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<html>
<head><title>
HDF5/H5A API Specification
</title></head>
<body bgcolor="#FFFFFF">
<hr>
<center>
<table border=0 width=98%>
<tr><td valign=top align=left>
<a href="H5.intro.html">Introduction to HDF5</a> <br>
<a href="H5.user.html">HDF5 User Guide</a> <br>
<a href="index.html">Other HDF5 documents and links</a> <br>
<!--
<a href="Glossary.html">Glossary</a><br>
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</td>
<td valign=top align=right>
And in this document, the
<a href="RM_H5Front.html">HDF5 Reference Manual</a>
<br>
<a href="RM_H5.html">H5</a>
H5A
<a href="RM_H5D.html">H5D</a>
<a href="RM_H5E.html">H5E</a>
<a href="RM_H5F.html">H5F</a>
<a href="RM_H5G.html">H5G</a>
<a href="RM_H5I.html">H5I</a>
<br>
<a href="RM_H5P.html">H5P</a>
<a href="RM_H5R.html">H5R</a>
<a href="RM_H5RA.html">H5RA</a>
<a href="RM_H5S.html">H5S</a>
<a href="RM_H5T.html">H5T</a>
<a href="RM_H5Z.html">H5Z</a>
<a href="Tools.html">Tools</a>
</td></tr>
</table>
</center>
<hr>
<center>
<h1>H5A: Attribute Interface</h1>
</center>
<h2>Attribute API Functions</h2>
These functions create and manipulate attributes
and information about attributes.
<table border=0>
<tr><td valign=top>
<ul>
<li><a href="#Annot-Create">H5Acreate</a>
<li><a href="#Annot-Write">H5Awrite</a>
<li><a href="#Annot-Read">H5Aread</a>
<li><a href="#Annot-Close">H5Aclose</a>
</ul>
</td><td> </td><td valign=top>
<ul>
<li><a href="#Annot-GetName">H5Aget_name</a>
<li><a href="#Annot-OpenName">H5Aopen_name</a>
<li><a href="#Annot-OpenIdx">H5Aopen_idx</a>
<li><a href="#Annot-GetSpace">H5Aget_space</a>
</ul>
</td><td> </td><td valign=top>
<ul>
<li><a href="#Annot-GetType">H5Aget_type</a>
<li><a href="#Annot-NumAttrs">H5Aget_num_attrs</a>
<li><a href="#Annot-Iterate">H5Aiterate</a>
<li><a href="#Annot-Delete">H5Adelete</a>
</ul>
</td></tr>
</table>
<p>
The Attribute interface, H5A, is primarily designed to easily allow
small datasets to be attached to primary datasets as metadata information.
Additional goals for the H5A interface include keeping storage requirement
for each attribute to a minimum and easily sharing attributes among
datasets.
<p>
Because attributes are intended to be small objects, large datasets
intended as additional information for a primary dataset should be
stored as supplemental datasets in a group with the primary dataset.
Attributes can then be attached to the group containing everything
to indicate a particular type of dataset with supplemental datasets
is located in the group. How small is "small" is not defined by the
library and is up to the user's interpretation.
<p>
See <a href="Attributes.html"><cite>Attributes</cite></a> in the
<a href="H5.user.html"><cite>HDF5 User's Guide</cite></a> for further information.
<hr>
<dl>
<dt><strong>Name:</strong> <a name="Annot-Create">H5Acreate</a>
<dt><strong>Signature:</strong>
<dd><em>hid_t</em> <code>H5Acreate</code>(<em>hid_t</em> <code>loc_id</code>,
<em>const char *</em><code>name</code>,
<em>hid_t</em> <code>type_id</code>,
<em>hid_t</em> <code>space_id</code>,
<em>hid_t</em> <code>create_plist</code>
)
<dt><strong>Purpose:</strong>
<dd>Creates a dataset as an attribute of another group, dataset,
or named datatype.
