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<html>
<head><title>
HDF5/H5G API Specification
</title></head>
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<hr>
<center>
<h1>H5G: Group Interface</h1>
</center>
<h2>Group Object API Functions</h2>
The Group interface functions create and manipulate groups
of objects in an HDF5 file.
<table border=0>
<tr><td valign=top>
<ul>
<li><a href="#Group-Create">H5Gcreate</a>
<li><a href="#Group-Open">H5Gopen</a>
<li><a href="#Group-Close">H5Gclose</a>
</ul>
</td><td> </td><td valign=top>
<ul>
<li><a href="#Group-Link">H5Glink</a>
<li><a href="#Group-Unlink">H5Gunlink</a>
<li><a href="#Group-Iterate">H5Giterate</a>
<li><a href="#Group-Move">H5Gmove</a>
</ul>
</td><td> </td><td valign=top>
<ul>
<li><a href="#Group-GetObjinfo">H5Gget_objinfo</a>
<li><a href="#Group-GetLinkval">H5Gget_linkval</a>
<li><a href="#Group-SetComment">H5Gset_comment</a>
<li><a href="#Group-GetComment">H5Gget_comment</a>
</ul>
</td></tr>
</table>
<p>
A group associates names with objects and provides a mechanism
for mapping a name to an object. Since all objects appear in at
least one group (with the possible exception of the root object)
and since objects can have names in more than one group, the set
of all objects in an HDF5 file is a directed graph. The internal
nodes (nodes with out-degree greater than zero) must be groups
while the leaf nodes (nodes with out-degree zero) are either empty
groups or objects of some other type. Exactly one object in every
non-empty file is the root object. The root object always has a
positive in-degree because it is pointed to by the file boot block.
<p>
An object name consists of one or more components separated from
one another by slashes. An absolute name begins with a slash and the
object is located by looking for the first component in the root
object, then looking for the second component in the first object, etc.,
until the entire name is traversed. A relative name does not begin
with a slash and the traversal begins at the location specified by the
create or access function.
<p>
<hr>
<dl>
<dt><strong>Name:</strong> <a name="Group-Create">H5Gcreate</a>
<dt><strong>Signature:</strong>
<dd><em>hid_t </em><code>H5Gcreate</code>(<em>hid_t</em> <code>loc_id</code>,
<em>const char *</em><code>name</code>,
<em>size_t</em> <code>size_hint</code>
)
<dt><strong>Purpose:</strong>
<dd>Creates a new empty group and gives it a name.
<dt><strong>Description:</strong>
<dd><code>H5Gcreate</code> creates a new group with the specified
name at the specified location, <code>loc_id</code>.
The location is identified by a file or group identifier.
The name, <code>name</code>, must not already be taken by some
other object and all parent groups must already exist.
<p>
<code>size_hint</code> 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 <code>size_hint</code>
is usually adequate since the library is able to dynamically
resize the name heap, but a correct hint may result in better
performance.
If a non-positive value is supplied for <code>size_hint</code>,
then a default size is chosen.
<p>
The return value is a group identifier for the open group.
This group identifier should be closed by calling
<code>H5Gclose()</code> when it is no longer needed.
<dt><strong>Parameters:</strong>
<dl>
<dt><em>hid_t</em> <code>loc_id</code>
<dd>The file or group identifier.
<dt><em>const char *</em><code>name</code>
<dd>The absolute or relative name of the new group.
<dt><em>size_t</em> <code>size_hint</code>
<dd>An optional parameter indicating the number of bytes
to reserve for the names that will appear in the group.
A conservative estimate could result in multiple
system-level I/O requests to read the group name heap;
a liberal estimate could result in a single large
I/O request even when the group has just a few names.
HDF5 stores each name with a null terminator.
</dl>
<dt><strong>Returns:</strong>
<dd>Returns a valid group identifier for the open group if successful;
otherwise returns a negative value.
</dl>
<hr>
<dl>
<dt><strong>Name:</strong> <a name="Group-Open">H5Gopen</a>
<dt><strong>Signature:</strong>
<dd><em>hid_t </em><code>H5Gopen</code>(<em>hid_t</em> <code>loc_id</code>,
<em>const char *</em><code>name</code>
)
<dt><strong>Purpose:</strong>
<dd>Opens an existing group for modification and returns a group
identifier for that group.
<dt><strong>Description:</strong>
<dd><code>H5Gopen</code> opens an existing group with the specified
name at the specified location, <code>loc_id</code>.
