.html
Essentially a new section. It was in the tree previously,
but it did not actually have content.
RM_H5S.html
Changed H5Sget_select_npoints return type to hssize_t.
Tools.html
Updated h5dump documentation.
279 lines
9.6 KiB
HTML
279 lines
9.6 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN">
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<html>
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<head>
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<title>Groups</title>
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</head>
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<body>
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<h1>Groups</h1>
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<h2>1. Introduction</h2>
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<p>An object in HDF5 consists of an object header at a fixed file
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address that contains messages describing various properties of
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the object such as its storage location, layout, compression,
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etc. and some of these messages point to other data such as the
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raw data of a dataset. The address of the object header is also
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known as an <em>OID</em> and HDF5 has facilities for translating
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names to OIDs.
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<p>Every HDF5 object has at least one name and a set of names can
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be stored together in a group. Each group implements a name
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space where the names are any length and unique with respect to
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other names in the group.
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<p>Since a group is a type of HDF5 object it has an object header
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and a name which exists as a member of some other group. In this
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way, groups can be linked together to form a directed graph.
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One particular group is called the <em>Root Group</em> and is
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the group to which the HDF5 file boot block points. Its name is
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"/" by convention. The <em>full name</em> of an object is
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created by joining component names with slashes much like Unix.
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<p>
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<center>
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<img alt="Group Graph Example" src="group_p1.gif">
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</center>
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<p>However, unlike Unix which arranges directories hierarchically,
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HDF5 arranges groups in a directed graph. Therefore, there is
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no ".." entry in a group since a group can have more than one
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parent. There is no "." entry either but the library understands
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it internally.
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<h2>2. Names</h2>
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<p>HDF5 places few restrictions on names: component names may be
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any length except zero and may contain any character except
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slash ("/") and the null terminator. A full name may be
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composed of any number of component names separated by slashes,
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with any of the component names being the special name ".". A
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name which begins with a slash is an <em>absolute</em> name
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which is looked up beginning at the root group of the file while
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all other <em>relative</em> names are looked up beginning at the
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specified group.
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Multiple consecutive slashes in a full name are treated as
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single slashes and trailing slashes are not significant. A
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special case is the name "/" (or equivalent) which refers to the
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root group.
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<p>Functions which operate on names generally take a location
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identifier which is either a file ID or a group ID and perform
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the lookup with respect to that location. Some possibilities
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are:
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<p>
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<center>
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<table border cellpadding=4>
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<tr>
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<th>Location Type</th>
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<th>Object Name</th>
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<th>Description</th>
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</tr>
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<tr>
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<td>File ID</td>
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<td><code>/foo/bar</code></td>
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<td>The object <code>bar</code> in group <code>foo</code>
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in the root group of the specified file.</td>
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</tr>
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<tr>
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<td>Group ID</td>
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<td><code>/foo/bar</code></td>
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<td>The object <code>bar</code> in group <code>foo</code>
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in the root group of the file containing the specified
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group. In other words, the group ID's only purpose is
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to supply a file.</td>
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</tr>
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<tr>
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<td>File ID</td>
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<td><code>/</code></td>
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<td>The root group of the specified file.</td>
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</tr>
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<tr>
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<td>Group ID</td>
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<td><code>/</code></td>
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<td>The root group of the file containing the specified
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group.</td>
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</tr>
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<tr>
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<td>File ID</td>
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<td><code>foo/bar</code></td>
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<td>The object <code>bar</code> in group <code>foo</code>
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in the specified group.</td>
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</tr>
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<tr>
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<td>Group ID</td>
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<td><code>foo/bar</code></td>
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<td>The object <code>bar</code> in group <code>foo</code>
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in the specified group.</td>
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</tr>
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<tr>
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<td>File ID</td>
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<td><code>.</code></td>
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<td>The specified file.</td>
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</tr>
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<tr>
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<td>Group ID</td>
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<td><code>.</code></td>
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<td>The specified group.</td>
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</tr>
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<tr>
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<td>Other ID</td>
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<td><code>.</code></td>
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<td>The specified object.</td>
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</tr>
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</table>
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</center>
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<h2>3. Creating, Opening, and Closing Groups</h2>
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<p>Groups are created with the <code>H5Gcreate()</code> function,
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and existing groups can be access with
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<code>H5Gopen()</code>. Both functions return an object ID which
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should be eventually released by calling
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<code>H5Gclose()</code>.