<dt><strong>Description:</strong>
<dd><code>H5Acreate</code> creates an attribute which is attached
to the object specified with <code>loc_id</code>.
<code>loc_id</code> is an identifier of a group, dataset,
or named datatype. The name specified with <code>name</code>
for each attribute for an object must be unique for that object.
The datatype and dataspace identifiers of the attribute,
<code>type_id</code> and <code>space_id</code>, respectively,
are created with the H5T and H5S interfaces, respectively.
Currently only simple dataspaces are allowed for attribute
dataspaces. The <code>create_plist_id</code> property list
is currently unused, but will be used int the future for optional
properties of attributes. The attribute identifier returned from
this function must be released with <code>H5Aclose</code> or
resource leaks will develop. Attempting to create an attribute
with the same name as an already existing attribute will fail,
leaving the pre-existing attribute in place.
<dt><strong>Parameters:</strong>
<dl>
<dt><em>hid_t</em> <code>loc_id</code>
<dd>IN: Object (dataset, group, or named datatype) to be attached to.
<dt><em>const char *</em><code>name</code>
<dd>IN: Name of attribute to create.
<dt><em>hid_t</em> <code>type_id</code>
<dd>IN: Identifier of datatype for attribute.
<dt><em>hid_t</em> <code>space_id</code>
<dd>IN: Identifier of dataspace for attribute.
<dt><em>hid_t</em> <code>create_plist</code>
<dd>IN: Identifier of creation property list (currently not used).
</dl>
<dt><strong>Returns:</strong>
<dd>Returns an attribute identifier if successful;
otherwise returns a negative value.
</dl>
<hr>
<dl>
<dt><strong>Name:</strong> <a name="Annot-OpenName">H5Aopen_name</a>
<dt><strong>Signature:</strong>
<dd><em>hid_t</em> <code>H5Aopen_name</code>(<em>hid_t</em> <code>loc_id</code>,
<em>const char *</em><code>name</code>
)
<dt><strong>Purpose:</strong>
<dd> Opens an attribute specified by name.
<dt><strong>Description:</strong>
<dd><code>H5Aopen_name</code> opens an attribute specified by
its name, <code>name</code>, which is attached to the
object specified with <code>loc_id</code>.
The location object may be either a group, dataset, or
named datatype, which may have any sort of attribute.
The attribute identifier returned from this function must
be released with <code>H5Aclose</code> or resource leaks
will develop.
<dt><strong>Parameters:</strong>
<dl>
<dt><em>hid_t</em> <code>loc_id</code>
<dd>IN: Identifier of a group, dataset, or named datatype
atttribute to be attached to.
<dt><em>const char *</em><code>name</code>
<dd>IN: Attribute name.
</dl>
<dt><strong>Returns:</strong>
<dd>Returns attribute identifier if successful;
otherwise returns a negative value.
</dl>
<hr>
<dl>
<dt><strong>Name:</strong> <a name="Annot-OpenIdx">H5Aopen_idx</a>
<dt><strong>Signature:</strong>
<dd><em>hid_t</em> <code>H5Aopen_idx</code>(<em>hid_t</em> <code>loc_id</code>,
<em>unsigned int</em> <code>idx</code>
)
<dt><strong>Purpose:</strong>
<dd>Opens the attribute specified by its index.
<dt><strong>Description:</strong>
<dd><code>H5Aopen_idx</code> opens an attribute which is attached
to the object specified with <code>loc_id</code>.
The location object may be either a group, dataset, or
named datatype, all of which may have any sort of attribute.
The attribute specified by the index, <code>idx</code>,
indicates the attribute to access.
The value of <code>idx</code> is a 0-based, non-negative integer.
The attribute identifier returned from this function must be
released with <code>H5Aclose</code> or resource leaks will develop.
<dt><strong>Parameters:</strong>
<dl>
<dt><em>hid_t</em> <code>loc_id</code>
<dd>IN: Identifier of the group, dataset, or named datatype
attribute to be attached to.
<dt><em>unsigned int</em> <code>idx</code>
<dd>IN: Index of the attribute to open.
