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<div class="headertitle"><div class="title">Creating an Attribute</div></div>
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<div class="textblock"><p>Navigate back: <a class="el" href="index.html">Main</a> / <a class="el" href="_getting_started.html">Getting Started with HDF5</a> / <a class="el" href="_learn_basics.html">Learning the Basics</a> </p><hr />
<p>Attributes are small datasets that can be used to describe the nature and/or the intended usage of the object they are attached to. In this section, we show how to create, read, and write an attribute.</p>
<h1><a class="anchor" id="secLBAttrCreate"></a>
Creating an attribute</h1>
<p>Creating an attribute is similar to creating a dataset. To create an attribute, the application must specify the object which the attribute is attached to, the datatype and dataspace of the attribute data, and the attribute creation property list.</p>
<p>The steps to create an attribute are as follows: </p><ol>
<li>
Obtain the object identifier that the attribute is to be attached to. </li>
<li>
Define the characteristics of the attribute and specify the attribute creation property list. <ul>
<li>
Define the datatype. </li>
<li>
Define the dataspace. </li>
<li>
Specify the attribute creation property list. </li>
</ul>
</li>
<li>
Create the attribute. </li>
<li>
Close the attribute and datatype, dataspace, and attribute creation property list, if necessary. </li>
</ol>
<p>To create and close an attribute, the calling program must use <a class="el" href="group___h5_a.html#ga4a76e4e5ab6eb0fd2aa7990d38d55f24">H5Acreate</a> and <a class="el" href="group___h5_a.html#gaef4394b661e2c930879e9868e122bdda" title="Closes the specified attribute.">H5Aclose</a>. For example:</p>
<p><em>C</em> </p><div class="fragment"><div class="line">attr_id = <a class="code hl_define" href="group___h5_a.html#ga4a76e4e5ab6eb0fd2aa7990d38d55f24">H5Acreate</a> (dataset_id, <span class="stringliteral">"Units"</span>, <a class="code hl_define" href="group___p_d_t_s_t_d.html#ga37e8a6be7ee64587c2a282b965019bb8">H5T_STD_I32BE</a>, dataspace_id, <a class="code hl_define" href="_h5_ppublic_8h.html#afa85e97bfbf9bf1c58e39263846c568f">H5P_DEFAULT</a>, <a class="code hl_define" href="_h5_ppublic_8h.html#afa85e97bfbf9bf1c58e39263846c568f">H5P_DEFAULT</a>)</div>
<div class="line">status = <a class="code hl_function" href="group___h5_a.html#gaef4394b661e2c930879e9868e122bdda">H5Aclose</a> (attr_id);</div>
<div class="ttc" id="a_h5_ppublic_8h_html_afa85e97bfbf9bf1c58e39263846c568f"><div class="ttname"><a href="_h5_ppublic_8h.html#afa85e97bfbf9bf1c58e39263846c568f">H5P_DEFAULT</a></div><div class="ttdeci">#define H5P_DEFAULT</div><div class="ttdef"><b>Definition</b> H5Ppublic.h:102</div></div>
<div class="ttc" id="agroup___h5_a_html_ga4a76e4e5ab6eb0fd2aa7990d38d55f24"><div class="ttname"><a href="group___h5_a.html#ga4a76e4e5ab6eb0fd2aa7990d38d55f24">H5Acreate</a></div><div class="ttdeci">#define H5Acreate</div><div class="ttdef"><b>Definition</b> H5version.h:868</div></div>
<div class="ttc" id="agroup___h5_a_html_gaef4394b661e2c930879e9868e122bdda"><div class="ttname"><a href="group___h5_a.html#gaef4394b661e2c930879e9868e122bdda">H5Aclose</a></div><div class="ttdeci">herr_t H5Aclose(hid_t attr_id)</div><div class="ttdoc">Closes the specified attribute.</div></div>
<div class="ttc" id="agroup___p_d_t_s_t_d_html_ga37e8a6be7ee64587c2a282b965019bb8"><div class="ttname"><a href="group___p_d_t_s_t_d.html#ga37e8a6be7ee64587c2a282b965019bb8">H5T_STD_I32BE</a></div><div class="ttdeci">#define H5T_STD_I32BE</div><div class="ttdef"><b>Definition</b> H5Tpublic.h:305</div></div>
</div><!-- fragment --><p><em>Fortran</em> </p><div class="fragment"><div class="line">CALL h5acreate_f (dset_id, attr_nam, type_id, space_id, attr_id, &</div>
<div class="line"> hdferr, creation_prp=creat_plist_id)</div>
<div class="line">or</div>
<div class="line">CALL h5acreate_f (dset_id, attr_nam, type_id, space_id, attr_id, hdferr)</div>
