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<html>
<head><title>
HDF5/H5A 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;
H5A&nbsp;&nbsp;
<a href="RM_H5D.html">H5D</a>&nbsp;&nbsp;
<a href="RM_H5E.html">H5E</a>&nbsp;&nbsp;
<a href="RM_H5F.html">H5F</a>&nbsp;&nbsp;
<a href="RM_H5G.html">H5G</a>&nbsp;&nbsp;
<a href="RM_H5P.html">H5P</a>&nbsp;&nbsp;
<a href="RM_H5S.html">H5S</a>&nbsp;&nbsp;
<a href="RM_H5T.html">H5T</a>&nbsp;&nbsp;
<a href="RM_H5Z.html">H5Z</a>&nbsp;&nbsp;
<a href="Tools.html">Tools</a>&nbsp;&nbsp;
<!--
<a href="Glossary.html">Glossary</a>
-->
</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>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</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>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</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 FAIL (-1).
</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 FAIL (-1).
</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 FAIL (-1).
</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 SUCCEED (0) if successful;
        otherwise FAIL (-1).
</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 SUCCEED (0) if successful;
        otherwise FAIL (-1).
</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 FAIL (-1).
</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 FAIL (-1).
</dl>


<hr>
<dl>
<dt><strong>Name:</strong> <a name="Annot-GetName">H5Aget_name</a>
<dt><strong>Signature:</strong>
    <dd><em>size_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 FAIL (-1).
</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 FAIL (-1).
</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 FAIL (-1).
</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 SUCCEED (0) if successful;
        otherwise FAIL (-1).
</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 SUCCEED (0) if successful;
        otherwise FAIL (-1).
</dl>


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Last modified:  1 September 1998

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