</dl>
<dt><strong>Returns:</strong>
<dd>Returns attribute identifier if successful;
otherwise returns a negative value.
</dl>
<hr>
<dl>
<dt><strong>Name:</strong> <a name="Annot-Write">H5Awrite</a>
<dt><strong>Signature:</strong>
<dd><em>herr_t</em> <code>H5Awrite</code>(<em>hid_t</em> <code>attr_id</code>,
<em>hid_t</em> <code>mem_type_id</code>,
<em>void *</em><code>buf</code>
)
<dt><strong>Purpose:</strong>
<dd>Writes data to an attribute.
<dt><strong>Description:</strong>
<dd><code>H5Awrite</code> writes an attribute, specified with
<code>attr_id</code>. The attribute's memory datatype
is specified with <code>mem_type_id</code>. The entire
attribute is written from <code>buf</code> to the file.
<p>
Datatype conversion takes place at the time of a read or write
and is automatic. See the
<a href="Datatypes.html#Datatypes-DataConversion">Data Conversion</a>
section of <cite>The Data Type Interface (H5T)</cite> in the
<cite>HDF5 User's Guide</cite> for a discussion of
data conversion, including the range of conversions currently
supported by the HDF5 libraries.
<dt><strong>Parameters:</strong>
<dl>
<dt><em>hid_t</em> <code>attr_id</code>
<dd>IN: Identifier of an attribute to write.
<dt><em>hid_t</em> <code>mem_type_id</code>
<dd>IN: Identifier of the attribute datatype (in memory).
<dt><em>void *</em><code>buf</code>
<dd>IN: Data to be written.
</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="Annot-Read">H5Aread</a>
<dt><strong>Signature:</strong>
<dd><em>herr_t</em> <code>H5Aread</code>(<em>hid_t</em> <code>attr_id</code>,
<em>hid_t</em> <code>mem_type_id</code>,
<em>void *</em><code>buf</code>
)
<dt><strong>Purpose:</strong>
<dd>Reads an attribute.
<dt><strong>Description:</strong>
<dd><code>H5Aread</code> reads an attribute, specified with
<code>attr_id</code>. The attribute's memory datatype
is specified with <code>mem_type_id</code>. The entire
attribute is read into <code>buf</code> from the file.
<p>
Datatype conversion takes place at the time of a read or write
and is automatic. See the
<a href="Datatypes.html#Datatypes-DataConversion">Data Conversion</a>
section of <cite>The Data Type Interface (H5T)</cite> in the
<cite>HDF5 User's Guide</cite> for a discussion of
data conversion, including the range of conversions currently
supported by the HDF5 libraries.
<dt><strong>Parameters:</strong>
<dl>
<dt><em>hid_t</em> <code>attr_id</code>
<dd>IN: Identifier of an attribute to read.
<dt><em>hid_t</em> <code>mem_type_id</code>
<dd>IN: Identifier of the attribute datatype (in memory).
<dt><em>void *</em><code>buf</code>
<dd>IN: Buffer for data to be read.
</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="Annot-GetSpace">H5Aget_space</a>
<dt><strong>Signature:</strong>
<dd><em>hid_t</em> <code>H5Aget_space</code>(<em>hid_t</em> <code>attr_id</code>)
<dt><strong>Purpose:</strong>
<dd>Gets a copy of the dataspace for an attribute.
<dt><strong>Description:</strong>
<dd><code>H5Aget_space</code> retrieves a copy of the dataspace
for an attribute. The dataspace identifier returned from
this function must be released with <code>H5Sclose</code>
or resource leaks will develop.
<dt><strong>Parameters:</strong>
<dl>
<dt><em>hid_t</em> <code>attr_id</code>
<dd>IN: Identifier of an attribute.
</dl>
<dt><strong>Returns:</strong>
<dd>Returns attribute dataspace identifier if successful;
otherwise returns a negative value.