<div class="line"> </div>
<div class="line">CALL h5aclose_f (attr_id, hdferr)</div>
</div><!-- fragment --><h1><a class="anchor" id="secLBAttrCreateRW"></a>
Reading/Writing an attribute</h1>
<p>Attributes may only be read or written as an entire object; no partial I/O is supported. Therefore, to perform I/O operations on an attribute, the application needs only to specify the attribute and the attribute's memory datatype.</p>
<p>The steps to read or write an attribute are as follows. </p><ol>
<li>
Obtain the attribute identifier. </li>
<li>
Specify the attribute's memory datatype. </li>
<li>
Perform the desired operation. </li>
<li>
Close the memory datatype if necessary. </li>
</ol>
<p>To read and/or write an attribute, the calling program must contain the <a class="el" href="group___h5_a.html#gaacb27a997f7c98e8a833d0fd63b58f1c" title="Reads the value of an attribute.">H5Aread</a> and/or <a class="el" href="group___h5_a.html#gab70871e205d57450c83efd9912be2b5c" title="Writes data to an attribute.">H5Awrite</a> routines. For example:</p>
<p><em>C</em> </p><div class="fragment"><div class="line">status = <a class="code hl_function" href="group___h5_a.html#gaacb27a997f7c98e8a833d0fd63b58f1c">H5Aread</a> (attr_id, mem_type_id, buf);</div>
<div class="line">status = <a class="code hl_function" href="group___h5_a.html#gab70871e205d57450c83efd9912be2b5c">H5Awrite</a> (attr_id, mem_type_id, buf);</div>
<div class="ttc" id="agroup___h5_a_html_gaacb27a997f7c98e8a833d0fd63b58f1c"><div class="ttname"><a href="group___h5_a.html#gaacb27a997f7c98e8a833d0fd63b58f1c">H5Aread</a></div><div class="ttdeci">herr_t H5Aread(hid_t attr_id, hid_t type_id, void *buf)</div><div class="ttdoc">Reads the value of an attribute.</div></div>
<div class="ttc" id="agroup___h5_a_html_gab70871e205d57450c83efd9912be2b5c"><div class="ttname"><a href="group___h5_a.html#gab70871e205d57450c83efd9912be2b5c">H5Awrite</a></div><div class="ttdeci">herr_t H5Awrite(hid_t attr_id, hid_t type_id, const void *buf)</div><div class="ttdoc">Writes data to an attribute.</div></div>
</div><!-- fragment --><p><em>Fortran</em> </p><div class="fragment"><div class="line">CALL h5awrite_f (attr_id, mem_type_id, buf, dims, hdferr) </div>
<div class="line">CALL h5aread_f (attr_id, mem_type_id, buf, dims, hdferr)</div>
</div><!-- fragment --><h1><a class="anchor" id="secLBAttrCreateHL"></a>
High Level APIs</h1>
<p>The High Level <a class="el" href="group___h5_l_t.html">HDF5 Lite APIs (H5LT,H5LD)</a> include functions that simplify and condense the steps for creating and reading datasets. Please be sure to review them, in addition to this tutorial.</p>
<h1><a class="anchor" id="secLBAttrCreateRWEx"></a>
Programming Example</h1>
<h2><a class="anchor" id="secLBAttrCreateRWExDesc"></a>
Description</h2>
<p>See <a class="el" href="_l_b_examples.html">Examples from Learning the Basics</a> for the examples used in the <a class="el" href="_learn_basics.html">Learning the Basics</a> tutorial.</p>
<p>The example shows how to create and write a dataset attribute. It opens an existing file <code style="background-color:whitesmoke;">dset.h5</code> in C (<code style="background-color:whitesmoke;">dsetf.h5</code> in FORTRAN), obtains the identifier of the dataset <code style="background-color:whitesmoke;">/dset</code>, defines the attribute's dataspace, creates the dataset attribute, writes the attribute, and then closes the attribute's dataspace, attribute, dataset, and file.</p>
<p>For details on compiling an HDF5 application: [ <a class="el" href="_l_b_compiling.html">Compiling HDF5 Applications</a> ]</p>
<h2><a class="anchor" id="secLBAttrCreateRWExRem"></a>
Remarks</h2>
<p><a class="el" href="group___h5_a.html#ga4a76e4e5ab6eb0fd2aa7990d38d55f24">H5Acreate</a> creates an attribute which is attached to the object specified by the first parameter, and returns an identifier.</p>
<p><a class="el" href="group___h5_a.html#gab70871e205d57450c83efd9912be2b5c" title="Writes data to an attribute.">H5Awrite</a> writes the entire attribute, and returns the status of the write.</p>
