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<div id="projectname">Hardware Locality (hwloc)
 <span id="projectnumber">2.4.1</span>
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<div class="title">Thread Safety </div> </div>
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<p>Like most libraries that mainly fill data structures, hwloc is not thread safe but rather reentrant: all state is held in a <a class="el" href="a00192.html#ga9d1e76ee15a7dee158b786c30b6a6e38" title="Topology context.">hwloc_topology_t</a> instance without mutex protection. That means, for example, that two threads can safely operate on and modify two different <a class="el" href="a00192.html#ga9d1e76ee15a7dee158b786c30b6a6e38" title="Topology context.">hwloc_topology_t</a> instances, but they should not simultaneously invoke functions that modify the <em>same</em> instance. Similarly, one thread should not modify a <a class="el" href="a00192.html#ga9d1e76ee15a7dee158b786c30b6a6e38" title="Topology context.">hwloc_topology_t</a> instance while another thread is reading or traversing it. However, two threads can safely read or traverse the same <a class="el" href="a00192.html#ga9d1e76ee15a7dee158b786c30b6a6e38" title="Topology context.">hwloc_topology_t</a> instance concurrently.</p>
<p>When running in multiprocessor environments, be aware that proper thread synchronization and/or memory coherency protection is needed to pass hwloc data (such as <a class="el" href="a00192.html#ga9d1e76ee15a7dee158b786c30b6a6e38" title="Topology context.">hwloc_topology_t</a> pointers) from one processor to another (e.g., a mutex, semaphore, or a memory barrier). Note that this is not a hwloc-specific requirement, but it is worth mentioning.</p>
<p>For reference, <a class="el" href="a00192.html#ga9d1e76ee15a7dee158b786c30b6a6e38" title="Topology context.">hwloc_topology_t</a> modification operations include (but may not be limited to):</p>
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<dt>Creation and destruction </dt>
<dd><p class="startdd"><code><a class="el" href="a00192.html#ga03fd4a16d8b9ee1ffc32b25fd2f6bdfa" title="Allocate a topology context.">hwloc_topology_init()</a>, <a class="el" href="a00192.html#gabdf58d87ad77f6615fccdfe0535ff826" title="Build the actual topology.">hwloc_topology_load()</a>, <a class="el" href="a00192.html#ga9f34a640b6fd28d23699d4d084667b15" title="Terminate and free a topology context.">hwloc_topology_destroy()</a></code> (see <a class="el" href="a00192.html">Topology Creation and Destruction</a>) imply major modifications of the structure, including freeing some objects. No other thread cannot access the topology or any of its objects at the same time.</p>
<p class="interdd">Also references to objects inside the topology are not valid anymore after these functions return. </p>
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<dt>Runtime topology modifications </dt>
<dd><p class="startdd"><code><a class="el" href="a00200.html#gad980782ade737900c5cf208946768c30" title="Add a MISC object as a leaf of the topology.">hwloc_topology_insert_misc_object()</a></code>, <code><a class="el" href="a00200.html#ga4cea4741165faf5323931a9ed8786ef7" title="Allocate a Group object to insert later with hwloc_topology_insert_group_object().">hwloc_topology_alloc_group_object()</a></code>, and <code><a class="el" href="a00200.html#ga1fc6012b3e1c249b83f48cb7bcacaa5b" title="Add more structure to the topology by adding an intermediate Group.">hwloc_topology_insert_group_object()</a></code> (see <a class="el" href="a00200.html">Modifying a loaded Topology</a>) may modify the topology significantly by adding objects inside the tree, changing the topology depth, etc.</p>
<p class="interdd"><code><a class="el" href="a00216.html#gac5a71d96cd86efe31d6f8d282aae3d97" title="Provide a new distance matrix.">hwloc_distances_add()</a></code> and <code><a class="el" href="a00216.html#gac188d9b64d9560255ce5f6d0a20f9c0a" title="Remove all distance matrices from a topology.">hwloc_distances_remove()</a></code> (see <a class="el" href="a00216.html">Add or remove distances between objects</a>) modify the list of distance structures in the topology, and the former may even insert new Group objects.</p>