<p>
The location is identified by a file or group identifier
<p>
<code>H5Gopen</code> returns a group identifier for the group
that was opened. This group identifier should be released by
calling <code>H5Gclose()</code> when it is no longer needed.
<dt><strong>Parameters:</strong>
<dl>
<dt><em>hid_t</em> <code>loc_id</code>
<dd>File or group identifier within which group is to be open.
<dt><em>const char *</em> <code>name</code>
<dd>Name of group to open.
</dl>
<dt><strong>Returns:</strong>
<dd>Returns a valid group identifier if successful;
otherwise returns a negative value.
</dl>
<hr>
<dl>
<dt><strong>Name:</strong> <a name="Group-Close">H5Gclose</a>
<dt><strong>Signature:</strong>
<dd><em>herr_t </em><code>H5Gclose</code>(<em>hid_t </em><code>group_id</code>)
<dt><strong>Purpose:</strong>
<dd>Closes the specified group.
<dt><strong>Description:</strong>
<dd><code>H5Gclose</code> releases resources used by a group which was
opened by <code>H5Gcreate()</code> or <code>H5Gopen()</code>.
After closing a group, the <code>group_id</code> cannot be used again.
<p>
Failure to release a group with this call will result in resource leaks.
<dt><strong>Parameters:</strong>
<dl>
<dt><em>hid_t</em> <code>group_id</code>
<dd>Group identifier to release.
</dl>
<dt><strong>Returns:</strong>
<dd>Returns a non-negative value if successful;
otherwise returns a negative value.
</dl>
<hr>
<dl>
<dt><strong>Name:</strong> <a name="Group-Link">H5Glink</a>
<dt><strong>Signature:</strong>
<dd><em>herr_t</em> <code>H5Glink</code>(<em>hid_t</em> <code>loc_id</code>,
<em>H5G_link_t</em> <code>link_type</code>,
<em>const char *</em><code>current_name</code>,
<em>const char *</em><code>new_name</code>
)
<dt><strong>Purpose:</strong>
<dd>Creates a link of the specified type from <code>new_name</code>
to <code>current_name</code>.
<dt><strong>Description:</strong>
<dd><code>H5Glink</code> creates a new name for an object that has some current
name, possibly one of many names it currently has.
<p>
If <code>link_type</code> is <code>H5G_LINK_HARD</code>, then
<code>current_name</code> must specify the name of an
existing object and both
names are interpreted relative to <code>loc_id</code>, which is
either a file identifier or a group identifier.
<p>
If <code>link_type</code> is <code>H5G_LINK_SOFT</code>, then
<code>current_name</code> can be anything and is interpreted at
lookup time relative to the group which contains the final
component of <code>new_name</code>. For instance, if
<code>current_name</code> is <code>./foo</code>,
<code>new_name</code> is <code>./x/y/bar</code>, and a request
is made for <code>./x/y/bar</code>, then the actual object looked
up is <code>./x/y/./foo</code>.
<dt><strong>Parameters:</strong>
<dl>
<dt><em>hid_t</em> <code>loc_id</code>
<dd>File or group identifier.
<dt><em>H5G_link_t</em> <code>link_type</code>
<dd>Link type.
Possible values are <code>H5G_LINK_HARD</code> and <code>H5G_LINK_SOFT</code>.
<dt><em>const char *</em> <code>current_name</code>
<dd>Name of the existing object if link is a hard link.
Can be anything for the soft link.
<dt><em>const char *</em> <code>new_name</code>
<dd>New name for the object.
</dl>
<dt><strong>Returns:</strong>
<dd>Returns a non-negative value if successful;
otherwise returns a negative value.
</dl>
<hr>
<dl>
<dt><strong>Name:</strong> <a name="Group-Unlink">H5Gunlink</a>
<dt><strong>Signature:</strong>
<dd><em>herr_t </em><code>H5Gunlink</code>(<em>hid_t</em> <code>loc_id</code>,
<em>const char *</em><code>name</code>
)
<dt><strong>Purpose:</strong>
<dd>Removes the specified <code>name</code> from the group graph and
decrements the link count for the object to which <code>name</code> points
<dt><strong>Description:</strong>
<dd><code>H5Gunlink</code> removes an association between a name and an object.
Object headers keep track of how many hard links refer to the object;
when the hard link count reaches zero, the object can be removed
from the file. Objects which are open are not removed until all
identifiers to the object are closed.