<p>When an attribute is no longer accessed by a program, <a class="el" href="group___h5_a.html#gaef4394b661e2c930879e9868e122bdda" title="Closes the specified attribute.">H5Aclose</a> must be called to release the attribute from use. An <a class="el" href="group___h5_a.html#gaef4394b661e2c930879e9868e122bdda" title="Closes the specified attribute.">H5Aclose</a>/h5aclose_f call is mandatory.</p>
<h2><a class="anchor" id="secLBAttrCreateRWExCont"></a>
File Contents</h2>
<p>Shown below is the contents and the attribute definition of <code style="background-color:whitesmoke;">dset.h5</code> (created by the C program).</p>
<p><em>dset.h5 in DDL</em> </p><div class="fragment"><div class="line">HDF5 <span class="stringliteral">"dset.h5"</span> {</div>
<div class="line">GROUP <span class="stringliteral">"/"</span> {</div>
<div class="line">DATASET <span class="stringliteral">"dset"</span> {</div>
<div class="line">DATATYPE { <a class="code hl_define" href="group___p_d_t_s_t_d.html#ga37e8a6be7ee64587c2a282b965019bb8">H5T_STD_I32BE</a> }</div>
<div class="line">DATASPACE { SIMPLE ( 4, 6 ) / ( 4, 6 ) }</div>
<div class="line">DATA {</div>
<div class="line"> 1, 2, 3, 4, 5, 6,</div>
<div class="line"> 7, 8, 9, 10, 11, 12,</div>
<div class="line"> 13, 14, 15, 16, 17, 18,</div>
<div class="line"> 19, 20, 21, 22, 23, 24</div>
<div class="line">}</div>
<div class="line">ATTRIBUTE <span class="stringliteral">"attr"</span> {</div>
<div class="line"> DATATYPE { <a class="code hl_define" href="group___p_d_t_s_t_d.html#ga37e8a6be7ee64587c2a282b965019bb8">H5T_STD_I32BE</a> }</div>
<div class="line"> DATASPACE { SIMPLE ( 2 ) / ( 2 ) }</div>
<div class="line"> DATA {</div>
<div class="line"> 100, 200</div>
<div class="line"> }</div>
<div class="line">}</div>
<div class="line">}</div>
<div class="line">}</div>
<div class="line">}</div>
</div><!-- fragment --><p>Shown below is the contents and the attribute definition of <code style="background-color:whitesmoke;">dsetf.h5</code> (created by the FORTRAN program).</p>
<p><em>dsetf.h5 in DDL</em> </p><div class="fragment"><div class="line">HDF5 <span class="stringliteral">"dsetf.h5"</span> {</div>
<div class="line">GROUP <span class="stringliteral">"/"</span> {</div>
<div class="line">DATASET <span class="stringliteral">"dset"</span> {</div>
<div class="line">DATATYPE { <a class="code hl_define" href="group___p_d_t_s_t_d.html#ga37e8a6be7ee64587c2a282b965019bb8">H5T_STD_I32BE</a> }</div>
<div class="line">DATASPACE { SIMPLE ( 6, 4 ) / ( 6, 4 ) }</div>
<div class="line">DATA {</div>
<div class="line"> 1, 7, 13, 19,</div>
<div class="line"> 2, 8, 14, 20,</div>
<div class="line"> 3, 9, 15, 21,</div>
<div class="line"> 4, 10, 16, 22,</div>
<div class="line"> 5, 11, 17, 23,</div>
<div class="line"> 6, 12, 18, 24</div>
<div class="line">}</div>
<div class="line">ATTRIBUTE <span class="stringliteral">"attr"</span> {</div>
<div class="line"> DATATYPE { <a class="code hl_define" href="group___p_d_t_s_t_d.html#ga37e8a6be7ee64587c2a282b965019bb8">H5T_STD_I32BE</a> }</div>
<div class="line"> DATASPACE { SIMPLE ( 2 ) / ( 2 ) }</div>
<div class="line"> DATA {</div>
<div class="line"> 100, 200</div>
<div class="line"> }</div>
<div class="line">}</div>
<div class="line">}</div>
<div class="line">}</div>
<div class="line">}</div>
</div><!-- fragment --><h2><a class="anchor" id="secLBAttrCreateRWExDDL"></a>
Attribute Definition in DDL</h2>
<p><em>HDF5 Attribute Definition</em> </p><div class="fragment"><div class="line"><attribute> ::= ATTRIBUTE <span class="stringliteral">"<attr_name>"</span> { <datatype></div>
<div class="line"> <dataspace></div>
<div class="line"> <data> }</div>
</div><!-- fragment --><hr />
<p> Navigate back: <a class="el" href="index.html">Main</a> / <a class="el" href="_getting_started.html">Getting Started with HDF5</a> / <a class="el" href="_learn_basics.html">Learning the Basics</a> </p>
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