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<div class="headertitle"><div class="title">Parallel <div class="ingroups"><a class="el" href="group___h5_f.html">Files (H5F)</a></div></div></div>
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<a name="details" id="details"></a><h2 id="header-details" class="groupheader">Detailed Description</h2>
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<tr class="heading"><td colspan="2"><h2 id="header-func-members" class="groupheader"><a id="func-members" name="func-members"></a>
Functions</h2></td></tr>
<tr class="memitem:ga5356a1bee667b2a7dd1b657c820755fd" id="r_ga5356a1bee667b2a7dd1b657c820755fd"><td class="memItemLeft"><a class="el" href="_h5public_8h.html#a3b079ecf932a5c599499cf7e298af160">herr_t</a>&#160;</td><td class="memItemRight"><a class="el" href="#ga5356a1bee667b2a7dd1b657c820755fd">H5Fget_mpi_atomicity</a> (<a class="el" href="_h5_ipublic_8h.html#a0045db7ff9c22ad35db6ae91662e1943">hid_t</a> file_id, bool *flag)</td></tr>
<tr class="memdesc:ga5356a1bee667b2a7dd1b657c820755fd"><td class="mdescLeft">&#160;</td><td class="mdescRight">Retrieves the atomicity mode in use. <br /></td></tr>
<tr class="memitem:gaaefabc54e36a2d13325e9a853400f359" id="r_gaaefabc54e36a2d13325e9a853400f359"><td class="memItemLeft"><a class="el" href="_h5public_8h.html#a3b079ecf932a5c599499cf7e298af160">herr_t</a>&#160;</td><td class="memItemRight"><a class="el" href="#gaaefabc54e36a2d13325e9a853400f359">H5Fset_mpi_atomicity</a> (<a class="el" href="_h5_ipublic_8h.html#a0045db7ff9c22ad35db6ae91662e1943">hid_t</a> file_id, bool flag)</td></tr>
<tr class="memdesc:gaaefabc54e36a2d13325e9a853400f359"><td class="mdescLeft">&#160;</td><td class="mdescRight">Sets the MPI atomicity mode. <br /></td></tr>
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<h2 class="memtitle"><span class="permalink"><a href="#ga5356a1bee667b2a7dd1b657c820755fd">&#9670;&#160;</a></span>H5Fget_mpi_atomicity()</h2>
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<td class="memname"><a class="el" href="_h5public_8h.html#a3b079ecf932a5c599499cf7e298af160">herr_t</a> H5Fget_mpi_atomicity </td>
<td>(</td>
<td class="paramtype"><a class="el" href="_h5_ipublic_8h.html#a0045db7ff9c22ad35db6ae91662e1943">hid_t</a></td> <td class="paramname"><span class="paramname"><em>file_id</em></span>, </td>
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<td class="paramtype">bool *</td> <td class="paramname"><span class="paramname"><em>flag</em></span>&#160;)</td>
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<p>Retrieves the atomicity mode in use. </p>
<dl class="params"><dt>Parameters</dt><dd>
<table class="params">
<tr><td class="paramdir">[in]</td><td class="paramname">file_id</td><td>File identifier </td></tr>
<tr><td class="paramdir">[out]</td><td class="paramname">flag</td><td>Logical flag for atomicity setting. Valid values are: <ul>
<li>1 &ndash; MPI file access is set to atomic mode. </li>
<li>0 &ndash; MPI file access is set to nonatomic mode. </li>
</ul>
</td></tr>
</table>
</dd>
</dl>
<dl class="section return"><dt>Returns</dt><dd>Returns a non-negative value if successful; otherwise, returns a negative value.</dd></dl>
<p><a class="el" href="#ga5356a1bee667b2a7dd1b657c820755fd" title="Retrieves the atomicity mode in use.">H5Fget_mpi_atomicity()</a> retrieves the current consistency semantics mode for data access for the file <code class="param">file_id</code>.</p>
<p>Upon successful return, <code class="param">flag</code> will be set to <code>1</code> if file access is set to atomic mode and <code>0</code> if file access is set to nonatomic mode.</p>
<dl class="section see"><dt>See also</dt><dd><a href="https://support.hdfgroup.org/releases/hdf5/documentation/rfc/RFC%20PHDF5%20Consistency%20Semantics%20MC%20120328.docx.pdf">Enabling a Strict Consistency Semantics Model in Parallel HDF5</a></dd></dl>
<dl class="section since"><dt>Since</dt><dd>1.8.9 </dd></dl>
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<a id="gaaefabc54e36a2d13325e9a853400f359" name="gaaefabc54e36a2d13325e9a853400f359"></a>
<h2 class="memtitle"><span class="permalink"><a href="#gaaefabc54e36a2d13325e9a853400f359">&#9670;&#160;</a></span>H5Fset_mpi_atomicity()</h2>
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<td class="memname"><a class="el" href="_h5public_8h.html#a3b079ecf932a5c599499cf7e298af160">herr_t</a> H5Fset_mpi_atomicity </td>
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<td class="paramtype"><a class="el" href="_h5_ipublic_8h.html#a0045db7ff9c22ad35db6ae91662e1943">hid_t</a></td> <td class="paramname"><span class="paramname"><em>file_id</em></span>, </td>
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<td class="paramkey"></td>
<td></td>
<td class="paramtype">bool</td> <td class="paramname"><span class="paramname"><em>flag</em></span>&#160;)</td>
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</table>
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<p>Sets the MPI atomicity mode. </p>