</dl>
<hr>
<dl>
<dt><strong>Name:</strong> <a name="Annot-GetType">H5Aget_type</a>
<dt><strong>Signature:</strong>
<dd><em>hid_t</em> <code>H5Aget_type</code>(<em>hid_t</em> <code>attr_id</code>)
<dt><strong>Purpose:</strong>
<dd>Gets an attribute datatype.
<dt><strong>Description:</strong>
<dd><code>H5Aget_type</code> retrieves a copy of the datatype
for an attribute.
<p>
The datatype is reopened if it is a named type before returning
it to the application. The datatypes returned by this function
are always read-only. If an error occurs when atomizing the
return datatype, then the datatype is closed.
<p>
The datatype identifier returned from this function must be
released with <code>H5Tclose</code> or resource leaks will develop.
<dt><strong>Parameters:</strong>
<dl>
<dt><em>hid_t</em> <code>attr_id</code>
<dd>IN: Identifier of an attribute.
</dl>
<dt><strong>Returns:</strong>
<dd>Returns a datatype identifier if successful;
otherwise returns a negative value.
</dl>
<hr>
<dl>
<dt><strong>Name:</strong> <a name="Annot-GetName">H5Aget_name</a>
<dt><strong>Signature:</strong>
<dd><em>hssize_t</em> <code>H5Aget_name</code>(<em>hid_t</em> <code>attr_id</code>,
<em>char *</em><code>buf</code>,
<em>size_t</em> <code>buf_size</code>
)
<dt><strong>Purpose:</strong>
<dd>Gets an attribute name.
<dt><strong>Description:</strong>
<dd><code>H5Aget_name</code> retrieves the name of an attribute
specified by the identifier, <code>attr_id</code>.
Up to <code>buf_size</code> characters are stored in
<code>buf</code> followed by a <code>\0</code> string
terminator. If the name of the attribute is longer than
<code>buf_size</code> -1, the string terminator is stored in the
last position of the buffer to properly terminate the string.
<dt><strong>Parameters:</strong>
<dl>
<dt><em>hid_t</em> <code>attr_id</code>
<dd>IN: Identifier of the attribute.
<dt><em>char *</em><code>buf</code>
<dd>IN: Buffer to store name in.
<dt><em>size_t</em> <code>buf_size</code>
<dd>IN: The size of the buffer to store the name in.
</dl>
<dt><strong>Returns:</strong>
<dd>Returns the length of the attribute's name, which may be
longer than <code>buf_size</code>, if successful.
Otherwise returns a negative value.
</dl>
<hr>
<dl>
<dt><strong>Name:</strong> <a name="Annot-NumAttrs">H5Aget_num_attrs</a>
<dt><strong>Signature:</strong>
<dd><em>int</em> <code>H5Aget_num_attrs</code>(<em>hid_t</em> <code>loc_id</code>)
<dt><strong>Purpose:</strong>
<dd>Determines the number of attributes attached to an object.
<dt><strong>Description:</strong>
<dd><code>H5Aget_num_attrs</code> returns the number of attributes
attached to the object specified by its identifier,
<code>loc_id</code>.
The object can be a group, dataset, or named datatype.
<dt><strong>Parameters:</strong>
<dl>
<dt><em>hid_t</em> <code>loc_id</code>
<dd>IN: Identifier of a group, dataset, or named datatype.
</dl>
<dt><strong>Returns:</strong>
<dd>Returns the number of attributes if successful;
otherwise returns a negative value.
</dl>
<hr>
<dl>
<dt><strong>Name:</strong> <a name="Annot-Iterate">H5Aiterate</a>
<dt><strong>Signature:</strong>
<dd><em>int</em> <code>H5Aiterate</code>(<em>hid_t</em> <code>loc_id</code>,
<em>unsigned *</em> <code>idx</code>,
<em>H5A_operator_t</em> <code>op</code>,
<em>void *</em><code>op_data</code>
)
<dt><strong>Purpose:</strong>
<dd>Calls a user's function for each attribute on an object.
