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<html>
<head><title>
HDF5/H5G Draft API Specification
</title></head>
<body>
<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>
<hr>
</b>UNUSED PORTIONS OF EMAIL NOTES.</b>
<br>
<code>....</code> in left margin indicates where material was pulled out for use.
<HR>
<HR>
<pre>
Hi Elena, here's a hodgepodge of stuff from html documents and
previous e-mails to answer your questions. Let me know if you need
further clarification.
....
.... [H5Gmove and H5Gunlink "NYI" comments were here]
....
Elena> We need little bit more user-friendly description of the H5Gstat and
Elena> H5Giterate functions.
>From a Mar 31 e-mail...
Robb>
....
.... [H5Giterate was here]
....
Robb>
Robb>
Robb> Group Information Functions:
....
.... [H5Gstat was here]
....
Robb>
Robb> herr_t H5Gname(hid_t group_id, char *name, size_t max_name_len);
Robb>
Robb> This function retrieves the name of the group for a group ID. The
Robb> name is returned in 'name' up to the length specified in 'max_name_len'.
Robb>
....
.... [H5Gget_linkval was here]
....
Robb>
Robb> H5Giterate example #1: The operator just looks at the member name and if it has an
Robb> `X' in it then it returns 1, otherwise return zero. Returning 1 (or any
Robb> positive value) causes the iterator to immediately return that value. If
Robb> none of the operators return 1 then the iterator eventually returns zero.
Robb>
Robb> 1 herr_t
Robb> 2 any_X (hid_t group_id/*unused*/,
Robb> 4 const char *member_name,
Robb> 5 void *op_data/*unused*/)
Robb> 6 {
Robb> 7 return strchr(member_name,'X') ? 1 : 0;
Robb> 8 }
Robb>
Robb> The iterator is invoked as:
Robb>
Robb> 9 int elmt=0;
Robb> 10 herr_t found = H5Giterate(file_id, "/foo", &elmt, any_X, NULL);
Robb> 11
Robb> 12 if (found<0) {
Robb> 13 printf ("Error iterating through group, at member %d\n", elmt);
Robb> 14 } else if (found) {
Robb> 15 printf ("Yes, member %d has an `X' in the name\n", elmt);
Robb> 16 } else {
Robb> 17 printf ("No member name contains an `X'\n");
Robb> 18 }
Robb>
Robb> H5Giterate example #2: An iterator to find an object whose name contains an `X'
Robb> This is the same as Example 1 except the operator also returns the name
Robb> that contains the `X'. We do this by passing a pointer to a `char*' as the
Robb> operator data and the operator will initialize that `char*' by strdup'ing
Robb> the object name.
Robb>
Robb> 1 herr_t
Robb> 2 find_X (hid_t group_id/*unused*/,
Robb> 4 const char *member_name,
Robb> 5 void *op_data/*out*/)
Robb> 6 {
Robb> 7 if (strchr(member_name,'X')) {
Robb> 8 *((char**)op_data) = strdup(member_name);
Robb> 9 return 1;
Robb> 10 }
Robb> 11 return 0;
Robb> 12 }
Robb>
Robb> To print all the names with an `X' the iterator is invoked
Robb> repeatedly until it returns zero.
Robb>
Robb> 13 int elmt = 0;
Robb> 14 char *name;
Robb> 15 while (H5Giterate(file_id, "/foo", &elmt, find_X, &name)) {
Robb> 16 puts (name);
Robb> 17 free (name);
Robb> 18 }
Robb>
Robb> H5Giterate example #3: Print all names that contain the specified character.
Robb> This is the same as Example 2 except we have to pass data into the operator
Robb> as well as get data out. We create a struct that contains the input data
Robb> (character to look for) and a slot for the output data (the name) and pass
Robb> that as the operator data.
Robb>
Robb> 1 typedef struct {
Robb> 2 char look_for;
Robb> 3 char *name;
Robb> 4 } find_char_t;
Robb> 5
Robb> 6 herr_t
Robb> 7 find_char (hid_t group_id/*unused*/,
Robb> 9 const char *member_name,
Robb> 10 find_char_t *op_data/*in,out*/)
Robb> 11 {
Robb> 13 if (strchr(member_name, op_data->look_for)) {
Robb> 14 op_data->name = strdup (member_name);
Robb> 15 return 1;
Robb> 16 }
Robb> 17 return 0;
Robb> 18 }
Robb>
Robb> To print all names that have a `Y' one would say
Robb>
Robb> 19 find_char_t op_data;
Robb> 20 int elmt = 0;
Robb> 21 op_data->look_for = 'Y';
Robb> 22 while (H5Giterate(file_id, "/foo", &elmt, find_X, &op_data)) {
Robb> 23 puts (op_data->name);
Robb> 24 free (op_data->name);
Robb> 25 }
Robb>
Robb> H5Giterate example #4: Efficient version of Example 3.
Robb> Examples 2 and 3 weren't too efficient because we kept interrupting the
Robb> iterator and it had to start over, albeit from the middle of the search. If
Robb> we could allow the iterator to go further then we wouldn't end up scanning
Robb> through the leaf nodes of the symbol table so often (searching for a
Robb> particular index, n, in a B-tree is an O(n) operation).
Robb> The H5Glist() function defined earlier returns the names of all the group
Robb> members which belong to a certain class, H5G_ALL, H5G_DATASET, or H5G_GROUP.
Robb> This example shows how to implement that functionality.