<p>
If the link count reaches zero, all file-space associated with
the object will be reclaimed. If the object is open, the
reclamation of the file space is delayed until all handles to the
object are closed.
<dt><strong>Parameters:</strong>
<dl>
<dt><em>hid_t</em> <code>loc_id</code>
<dd>Identifier of the file containing the object.
<dt><em>const char *</em> <code>name</code>
<dd>Name of the object to unlink.
</dl>
<dt><strong>Returns:</strong>
<dd>Returns a non-negative value if successful;
otherwise returns a negative value.
</dl>
<hr>
<dl>
<dt><strong>Name:</strong> <a name="Group-Iterate">H5Giterate</a>
<dt><strong>Signature:</strong>
<dd><em>int</em> <code>H5Giterate</code>(<em>hid_t</em> <code>loc_id</code>,
<em>const char</em> <code>*name</code>,
<em>int</em> <code>*idx</code>,
<em>H5G_operator_t</em> <code>operator</code>,
<em>void</em> <code>*operator_data</code>
)
<dt><strong>Purpose:</strong>
<dd>Iterates an operation over the entries of a group.
<dt><strong>Description:</strong>
<dd><code>H5Giterate</code> iterates over the members of
<code>name</code> in the file or group specified with
<code>loc_id</code>.
For each object in the group, the <code>operator_data</code>
and some additional information, specified below, are
passed to the <code>operator</code> function.
The iteration begins with the <code>idx</code> object in the
group and the next element to be processed by the operator is
returned in <code>idx</code>. If <code>idx</code>
is NULL, then the iterator starts at the first group member;
since no stopping point is returned in this case, the iterator
cannot be restarted if one of the calls to its operator returns
non-zero.
<p>
The prototype for <code>H5G_operator_t</code> is:
<ul><dl>
<dd><code>typedef</code> <em>herr_t *</em>(<code>H5G_operator_t</code>)(<em>hid_t</em> <code>group_id</code>,
<em>const char *</em><code>member_name</code>, <em>void *</em><code>operator_data/*in,out*/</code>);
</dl></ul>
<dd>The operation receives the group identifier for the group being
iterated over, <code>group_id</code>, the name of the current
object within the group, <code>member_name</code>, and the
pointer to the operator data passed in to <code>H5Giterate</code>,
<code>operator_data</code>.
<p>
The return values from an operator are:
<ul>
<li>Zero causes the iterator to continue, returning
zero when all group members have been processed.
<li>Positive causes the iterator to immediately return that positive
value, indicating short-circuit success. The iterator can be
restarted at the next group member.
<li>Negative causes the iterator to immediately return that value,
indicating failure. The iterator can be restarted at the next
group member.
</ul>
<dt><strong>Parameters:</strong>
<dl>
<dt><em>hid_t</em> <code>loc_id</code>
<dd>IN: File or group identifier.
<dt><em>const char</em> <code>*name</code>
<dd>IN: Group over which the iteration is performed.
<dt><em>int</em> <code>*idx</code>
<dd>IN/OUT: Location at which to begin the iteration.
<dt><em>H5G_iterate_t</em> <code>operator</code>
<dd>IN: Operation to be performed on an object at each step of
the iteration.
<dt><em>void</em> <code>*operator_data</code>
<dd>IN/OUT: Data associated with the operation.
</dl>
<dt><strong>Returns:</strong>
<dd>Returns the return value of the last operator if it was non-zero,
or zero if all group members were processed.
Otherwise returns a negative value.
</dl>
<hr>
<dl>
<dt><strong>Name:</strong> <a name="Group-Move">H5Gmove</a>
<dt><strong>Signature:</strong>
<dd><em>herr_t</em> <code>H5Gmove</code>(<em>hid_t</em> <code>loc_id</code>,
<em>const char</em> <code>*src</code>,
<em>const char</em> <code>*dst</code>
)
<dt><strong>Purpose:</strong>
<dd>Renames an object within an HDF5 file.
<dt><strong>Description:</strong>
<dd><code>H5Gmove</code> renames an object within an HDF5 file.
The original name, <code>src</code>, is unlinked from the
group graph and the new name, <code>dst</code>, is inserted
as an atomic operation. Both names are interpreted relative
to <code>loc_id</code>, which is either a file or a group
identifier.