<dl class="params"><dt>Parameters</dt><dd>
<table class="params">
<tr><td class="paramdir">[in]</td><td class="paramname">file_id</td><td>File identifier </td></tr>
<tr><td class="paramdir">[in]</td><td class="paramname">flag</td><td>Logical flag for atomicity setting. Valid values are: <ul>
<li><code>1</code> &ndash; Sets MPI file access to atomic mode. </li>
<li><code>0</code> &ndash; Sets MPI file access to nonatomic mode. </li>
</ul>
</td></tr>
</table>
</dd>
</dl>
<dl class="section return"><dt>Returns</dt><dd>Returns a non-negative value if successful; otherwise, returns a negative value.</dd></dl>
<dl class="section user"><dt>Motivation</dt><dd><a class="el" href="#gaaefabc54e36a2d13325e9a853400f359" title="Sets the MPI atomicity mode.">H5Fset_mpi_atomicity()</a> is applicable only in parallel environments using MPI I/O. The function is one of the tools used to ensure sequential consistency. This means that a set of operations will behave as though they were performed in a serial order consistent with the program order.</dd></dl>
<p><a class="el" href="#gaaefabc54e36a2d13325e9a853400f359" title="Sets the MPI atomicity mode.">H5Fset_mpi_atomicity()</a> sets MPI consistency semantics for data access to the file, <code class="param">file_id</code>.</p>
<p>If <code class="param">flag</code> is set to <code>1</code>, all file access operations will appear atomic, guaranteeing sequential consistency. If <code class="param">flag</code> is set to <code>0</code>, enforcement of atomic file access will be turned off.</p>
<p><a class="el" href="#gaaefabc54e36a2d13325e9a853400f359" title="Sets the MPI atomicity mode.">H5Fset_mpi_atomicity()</a> is a collective function and all participating processes must pass the same values for <code class="param">file_id</code> and <code class="param">flag</code>.</p>
<p>This function is available only when the HDF5 library is configured with parallel support (<span class="tt">HDF5_ENABLE_PARALLEL</span>). It is useful only when used with the <a class="el" href="_h5_f_dmpio_8h.html#a7a231bc1d78744088a4e1d297284cabf">H5FD_MPIO</a> driver (see <a class="el" href="group___f_a_p_l.html#ga7519d659a83ef5717d7a5d95baf0e9b1" title="Stores MPI IO communicator information to the file access property list.">H5Pset_fapl_mpio()</a>). </p>
<dl class="section attention"><dt>Attention</dt><dd><p class="startdd"><a class="el" href="#gaaefabc54e36a2d13325e9a853400f359" title="Sets the MPI atomicity mode.">H5Fset_mpi_atomicity()</a> calls <span class="tt">MPI_File_set_atomicity</span> underneath and is not supported if the execution platform does not support <span class="tt">MPI_File_set_atomicity</span>. When it is supported and used, the performance of data access operations may drop significantly.</p>
<p class="enddd">In certain scenarios, even when <span class="tt">MPI_File_set_atomicity</span> is supported, setting atomicity with <a class="el" href="#gaaefabc54e36a2d13325e9a853400f359" title="Sets the MPI atomicity mode.">H5Fset_mpi_atomicity()</a> and <code class="param">flag</code> set to 1 does not always yield strictly atomic updates. For example, some <a class="el" href="group___h5_d.html#ga98f44998b67587662af8b0d8a0a75906" title="Writes raw data from a buffer to a dataset.">H5Dwrite()</a> calls translate to multiple <span class="tt">MPI_File_write_at</span> calls. This happens in all cases where the high-level file access routine translates to multiple lower level file access routines. The following scenarios will raise this issue: </p><ul>
<li>Non-contiguous file access using independent I/O </li>
<li>Partial collective I/O using chunked access </li>
<li>Collective I/O using filters or when data conversion is required</li>
</ul>
<p>This issue arises because MPI atomicity is a matter of MPI file access operations rather than HDF5 access operations. But the user is normally seeking atomicity at the HDF5 level. To accomplish this, the application must set a barrier after a write, <a class="el" href="group___h5_d.html#ga98f44998b67587662af8b0d8a0a75906" title="Writes raw data from a buffer to a dataset.">H5Dwrite()</a>, but before the next read, <a class="el" href="group___h5_d.html#ga8287d5a7be7b8e55ffeff68f7d26811c" title="Reads raw data from a dataset into a provided buffer.">H5Dread()</a>, in addition to calling <a class="el" href="#gaaefabc54e36a2d13325e9a853400f359" title="Sets the MPI atomicity mode.">H5Fset_mpi_atomicity()</a>.The barrier will guarantee that all underlying write operations execute atomically before the read operations starts. This ensures additional ordering semantics and will normally produce the desired behavior. </p>
</dd></dl>
<dl class="section see"><dt>See also</dt><dd><a href="https://support.hdfgroup.org/releases/hdf5/documentation/rfc/RFC%20PHDF5%20Consistency%20Semantics%20MC%20120328.docx.pdf">Enabling a Strict Consistency Semantics Model in Parallel HDF5</a></dd></dl>
<dl class="section since"><dt>Since</dt><dd>1.8.9 </dd></dl>
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