Robb> First we need a struct to describe the operator data and it's return
Robb> value(s). These correspond to the arguments presented for H5Glist, whose
Robb> definition is at the end of this example. (Since most of the work is done by
Robb> the operator we have to pass all this stuff back and forth through the
Robb> iterator).
Robb>
Robb> 1 typedef struct {
Robb> 2 size_t name_heap_size;
Robb> 3 char *name_heap;
Robb> 4 unsigned max_entries;
Robb> 5 char *names[];
Robb> 6 int type;
Robb> 7 unsigned cur_entries; /* How many names read so far */
Robb> 8 size_t cur_heap_size; /* How large is the heap */
Robb> 9 } H5G_list_t;
Robb>
Robb> The operator checks if an object is the right type, and if so adds it to
Robb> the return value arrays in the op_data struct. If the arrays become full
Robb> then the operator returns 1 to stop the iterator. The H5*isa() functions
Robb> return true iff the named object is of the * type (sans error handling).
Robb>
Robb> 10 herr_t
Robb> 11 H5G_list_op (hid_t grp_id, const char *memb_name,
Robb> 12 H5G_list_t *op_data)
Robb> 13 {
Robb> 14 char *out_name;
Robb> 15
Robb> 16 if (H5G_ALL==op_data->type ||
Robb> 17 (H5G_DATASET==op_data->type && H5Disa(grp_id,memb_name)) ||
Robb> 18 (H5G_GROUP==op_data->type && H5Gisa(grp_id, memb_name))) {
Robb> 19
Robb> 20 /* Is there enough room for the name in the heap? */
Robb> 21 if (op_data->cur_heap_size + strlen(memb_name) + 1 >
Robb> 22 op_data->name_heap_size) {
Robb> 23 return 2; /*try again later, see lines 59-62*/
Robb> 24 }
Robb> 25
Robb> 26 /* Add name to op_data return value arrays */
Robb> 27 out_name = op_data->name_heap + op_data->cur_heap_size;
Robb> 28 strcpy (out_name, memb_name);
Robb> 29 op_data->names[op_data->cur_entries] = out_name;
Robb> 30 op_data->cur_heap_size += strlen(memb_name) + 1;
Robb> 31
Robb> 32 /* Is the output full? */
Robb> 33 if (op_data->cur_entries >= op_data->max_entries) {
Robb> 34 return 1;
Robb> 35 }
Robb> 36 }
Robb> 37 return 0;
Robb> 38 }
Robb>
Robb> And finally, the definition of H5Glist():
Robb>
Robb> 39 int
Robb> 40 H5Glist (hid_t group_id, size_t name_heap_size, char *name_heap,
Robb> 41 int *start_idx, unsigned max_entries, char *names[],
Robb> 42 int type)
Robb> 43 {
Robb> 44 H5G_list_t op_data;
Robb> 45 herr_t status;
Robb> 46
Robb> 47 op_data->name_heap_size = name_heap_size;
Robb> 48 op_data->name_heap = name_heap;
Robb> 49 op_data->max_entries = max_entries;
Robb> 50 op_data->names = names;
Robb> 51 op_data->type = type;
Robb> 52 op_data->cur_entries = 0;
Robb> 53 op_data->cur_heap_size = 0;
Robb> 54
Robb> 55 if (0==cur_entries) return 0;
Robb> 56 status = H5Giterate (group_id, ".", start_idx, H5G_list_op,
Robb> 57 &op_data);
Robb> 58
Robb> 59 if (2==status && 0==op_data->cur_entries) {
Robb> 60 return -1; /*heap not large enough for even one name*/
Robb> 61 } else if (2==status) {
Robb> 62 --*start_idx; /*heap overflow, try again later*/
Robb> 63 } else if (status<0) {
Robb> 64 return status; /*iterator or operator error*/
Robb> 65 }
Robb> 66 return op_data->cur_entries;
Robb> 67 }
Robb>
Robb> The only other really interesting thing in this example are lines 23, 61,
Robb> and 62. We don't know if the name heap will overflow until we find a name to
Robb> put there. And then if it does, we need to be able to restart the iterator at
Robb> the entry that failed instead of the following entry.
Robb>
Robb> H5Giterate example #5: Print all the names of objects in a group, without any
Robb> buffers.
Robb> ---------------------
Robb>
Robb> herr_t print_names (hid_t grp_id, const char *name, void *opdata)
Robb> {
Robb> puts (name);
Robb> return 0;
Robb> }
Robb>
Robb> {
Robb> H5Giterate (file_id, "/foo/bar", NULL, print_names, NULL);
Robb> }
Elena> I believe there is a typo in the H5Pget_layout function in the source
Elena> code. Second argument ( H5D_layout_t *layout is missing....)
It's returned by value, not reference. It returns only the class of
layout and then, based on the class, you must call some other H5Pget
function like H5Pget_chunk() like this:
hid_t dcpl; /*dataset creation property list*/
hsize_t dims[32]; /*chunk dimensions*/
...
switch (H5Pget_layout(dcpl)) {
case H5D_CHUNKED:
H5Pget_chunk(dcpl, NELMTS(dims), dims);
break;
...
case H5D_LAYOUT_ERROR:
...
...
}
Quincy and html>
Quincy and html>
....
.... [Datatype material move to H5T.html]
....
Quincy and html>
Quincy and html>
</pre>
<hr>
<hr>
<hr>
<address>
<a href="mailto:fbaker@ncsa.uiuc.edu">Frank Baker</a>
<br>
<a href="mailto:h5docs@ncsa.uiuc.edu">HDF5 Documentation</a>
<br>
Last modified: 8 July 1998
</body>
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