<dt><strong>Description:</strong>
<dd><code>H5Aiterate</code> iterates over the attributes of
the object specified by its identifier, <code>loc_id</code>.
The object can be a group, dataset, or named datatype.
For each attribute of the object, the <code>op_data</code>
and some additional information specified below are passed
to the operator function <code>op</code>.
The iteration begins with the attribute specified by its
index, <code>idx</code>; the index for the next attribute
to be processed by the operator, <code>op</code>, is
returned in <code>idx</code>.
If <code>idx</code> is the null pointer, then all attributes
are processed.
<p>
The prototype for <code>H5A_operator_t</code> is: <br>
<code>typedef herr_t (*H5A_operator_t)(hid_t <em>loc_id</em>,
const char *<em>attr_name</em>,
void *<em>operator_data</em>);
</code>
<p>
The operation receives the identifier for the group, dataset
or named datatype being iterated over, <code>loc_id</code>, the
name of the current attribute about the object, <code>attr_name</code>,
and the pointer to the operator data passed in to <code>H5Aiterate</code>,
<code>op_data</code>. The return values from an operator are:
<ul>
<li>Zero causes the iterator to continue, returning zero when all
attributes 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 attribute.
<li>Negative causes the iterator to immediately return that value,
indicating failure. The iterator can be restarted at the next
attribute.
</ul>
<dt><strong>Parameters:</strong>
<dl>
<dt><em>hid_t</em> <code>loc_id</code>
<dd>IN: Identifier of a group, dataset or named datatype.
<dt><em>unsigned *</em> <code>idx</code>
<dd>IN/OUT: Starting (IN) and ending (OUT) attribute index.
<dt><em>H5A_operator_t</em> <code>op</code>
<dd>IN: User's function to pass each attribute to
<dt><em>void *</em><code>op_data</code>
<dd>IN/OUT: User's data to pass through to iterator operator function
</dl>
<dt><strong>Returns:</strong>
<dd>If successful, returns the return value of the last operator
if it was non-zero, or zero if all attributes were processed.
Otherwise returns a negative value.
</dl>
<hr>
<dl>
<dt><strong>Name:</strong> <a name="Annot-Delete">H5Adelete</a>
<dt><strong>Signature:</strong>
<dd><em>herr_t</em> <code>H5Adelete</code>(<em>hid_t</em> <code>loc_id</code>,
<em>const char *</em><code>name</code>
)
<dt><strong>Purpose:</strong>
<dd>Deletes an attribute from a location.
<dt><strong>Description:</strong>
<dd><code>H5Adelete</code> removes the attribute specified by its
name, <code>name</code>, from a dataset, group, or named datatype.
This function should not be used when attribute identifiers are
open on <code>loc_id</code> as it may cause the internal indexes
of the attributes to change and future writes to the open
attributes to produce incorrect results.
<dt><strong>Parameters:</strong>
<dl>
<dt><em>hid_t</em> <code>loc_id</code>
<dd>IN: Identifier of the dataset, group, or named datatype
to have the attribute deleted from.
<dt><em>const char *</em><code>name</code>
<dd>IN: Name of the attribute to delete.
</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="Annot-Close">H5Aclose</a>
<dt><strong>Signature:</strong>
<dd><em>herr_t</em> <code>H5Aclose</code>(<em>hid_t</em> <code>attr_id</code>)
<dt><strong>Purpose:</strong>
<dd>Closes the specified attribute.
<dt><strong>Description:</strong>
<dd><code>H5Aclose</code> terminates access to the attribute
specified by its identifier, <code>attr_id</code>.
Further use of the attribute identifier will result in
undefined behavior.
<dt><strong>Parameters:</strong>
<dl>
<dt><em>hid_t</em> <code>attr_id</code>
<dd>IN: Attribute to release access to.
</dl>
<dt><strong>Returns:</strong>
<dd>Returns a non-negative value if successful;
otherwise returns a negative value.
</dl>
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Last modified: 30 October 1998
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