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<HTML><HEAD>
<TITLE>HDF5 Tutorial - Creating a Group
</TITLE>
</HEAD>
<body bgcolor="#ffffff">
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[ <A HREF="title.html"><I>HDF5 Tutorial Top</I></A> ]
<H1>
<BIG><BIG><BIG><FONT COLOR="#c101cd">Creating a Group</FONT>
</BIG></BIG></BIG></H1>
<hr noshade size=1>
<BODY>
<H2>Contents:</H2>
<UL>
<LI> <A HREF="#def">What is a Group</A>?
<LI> Programming Example
<UL>
<LI> <A HREF="#desc">Description</A>
<LI> <A HREF="#rem">Remarks</A>
<LI> <A HREF="#fc">File Contents</A>
</UL>
</UL>
<HR>
<A NAME="def">
<H2>What is a Group?</h2>
<P>
An HDF5 group is a structure containing zero or more HDF5 objects. The two
primary HDF5 objects are groups and datasets. To create a group, the calling
program must:
<OL>
<LI> Obtain the location identifier where the group is to be created.
<LI> Create the group.
<LI> Close the group.
</OL>
To create a group, the calling program must call
<code>H5Gcreate</code>/<code>h5gcreate_f</code>.
To close the group, <code>H5Gclose</code>/<code>h5gclose_f</code>
must be called. For example:
<P>
<I>C:</I>
<PRE>
group_id = H5Gcreate (loc_id, name, size_hint);
status = H5Gclose (group_id);
</PRE>
<I>FORTRAN:</I>
<PRE>
CALL h5gcreate_f (loc_id, name, group_id, error, size_hint=size)
<i>or</i>
CALL h5gcreate_f (loc_id, name, group_id, error)
CALL h5gclose_f (group_id, error)
</PRE>
<P>
<H2> Programming Example</H2>
<A NAME="desc">
<H3><U>Description</U></H3>
The following example shows how to create and close a group. It creates a file
called <code>group.h5</code> (<code>groupf.h5</code> for FORTRAN),
creates a group called <code>MyGroup</code> in the root group,
and then closes the group and file. <BR>
<UL>
[ <A HREF="examples/h5_crtgrp.c">C Example</A> ]
- <code>h5_crtgrp.c</code><BR>
[ <A HREF="examples/groupexample.f90">FORTRAN Example</A> ]
- <code>groupexample.f90</code><BR>
[ <A HREF="examples/java/CreateGroup.java">Java Example</A> ]
- <code>CreateGroup.java</code>
</UL>
<B>NOTE:</B> To download a tar file of the examples, including a Makefile,
please go to the <A HREF="references.html">References</A> page.
</PRE>
<A NAME="rem">
<H3><U>Remarks</U></H3>
<UL>
<LI><code>H5Gcreate</code>/<code>h5gcreate_f</code> creates
a new empty group, named <code>MyGroup</code> and located in the
root group, and returns a group identifier.
<P>
<I>C:</I>
<PRE>
hid_t H5Gcreate (hid_t loc_id, const char *name, size_t size_hint)
</PRE>
<I>FORTRAN:</I>
<PRE>
h5gcreate_f (loc_id, name, group_id, hdferr, size_hint)
loc_id INTEGER(HID_T)
name CHARACTER(LEN=*)
group_id INTEGER(HID_T)
hdferr INTEGER
(Possible values: 0 on success and -1 on failure)
size_hint INTEGER(SIZE_T), OPTIONAL
(Default value: OBJECT_NAMELEN_DEFAULT_F)
</PRE>
<UL>
<LI>The <I>loc_id</I> parameter specifies the location at which
to create the group.
<P>
<LI> The <I>name</I> parameter specifies the name of the group to be created.
<P>
<LI> The <I>size_hint</I> parameter specifies how much file space to
reserve to store the
names that will appear in the group. If a non-positive value is supplied,
then a default size is used. Passing a value of zero is usually adequate
since the library is able to dynamically resize the name heap.
<P>
<LI>In FORTRAN, the return value for the routine is passed in
<I>hdferr</I>: 0 if successful, -1 otherwise. The group identifier
is passed back in <I>group_id</I>. In C, the function returns a valid
group identifier if successful and a negative value otherwise.
</UL>
<P>
<LI><code>H5Gclose</code>/<code>h5gclose_f</code> closes the group.
This call is mandatory.
<P>
<I>C:</I>
<PRE>
herr_t H5Gclose (hid_t group_id)
</PRE>
<I>FORTRAN:</I>
<PRE>
h5gclose_f (group_id, hdferr)
group_id INTEGER(HID_T)
hdferr INTEGER
(Possible values: 0 on success and -1 on failure)
</PRE>
</UL>
<A NAME="fc">
<H3><U>File Contents</U></H3>
The contents of <code>group.h5</code> and the
definition of the group are shown below. (The FORTRAN program
creates the HDF5 file <code>groupf.h5</code> and the resulting DDL shows
<code>groupf.h5</code> in the first line.)
<P>
<table width="80%" border="1" bordercolor="#FFFFFF" cellpadding="6" cellspacing="6">
<tr valign=top>
<td width="43%"><b>Fig. 8.1</b> <i>The Contents of <code>group.h5</code>.</i>
</td>
<td width="10%">
</td>
<td width="47%"><b>Fig. 8.2</b> <i><code>group.h5</code> in DDL</i> </td>
</tr>
<tr bordercolor="#000000">
<!-- <td width="47%"><IMG src="grouph5.jpg" width="205" height="333"></td> -->
<td align=center><IMG src="img003.gif"></td>
<td bordercolor="#FFFFFF"> </td>
<td valign="top">
<pre>
HDF5 "group.h5" {
GROUP "/" {
GROUP "MyGroup" {
}
}
}
</pre>
</td>
</tr>
</table>
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Describes HDF5 Release 1.2.2, June 2000
<BR> <H6>Last Modified: April 5, 2000</H6><BR>
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