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<title>Attribute Interface (H5A)</title>
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<!-- #EndLibraryItem --><h1>The Attribute Interface (H5A)</h1>
<h2>1. Introduction</h2>
<p>The attribute API (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
requirements 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>Attributes are not separate objects in the file, they are always
contained in the object header of the object they are attached to. The
I/O functions defined below are required to read or write attribute
information, not the H5D I/O routines.
<h2>2. Creating, Opening, Closing and Deleting Attributes</h2>
<p>Attributes are created with the <code>H5Acreate()</code> function,
and existing attributes can be accessed with either the
<code>H5Aopen_name()</code> or <code>H5Aopen_idx()</code> functions. All
three functions return an object ID which should be eventually released
by calling <code>H5Aclose()</code>.
<dl>
<dt><code>hid_t H5Acreate (hid_t <em>loc_id</em>, const char
*<em>name</em>, hid_t <em>type_id</em>, hid_t <em>space_id</em>,
hid_t <em>create_plist_id</em>)</code>
<dd>This function creates an attribute which is attached to the object
specified with <em>loc_id</em>. The name specified with <em>name</em>
for each attribute for an object must be unique for that object. The <em>type_id</em>
and <em>space_id</em> are created with the H5T and H5S interfaces
respectively. Currently only simple dataspaces are allowed for attribute
dataspaces. The <em>create_plist_id</em> property list is currently
unused, but will be used in the future for optional properties of
attributes. The attribute ID returned from this function must be released
with H5Aclose 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.
This function returns an attribute ID for success or negative for failure.
<br><br>
<dt><code>hid_t H5Aopen_name (hid_t <em>loc_id</em>, const char
*<em>name</em>)</code>
<dd> This function opens an attribute which is attached to the object
specified with <em>loc_id</em>. The name specified with <em>name</em>
indicates the attribute to access. The attribute ID returned from this
function must be released with H5Aclose or resource leaks will develop.
This function returns an attribute ID for success or negative for failure.
<br><br>
<dt><code>hid_t H5Aopen_idx (hid_t <em>loc_id</em>, unsigned
<em>idx</em>)</code>
<dd>This function opens an attribute which is attached to the object
specified with <em>loc_id</em>. The attribute specified with <em>idx</em>
indicates the <em>idx</em>th attribute to access, starting with '0'. The
attribute ID returned from this function must be released with H5Aclose or
resource leaks will develop.
This function returns an attribute ID for success or negative for failure.
<br><br>
<dt><code>herr_t H5Aclose (hid_t <em>attr_id</em>)</code>
<dd>This function releases an attribute from use. Further use of the
attribute ID will result in undefined behavior.
This function returns non-negative on success, negative on failure.
<br><br>
<dt><code>herr_t H5Adelete (hid_t <em>loc_id</em>,
const char *<em>name</em>)</code>
<dd>This function removes the named attribute from a dataset or group.
This function should not be used when attribute IDs are open on <em>loc_id</em>
as it may cause the internal indexes of the attributes to change and future
writes to the open attributes to produce incorrect results.
Returns non-negative on success, negative on failure.
</dl>
<h2>3. Attribute I/O Functions</h2>
<p>Attributes may only be written as an entire object, no partial I/O
is currently supported.
<dl>
<dt><code>herr_t H5Awrite (hid_t <em>attr_id</em>,
hid_t <em>mem_type_id</em>, void *<em>buf</em>)</code>
<dd>This function writes an attribute, specified with <em>attr_id</em>,
with <em>mem_type_id</em> specifying the datatype in memory. The entire
attribute is written from <em>buf</em> to the file.
This function returns non-negative on success, negative on failure.
<br><br>
<dt><code>herr_t H5Aread (hid_t <em>attr_id</em>,
hid_t <em>mem_type_id</em>, void *<em>buf</em>)</code>
<dd>This function read an attribute, specified with <em>attr_id</em>, with
<em>mem_type_id</em> specifying the datatype in memory. The entire
attribute is read into <em>buf</em> from the file.
This function returns non-negative on success, negative on failure.
</dl>
<h2>4. Attribute Inquiry Functions</h2>
<dl>
<dt><code>herr_t H5Aiterate (hid_t <em>loc_id</em>,
unsigned *<em>attr_number</em>,
H5A_operator <em>operator</em>,
void *<em>operator_data</em>)</code>
<dd> This function iterates over the attributes of the dataset or group
specified with <em>loc_id</em>. For each attribute of the object, the
<em>operator_data</em> and some additional information (specified below)
are passed to the <em>operator</em> function. The iteration begins with
the <em>*attr_number</em> object in the group and the next attribute to be
processed by the operator is returned in <em>*attr_number</em>.
<P>The iterator returns a negative value if something is wrong, the return
value of the last operator if it was non-zero, or zero if all attributes
were processed.
<P>The prototype for H5A_operator_t 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 ID for the group or dataset being iterated over
(<em>loc_id</em>), the name of the current attribute about the object (<em>attr_name</em>)
and the pointer to the operator data passed in to H5Aiterate
(<em>operator_data</em>). 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>
<br><br>
<dt><code>hid_t H5Aget_space (hid_t <em>attr_id</em>)</code>
<dd>This function retrieves a copy of the dataspace for an attribute.
The dataspace ID returned from this function must be released with H5Sclose
or resource leaks will develop.
This function returns a dataspace ID for success or negative for failure.
<br><br>
<dt><code>hid_t H5Aget_type (hid_t <em>attr_id</em>)</code>
<dd>This function retrieves a copy of the datatype for an attribute.
The datatype ID returned from this function must be released with H5Tclose
or resource leaks will develop.
This function returns a datatype ID for success or negative for failure.
<br><br>
<dt><code>ssize_t H5Aget_name (hid_t <em>attr_id</em>,
size_t <em>buf_size</em>, char *<em>buf</em>)</code>
<dd>This function retrieves the name of an attribute for an attribute ID.
Up to <em>buf_size</em> characters are stored in <em>buf</em> followed by a
'\0' string terminator. If the name of the attribute is longer than
<em>buf_size</em>-1, the string terminator is stored in the last position
of the buffer to properly terminate the string.
This function returns the length of the attribute's name (which may be
longer than <em>buf_size</em>) on success or negative for failure.
<br><br>
<dt><code>int H5Aget_num_attrs (hid_t <em>loc_id</em>)</code>
<dd>This function returns the number of attributes attached to a dataset or
group, <em>loc_id</em>.
This function returns non-negative for success or negative for failure.
</dl>
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Describes HDF5 Release 1.4.5, February 2003
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Last modified: 6 July 2000
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