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<dl>
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<dt><code>hid_t H5Gcreate (hid_t <em>location_id</em>, const char
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*<em>name</em>, size_t <em>size_hint</em>)</code>
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<dd>This function creates a new group with the specified
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name at the specified location which is either a file ID or a
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group ID. The name must not already be taken by some other
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object and all parent groups must already exist. The
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<em>size_hint</em> is a hint for the number of bytes to
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reserve to store the names which will be eventually added to
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the new group. Passing a value of zero for <em>size_hint</em>
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is usually adequate since the library is able to dynamically
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resize the name heap, but a correct hint may result in better
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performance. The return value is a handle for the open group
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and it should be closed by calling <code>H5Gclose()</code>
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when it's no longer needed. A negative value is returned for
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failure.
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<br><br>
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<dt><code>hid_t H5Gopen (hid_t <em>location_id</em>, const char
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*<em>name</em>)</code>
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<dd>This function opens an existing group with the specified
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name at the specified location which is either a file ID or a
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group ID and returns an object ID. The object ID should be
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released by calling <code>H5Gclose()</code> when it is no
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longer needed. A negative value is returned for failure.
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<br><br>
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<dt><code>herr_t H5Gclose (hid_t <em>group_id</em>)</code>
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<dd>This function releases resources used by an group which was
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opened by <code>H5Gcreate()</code> or
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<code>H5Gopen()</code>. After closing a group the
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<em>group_id</em> should not be used again. This function
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returns zero for success or a negative value for failure.
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</dl>
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<h2>4. Objects with Multiple Names</h2>
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<p>An object (including a group) can have more than one
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name. Creating the object gives it the first name, and then
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functions described here can be used to give it additional
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names. The association between a name and the object is called
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a <em>link</em> and HDF5 supports two types of links: a <em>hard
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link</em> is a direct association between the name and the
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object where both exist in a single HDF5 address space, and a
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<em>soft link</em> is an indirect association.
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<p>
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<center>
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<img alt="Hard Link Example" src="group_p2.gif">
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</center>
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<p>
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<center>
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<img alt="Soft Link Example" src="group_p3.gif">
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</center>
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<dl>
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<dt>Object Creation</dt>
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<dd>The creation of an object creates a hard link which is
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indistinguishable from other hard links that might be added
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later.
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<br><br>
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<dt><code>herr_t H5Glink (hid_t <em>file_id</em>, H5G_link_t
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<em>link_type</em>, const char *<em>current_name</em>,
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const char *<em>new_name</em>)</code>
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<dd>Creates a new name for an object that has some current name
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(possibly one of many names it currently has). If the
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<em>link_type</em> is <code>H5G_LINK_HARD</code> then a new
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hard link is created. Otherwise if <em>link_type</em> is
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<code>H5T_LINK_SOFT</code> a soft link is created which is an
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alias for the <em>current_name</em>. When creating a soft
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link the object need not exist. This function returns zero
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for success or negative for failure.
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<br><br>
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<dt><code>herr_t H5Gunlink (hid_t <em>file_id</em>, const char
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*<em>name</em>)</code>
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<dd>This function removes an association between a name and an
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object. Object headers keep track of how many hard links refer
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to the object and when the hard link count reaches zero the
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object can be removed from the file (but objects which are
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open are not removed until all handles to the object are
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closed).
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</dl>
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<h2>5. Comments</h2>
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<p>Objects can have a comment associated with them. The comment
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is set and queried with these two functions:
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<dl>
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<dt><code>herr_t H5Gset_comment (hid_t <em>loc_id</em>, const
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char *<em>name</em>, const char *<em>comment</em>)</code>
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<dd>The previous comment (if any) for the specified object is
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replace with a new comment. If the <em>comment</em> argument
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is the empty string or a null pointer then the comment message
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is removed from the object. Comments should be relatively
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short, null-terminated, ASCII strings.
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<br><br>
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<dt><code>herr_t H5Gget_comment (hid_t <em>loc_id</em>, const
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char *<em>name</em>, size_t <em>bufsize</em>, char
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*<em>comment</em>)</code>
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<dd>The comment string for an object is returned through the
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<em>comment</em> buffer. At most <em>bufsize</em> characters
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including a null terminator are copied, and the result is
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not null terminated if the comment is longer than the supplied
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buffer. If an object doesn't have a comment then the empty
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string is returned.
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</dl>
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<!--
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<hr>
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<address><a href="mailto:matzke@llnl.gov">Robb Matzke</a></address>
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-->
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<!-- Created: Tue Jan 27 09:11:27 EST 1998 -->
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<!-- hhmts start -->
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<!--
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Last modified: Wed Jul 22 14:24:34 EDT 1998
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-->
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<!-- hhmts end -->
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<hr>
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<address>
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<a href="mailto:hdfhelp@ncsa.uiuc.edu">HDF Help Desk</a>
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</address>
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Last modified: 29 October 1998
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</body>
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</html>
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