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<html>
<head><title>
HDF5/H5G Draft API Specification
</title></head>
<body>
<hr>
<center>
<a href="RM_H5Front.html">HDF5 Reference Manual</a>
<a href="RM_H5.html">H5</a>
<a href="RM_H5A.html">H5A</a>
<a href="RM_H5D.html">H5D</a>
<a href="RM_H5E.html">H5E</a>
<a href="RM_H5F.html">H5F</a>
H5G
<a href="RM_H5P.html">H5P</a>
<a href="RM_H5S.html">H5S</a>
<a href="RM_H5T.html">H5T</a>
<a href="RM_H5Z.html">H5Z</a>
<a href="Glossary.html">Glossary</a>
</center>
<hr>
<center>
<h1>H5G: Group Interface</h1>
</center>
<h2>Group Object API Functions</h2>
The Group interface functions create and manipulate physical groups
of objects on disk.
<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-Set">H5Gset</a>
<li><a href="#Group-Close">H5Gclose</a>
</ul>
</td><td> </td><td valign=top>
<ul>
<li><a href="#Group-Push">H5Gpush</a>
<li><a href="#Group-Pop">H5Gpop</a>
<li><a href="#Group-Link">H5Glink</a>
<li><a href="#Group-Unlink">H5Gunlink</a> (NYI)
</ul>
</td><td> </td><td valign=top>
<ul>
<li><a href="#Group-Iterate">H5Giterate</a>
<li><a href="#Group-Move">H5Gmove</a> (NYI)
<li><a href="#Group-Stat">H5Gstat</a>
<li><a href="#Group-GetLinkval">H5Gget_linkval</a>
</ul>
</td></tr><tr><td colspan=5 align=right>
<font size=-2>(NYI = Not yet implemented)</font>
</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>
Every file identifier returned by <code>H5Fcreate</code> or
<code>H5Fopen</code> maintains an independent current working group
stack, the top item of which is the current working group. The
stack can be manipulated with <code>H5Gset</code>, <code>H5Gpush</code>,
and <code>H5Gpop</code>. The root object is the current working group
if the stack is empty.
<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 with the current working group.
<p>
The library does not maintain the full absolute name of its current
working group because (1) cycles in the graph can make the name length
unbounded and (2) a group does not necessarily have a unique name. A
more Unix-like hierarchical naming scheme can be implemented on top of
the directed graph scheme by creating a ".." entry in each group that
points to its single predecessor; a <code>getcwd</code> function would
then be trivial.
<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 size_hint,
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 FAIL (-1).
</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>.
The location is identified by a file or
group identifier, and returns a group identifier for the group.
The obtained 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 FAIL (-1).
</dl>
<hr>
<dl>
<dt><strong>Name:</strong> <a name="Group-Set">H5Gset</a>
<dt><strong>Signature:</strong>
<dd><em>herr_t </em><code>H5Gset</code> (<em>hid_t</em> <code>loc_id</code>,
<em>const char *</em><code>name</code>
)
<dt><strong>Purpose:</strong>
<dd>Sets the current working group within a file.
<dt><strong>Description:</strong>
<dd><code>H5Gset</code> sets the group with the specified name
to be the current working group for the file which contains it.
This function sets the current working group by modifying the
top element of the current working group stack or, if the
stack is empty, by pushing a new element onto the stack.
The initial current working group is the root group.
<p>
<code>loc_id</code> can be a file identifier or a group identifier.
<p>
<code>name</code> is an absolute or relative name and is resolved as follows. Each file identifier
has a current working group, initially the root group of the
file. Relative names do not begin with a slash and are relative
to the specified group or to the current working group.
Absolute names begin with a slash and are relative to the file's
root group. For instance, the name <code>/Foo/Bar/Baz</code> is
resolved by first looking up <code>Foo</code> in the root group;
the name <code>Foo/Bar/Baz</code> is resolved by first looking
up the name <code>Foo</code> in the current working group.
<p>
Each file identifier maintains its own notion of the current
working group. If <code>loc_id</code> is a group identifier, the
file identifier is derived from the group identifier.
<p>
If a single file is opened with multiple calls to <code>H5Fopen()</code>,
which would return multiple file identifiers, then each
identifier's current working group can be set independently
of the other file identifiers for that file.
<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 name of the new current working group.
</dl>
<dt><strong>Returns:</strong>
<dd>Returns SUCCEED (0) if successful;
otherwise FAIL (-1).
