[svn-r467] Restructuring documentation.
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Quincey Koziol
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doc/html/Attributes.html
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doc/html/Attributes.html
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<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN">
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<html>
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<head>
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<title>Attributes</title>
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</head>
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<body>
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<h1>Attributes</h1>
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<h2>1. Introduction</h2>
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<p>The appribute API (H5A) is primarily designed to easily allow small
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datasets to be attached to primary datasets as metadata information.
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Additional goals for the H5A interface include keeping storage
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requirement for each attribute to a minimum and easily sharing
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attributes among datasets.
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<p>Because attributes are intended to be small objects, large datasets
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intended as additional information for a primary dataset should be
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stored as supplemental datasets in a group with the primary dataset.
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Attributes can then be attached to the group containing everything to
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indicate a particular type of dataset with supplemental datasets is
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located in the group. How small is "small" is not defined by the
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library and is up to the user's interpretation.
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<p>Attributes are not seperate objects in the file, they are always
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contained in the object header of the object they are attached to. The
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I/O functions defined below are required to read or write attribute
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information, not the H5D I/O routines.
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<h2>2. Creating, Opening, Closing and Deleting Attributes</h2>
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<p>Attributes are created with the <code>H5Acreate()</code> function,
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and existing attributes can be accessed with either the
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<code>H5Aopen_name()</code> or <code>H5Aopen_idx()</code> functions. All
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three functions return an object ID which should be eventually released
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by calling <code>H5Aclose()</code>.
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<dl>
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<dt><code>hid_t H5Acreate (hid_t <em>loc_id</em>, const char
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*<em>name</em>, hid_t <em>type_id</em>, hid_t <em>space_id</em>,
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hid_t <em>create_plist_id</em>)</code>
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<dd>This function creates an attribute which is attached to the object
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specified with <em>loc_id</em>. The name specified with <em>name</em>
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for each attribute for an object must be unique for that object. The <em>type_id</em>
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and <em>space_id</em> are created with the H5T and H5S interfaces
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respectively. Currently only simple dataspaces are allowed for attribute
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dataspaces. The <em>create_plist_id</em> property list is currently
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unused, but will be used int the future for optional properties of
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attributes. The attribute ID returned from this function must be released
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with H5Aclose or resource leaks will develop. Attempting to create an
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attribute with the same name as an already existing attribute will fail,
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leaving the pre-existing attribute in place.
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This function returns a attribute ID for success or negative for failure.
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<br><br>
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<dt><code>hid_t H5Aopen_name (hid_t <em>loc_id</em>, const char
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*<em>name</em>)</code>
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<dd> This function opens an attribute which is attached to the object
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specified with <em>loc_id</em>. The name specified with <em>name</em>
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indicates the attribute to access. The attribute ID returned from this
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function must be released with H5Aclose or resource leaks will develop.
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This function returns a attribute ID for success or negative for failure.
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<br><br>
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<dt><code>hid_t H5Aopen_idx (hid_t <em>loc_id</em>, unsigned
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<em>idx</em>)</code>
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<dd>This function opens an attribute which is attached to the object
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specified with <em>loc_id</em>. The attribute specified with <em>idx</em>
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indicates the <em>idx</em>th attribute to access, starting with '0'. The
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attribute ID returned from this function must be released with H5Aclose or
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resource leaks will develop.
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This function returns a attribute ID for success or negative for failure.
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<br><br>
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<dt><code>herr_t H5Aclose (hid_t <em>attr_id</em>)</code>
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<dd>This function releases an attribute from use. Further use of the
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attribute ID will result in undefined behavior.
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This function returns non-negative on success, negative on failure.
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<br><br>
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<dt><code>herr_t H5Adelete (hid_t <em>loc_id</em>,
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const char *<em>name</em>)</code>
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<dd>This function removes the named attribute from a dataset or group.
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This function should not be used when attribute IDs are open on <em>loc_id</em>
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as it may cause the internal indexes of the attributes to change and future
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writes to the open attributes to produce incorrect results.
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Returns non-negative on success, negative on failure.
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</dl>
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<h2>3. Attribute I/O Functions</h2>
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<p>Attributes may only be written as an entire object, no partial I/O
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is currently supported.