<p class="interdd"><code><a class="el" href="a00218.html#ga770657d1e44b09e93e09f623936c1e5f" title="Register a new memory attribute.">hwloc_memattr_register()</a></code> and <code><a class="el" href="a00218.html#ga960529c08b25cf15825e0f72ecceb504" title="Set an attribute value for a specific target NUMA node.">hwloc_memattr_set_value()</a></code> (see <a class="el" href="a00218.html">Managing memory attributes</a>) modify the memory attributes of the topology.</p>
<p class="interdd"><code><a class="el" href="a00200.html#ga6db81ed13ac0a9d70cc80372ab537815" title="Restrict the topology to the given CPU set or nodeset.">hwloc_topology_restrict()</a></code> modifies the topology even more dramatically by removing some objects.</p>
<p class="interdd"><code><a class="el" href="a00200.html#ga698ecd640d2b76742bba3829a145cd9a" title="Refresh internal structures after topology modification.">hwloc_topology_refresh()</a></code> updates some internal cached structures. (see below).</p>
<p class="interdd">Although references to former objects <em>may</em> still be valid after insertion or restriction, it is strongly advised to not rely on any such guarantee and always re-consult the topology to reacquire new instances of objects. </p>
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</dd>
<dt>Consulting distances </dt>
<dd><p class="startdd"><code><a class="el" href="a00214.html#ga613e6b2a5d0f06626ee8d0c12fa46691" title="Retrieve distance matrices.">hwloc_distances_get()</a></code> and its variants are thread-safe except if the topology was recently modified (because distances may involve objects that were removed).</p>
<p class="interdd">Whenever the topology is modified (see above), <code><a class="el" href="a00200.html#ga698ecd640d2b76742bba3829a145cd9a" title="Refresh internal structures after topology modification.">hwloc_topology_refresh()</a></code> should be called in the same thread-safe context to force the refresh of internal distances structures. A call to <code><a class="el" href="a00214.html#ga613e6b2a5d0f06626ee8d0c12fa46691" title="Retrieve distance matrices.">hwloc_distances_get()</a></code> may also refresh distances-related structures.</p>
<p class="interdd">Once this refresh has been performed, multiple <code><a class="el" href="a00214.html#ga613e6b2a5d0f06626ee8d0c12fa46691" title="Retrieve distance matrices.">hwloc_distances_get()</a></code> may then be performed concurrently by multiple threads. </p>
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<dt>Consulting memory attributes </dt>
<dd><p class="startdd">Functions consulting memory attributes in <a class="el" href="a00140_source.html">hwloc/memattrs.h</a> are thread-safe except if the topology was recently modified (because memory attributes may involve objects that were removed).</p>
<p class="interdd">Whenever the topology is modified (see above), <code><a class="el" href="a00200.html#ga698ecd640d2b76742bba3829a145cd9a" title="Refresh internal structures after topology modification.">hwloc_topology_refresh()</a></code> should be called in the same thread-safe context to force the refresh of internal memory attribute structures. A call to <code><a class="el" href="a00217.html#ga297e4a9adc2272446a4c7449dacef0df" title="Return an attribute value for a specific target NUMA node.">hwloc_memattr_get_value()</a></code> or <code><a class="el" href="a00218.html#ga3177cc0ab47e4dd1fa69ca1df4c7cb1a" title="Return the target NUMA nodes that have some values for a given attribute.">hwloc_memattr_get_targets()</a></code> may also refresh internal structures for a given memory attribute.</p>
<p class="interdd">Once this refresh has been performed, multiple functions consulting memory attributes may then be performed concurrently by multiple threads. </p>
<p class="enddd"></p>
</dd>
<dt>Locating topologies </dt>
<dd><p class="startdd"><code>hwloc_topology_set_*</code> (see <a class="el" href="a00199.html">Topology Detection Configuration and Query</a>) do not modify the topology directly, but they do modify internal structures describing the behavior of the upcoming invocation of <code><a class="el" href="a00192.html#gabdf58d87ad77f6615fccdfe0535ff826" title="Build the actual topology.">hwloc_topology_load()</a></code>. Hence, all of these functions should not be used concurrently. </p>
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