</dl>
<hr>
<dl>
<dt><strong>Name:</strong> <a name="Group-Push">H5Gpush</a>
<dt><strong>Signature:</strong>
<dd><em>herr_t </em><code>H5Gpush</code> (<em>hid_t</em> <code>loc_id</code>,
<em>const char *</em><code>name</code>
)
<dt><strong>Purpose:</strong>
<dd>Sets the current working group by pushing a
new element onto the current working group stack.
<dt><strong>Description:</strong>
<dd>Each file identifier maintains a stack of groups, the top group
of which is the current working group. The stack initially
contains only the root group. <code>H5Gpush</code> pushes a new group
onto the stack, thus setting a new current working group.
<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>name</code>
<dd>The name of the new current working group. The name may be
an absolute or relative name.
</dl>
<dt><strong>Returns:</strong>
<dd>Returns SUCCEED (0) if successful;
otherwise FAIL (-1).
</dl>
<hr>
<dl>
<dt><strong>Name:</strong> <a name="Group-Pop">H5Gpop</a>
<dt><strong>Signature:</strong>
<dd><em>herr_t </em><code>H5Gpop</code> (<em>hid_t</em> <code>loc_id</code>)
<dt><strong>Purpose:</strong>
<dd>Removes the top, or latest, entry from the working group stack,
setting the current working group to the previous value.
<dt><strong>Description:</strong>
<dd><code>H5Gpop</code> restores the previous current working group by
popping an element from the current working group stack.
An empty stack implies that the current working group is the root
object. Attempting to pop an empty stack results in failure.
<p>
Each file identfier maintains its own notion of the current
working group. That is, if a single file is opened with
multiple calls to <code>H5Fopen()</code>, which returns multiple file
handles, then each identfier's current working group can be
set independently of the other file identfiers for that file.
<p>
If <code>loc_id</code> is a group identifier, it is used only to determine the
file identifier for the stack from which to pop the top entry.
<dt><strong>Parameters:</strong>
<dl>
<dt><em>hid_t</em> <code>loc_id</code>
<dd>The file or group identifier.
</dl>
<dt><strong>Returns:</strong>
<dd>Returns SUCCEED (0) if successful;
otherwise FAIL (-1).
</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 SUCCEED (0) if successful;
otherwise FAIL (-1).
</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 name 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 SUCCEED (0) if successful;
otherwise FAIL (-1).
</dl>
<hr>
<dl>
<dt><strong>Name:</strong> <a name="Group-Unlink">H5Gunlink</a>
<strong>(Not implemented in this release.)</strong>
<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 SUCCEED (0) if successful;
otherwise FAIL (-1).
</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 FAIL (-1).
</dl>
<hr>
<dl>
<dt><strong>Name:</strong> <a name="Group-Move">H5Gmove</a>
<strong>(Not implemented in this release.)</strong>
<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 SUCCEED (0) if successful;
otherwise FAIL (-1).
</dl>
<hr>
<dl>
<dt><strong>Name:</strong> <a name="Group-Stat">H5Gstat</a>
<dt><strong>Signature:</strong>
<dd><em>herr_t</em> <code>H5Gstat</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>H5Gstat</code> returns information about the
specified object through the <code>statbuf</code> argument.
<code>loc_id</code> (a file, group, or dataset 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>H5Gstat()</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;
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>,
or <code>H5G_LINK</code>.
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>Parameters:</strong>
<dl>
<dt><em>hid_t</em> <code>loc_id</code>
<dd>IN: File, group, or dataset 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 SUCCEED (0) with the fields of STATBUF (if non-null) initialized.
Otherwise returns FAIL (-1).
</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 link value.
<dt><strong>Description:</strong>
<dd><code>H5Gget_linkval</code> returns <code>size</code>
characters of the link value through the <code>value</code>
argument if <code>loc_id</code> (a file or group identifier)
and <code>name</code> specify a symbolic link.
If <code>size</code> is smaller than the link value, then
<code>value</code> will not be null terminated.
<p>
This function fails if the specified object 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>
Use <code>H5Gstat()</code> to get the size of a link value.
<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 link value is to be checked.
<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: Link value.
</dl>
<dt><strong>Returns:</strong>
<dd>Returns SUCCEED (0), with the link value in <code>value</code>,
if successful.
Otherwise returns FAIL (-1).
</dl>
<hr>
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