<dt><strong>Parameters:</strong>
<dl>
<dt><em>hid_t</em> <code>loc_id</code>
<dd>File or group identifier.
<dt><em>const char</em> <code>*src</code>
<dd>Object's original name.
<dt><em>const char</em> <code>*dst</code>
<dd>Object's new name.
</dl>
<dt><strong>Returns:</strong>
<dd>Returns a non-negative value if successful;
otherwise returns a negative value.
</dl>
<hr>
<dl>
<dt><strong>Name:</strong> <a name="Group-GetObjinfo">H5Gget_objinfo</a>
<dt><strong>Signature:</strong>
<dd><em>herr_t</em> <code>H5Gget_objinfo</code>(<em>hid_t</em> <code>loc_id</code>,
<em>const char *</em><code>name</code>,
<em>hbool_t</em> <code>follow_link</code>,
<em>H5G_stat_t *</em><code>statbuf</code>
)
<dt><strong>Purpose:</strong>
<dd>Returns information about an object.
<dt><strong>Description:</strong>
<dd><code>H5Gget_objinfo</code> returns information about the
specified object through the <code>statbuf</code> argument.
<code>loc_id</code> (a file or group identifier) and
<code>name</code> together determine the object.
If the object is a symbolic link and <code>follow_link</code> is
zero (<code>0</code>), then the information returned is that for the link itself;
otherwise the link is followed and information is returned about
the object to which the link points.
If <code>follow_link</code> is non-zero but the final symbolic link
is dangling (does not point to anything), then an error is returned.
The <code>statbuf</code> fields are undefined for an error.
The existence of an object can be tested by calling this function
with a null <code>statbuf</code>.
<p>
<code>H5Gget_objinfo()</code> fills in the following data structure:
<pre>
typedef struct H5G_stat_t {
unsigned long fileno[2];
unsigned long objno[2];
unsigned nlink;
H5G_type_t type;
time_t mtime;
size_t linklen;
} H5G_stat_t
</pre>
The <code>fileno</code> and <code>objno</code> fields contain
four values which uniquely itentify an object among those
HDF5 files which are open: if all four values are the same
between two objects, then the two objects are the same
(provided both files are still open).
The <code>nlink</code> field is the number of hard links to
the object or zero when information is being returned about a
symbolic link (symbolic links do not have hard links but
all other objects always have at least one).
The <code>type</code> field contains the type of the object,
one of
<code>H5G_GROUP</code>,
<code>H5G_DATASET</code>,
<code>H5G_LINK</code>, or
<code>H5G_TYPE</code>.
The <code>mtime</code> field contains the modification time.
If information is being returned about a symbolic link then
<code>linklen</code> will be the length of the link value
(the name of the pointed-to object with the null terminator);
otherwise <code>linklen</code> will be zero.
Other fields may be added to this structure in the future.
<dt><strong>Note:</strong>
<dd>Some systems will be able to record the time accurately but
unable to retrieve the correct time; such systems (e.g., Irix64)
will report an <code>mtime</code> value of 0 (zero).
<dt><strong>Parameters:</strong>
<dl>
<dt><em>hid_t</em> <code>loc_id</code>
<dd>IN: File or group identifier.
<dt><em>const char</em> <code>*name</code>
<dd>IN: Name of the object for which status is being sought.
<dt><em>hbool_t</em> <code>follow_link</code>
<dd>IN: Link flag.
<dt><em>H5G_stat_t</em> <code>*statbuf</code>
<dd>OUT: Buffer in which to return information about the object.
</dl>
<dt><strong>Returns:</strong>
<dd> Returns a non-negative value if successful, with the fields of
<code>statbuf</code> (if non-null) initialized.
Otherwise returns a negative value.
</dl>
<hr>
<dl>
<dt><strong>Name:</strong> <a name="Group-GetLinkval">H5Gget_linkval</a>
<dt><strong>Signature:</strong>
<dd><em>herr_t</em> <code>H5Gget_linkval</code>(<em>hid_t</em> <code>loc_id</code>,
<em>const char *</em><code>name</code>,
<em>size_t</em> <code>size</code>,
<em>char *</em><code>value</code>
)
<dt><strong>Purpose:</strong>
<dd>Returns the name of the object that the symbolic link points to.
<dt><strong>Description:</strong>
<dd><code>H5Gget_linkval</code> returns <code>size</code>
characters of the name of the object that the symbolic link <code>name</code> points to.