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<dl>
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<dt><code>herr_t H5Awrite (hid_t <em>attr_id</em>,
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hid_t <em>mem_type_id</em>, void *<em>buf</em>)</code>
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<dd>This function writes an attribute, specified with <em>attr_id</em>,
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with <em>mem_type_id</em> specifying the datatype in memory. The entire
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attribute is written from <em>buf</em> to the file.
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This function returns non-negative on success, negative on failure.
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<br><br>
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<dt><code>herr_t H5Aread (hid_t <em>attr_id</em>,
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hid_t <em>mem_type_id</em>, void *<em>buf</em>)</code>
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<dd>This function read an attribute, specified with <em>attr_id</em>, with
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<em>mem_type_id</em> specifying the datatype in memory. The entire
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attribute is read into <em>buf</em> from the file.
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This function returns non-negative on success, negative on failure.
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</dl>
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<h2>4. Attribute Inquiry Functions</h2>
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<dl>
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<dt><code>int H5Aiterate (hid_t <em>loc_id</em>,
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unsigned *<em>attr_number</em>,
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H5A_operator <em>operator</em>,
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void *<em>operator_data</em>)</code>
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<dd> This function interates over the attributes of dataset or group
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specified with <em>loc_id</em>. For each attribute of the object, the
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<em>operator_data</em> and some additional information (specified below)
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are passed to the <em>operator</em> function. The iteration begins with
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the <em>*attr_number</em> object in the group and the next attribute to be
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processed by the operator is returned in <em>*attr_number</em>.
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<P>The iterator returns a negative value if something is wrong, the return
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value of the last operator if it was non-zero, or zero if all attributes
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were processed.
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<P>The prototype for H5A_operator_t is: <br>
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<code>typedef herr_t (*H5A_operator_t)(hid_t <em>loc_id</em>,
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const char *<em>attr_name</em>, void *<em>operator_data</em>);</code>
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<P>The operation receives the ID for the group or dataset being iterated over
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(<em>loc_id</em>), the name of the current attribute about the object (<em>attr_name</em>)
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and the pointer to the operator data passed in to H5Aiterate
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(<em>operator_data</em>). The return values from an operator are:
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<ul>
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<li>Zero causes the iterator to continue, returning zero when all
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attributes have been processed.
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<li>Positive causes the iterator to immediately return that positive
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value, indicating short-circuit success. The iterator can be
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restarted at the next attribute.
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<li>Negative causes the iterator to immediately return that value,
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indicating failure. The iterator can be restarted at the next
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attribute.
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</ul>
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<br><br>
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<dt><code>hid_t H5Aget_space (hid_t <em>attr_id</em>)</code>
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<dd>This function retrieves a copy of the dataspace for an attribute.
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The dataspace ID returned from this function must be released with H5Sclose
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or resource leaks will develop.
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This function returns a dataspace ID for success or negative for failure.
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<br><br>
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<dt><code>hid_t H5Aget_type (hid_t <em>attr_id</em>)</code>
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<dd>This function retrieves a copy of the datatype for an attribute.
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The datatype ID returned from this function must be released with H5Tclose
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or resource leaks will develop.
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This function returns a datatype ID for success or negative for failure.
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<br><br>
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<dt><code>size_t H5Aget_name (hid_t <em>attr_id</em>,
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char *<em>buf</em>, size_t <em>buf_size</em>)</code>
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<dd>This function retrieves the name of an attribute for an attribute ID.
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Up to <em>buf_size</em> characters are stored in <em>buf</em> followed by a
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'\0' string terminator. If the name of the attribute is longer than
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<em>buf_size</em>-1, the string terminator is stored in the last position
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of the buffer to properly terminate the string.
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This function returns the length of the attribute's name (which may be
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longer than <em>buf_size</em>) on success or negative for failure.
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<br><br>
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<dt><code>int H5Anum_attrs (hid_t <em>loc_id</em>)</code>
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<dd>This function returns the number of attributes attached to a dataset or
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group, <em>loc_id</em>.
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This function returns non-negative for success or negative for failure.
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</dl>
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<hr>
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<address><a href="mailto:hdfhelp@ncsa.uiuc.edu">HDF Support</a></address>
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</body>
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</html>
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