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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>&nbsp;
<a href="RM_H5.html">H5</a>&nbsp;&nbsp;
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<!--
<a href="Glossary.html">Glossary</a>
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</center>
<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>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</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>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</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, group, dataset, or datatype 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, group, dataset, or 
        named datatype 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>, 
        or <code>H5G_LINK</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, group, dataset, or datatype 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 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>H5Gget_objinfo()</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, group, dataset, or datatype.
      <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 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, group, dataset, or datatype.
      <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, group, dataset, or datatype.
      <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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