<p>
The parameter <code>loc_id</code> is a file or group identifier.
<p>
The parameter <code>name</code> must be a symbolic link pointing to
the desired object and must be defined relative to <code>loc_id</code>.
<p>
If <code>size</code> is smaller than the size of the returned object name, then
the name stored in the buffer <code>value</code> will not be null terminated.
<p>
This function fails if <code>name</code> is not a symbolic link.
The presence of a symbolic link can be tested by passing zero for
<code>size</code> and NULL for <code>value</code>.
<p>
This function should be used only after <code>H5Gget_objinfo()</code> has been called
to verify that <code>name</code> is a symbolic link.
<dt><strong>Parameters:</strong>
<dl>
<dt><em>hid_t</em> <code>loc_id</code>
<dd>IN: Identifier of the file or group.
<dt><em>const char *</em><code>name</code>
<dd>IN: Symbolic link to the object whose name is to be returned.
<dt><em>size_t</em> <code>size</code>
<dd>IN: Maximum number of characters of <code>value</code>
to be returned.
<dt><em>char *</em><code>value</code>
<dd>OUT: A buffer to hold the name of the object being sought.
</dl>
<dt><strong>Returns:</strong>
<dd>Returns a non-negative value, with the link value in <code>value</code>,
if successful.
Otherwise returns a negative value.
</dl>
<hr>
<dl>
<dt><strong>Name:</strong> <a name="Group-SetComment">H5Gset_comment</a>
<dt><strong>Signature:</strong>
<dd><em>herr_t</em> <code>H5Gset_comment</code>(<em>hid_t</em> <code>loc_id</code>,
<em>const char *</em><code>name</code>,
<em>const char *</em><code>comment</code>
)
<dt><strong>Purpose:</strong>
<dd>Sets comment for specified object.
<dt><strong>Description:</strong>
<dd><code>H5Gset_comment</code> sets the comment for the the
object <code>name</code> to <code>comment</code>.
Any previously existing comment is overwritten.
<p>
If <code>comment</code> is the empty string or a
null pointer, the comment message is removed from the object.
<p>
Comments should be relatively short, null-terminated,
ASCII strings.
<p>
Comments can be attached to any object that has an object header,
e.g., data sets, groups, named data types, and data spaces, but
not symbolic links.
<dt><strong>Parameters:</strong>
<dl>
<dt><em>hid_t</em> <code>loc_id</code>
<dd>IN: Identifier of the file or group.
<dt><em>const char *</em><code>name</code>
<dd>IN: Name of the object whose comment is to be set or reset.
<dt><em>const char *</em><code>comment</code>
<dd>IN: The new comment.
</dl>
<dt><strong>Returns:</strong>
<dd>Returns a non-negative value if successful;
otherwise returns a negative value.
</dl>
<hr>
<dl>
<dt><strong>Name:</strong> <a name="Group-GetComment">H5Gget_comment</a>
<dt><strong>Signature:</strong>
<dd><em>herr_t</em> <code>H5Gget_comment</code>(<em>hid_t</em> <code>loc_id</code>,
<em>const char *</em><code>name</code>,
<em>size_t</em> <code>bufsize</code>,
<em>char *</em><code>comment</code>
)
<dt><strong>Purpose:</strong>
<dd>Retrieves comment for specified object.
<dt><strong>Description:</strong>
<dd><code>H5Gget_comment</code> retrieves the comment for the the
object <code>name</code>. The comment is returned in the buffer
<code>comment</code>.
<p>
At most <code>bufsize</code> characters, including a null
terminator, are copied. The result is not null terminated
if the comment is longer than the supplied buffer.
<p>
If an object does not have a comment, the empty string
is returned.
<dt><strong>Parameters:</strong>
<dl>
<dt><em>hid_t</em> <code>loc_id</code>
<dd>IN: Identifier of the file or group.
<dt><em>const char *</em><code>name</code>
<dd>IN: Name of the object whose comment is to be set or reset.
<dt><em>size_t</em> <code>bufsize</code>
<dd>IN: Anticipated size of the buffer required to hold
<code>comment</code>.
<dt><em>char *</em><code>comment</code>
<dd>OUT: The comment.
</dl>
<dt><strong>Returns:</strong>
<dd>Returns the number of characters in the comment,
counting the null terminator, if successful; the value
returned may be larger than <code>bufsize</code>.
Otherwise returns a negative value.
</dl>
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