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<h1 id="threads"><a class="header" href="#threads">Threads</a></h1>
<p>Threads <a name="a001"></a> provide a means of representing multiple parallel execution contexts.
<em>NNG</em> makes use of this concept internally, but also provides for user applications
to utilize the same thread facilities. This allows one form of <a name="a002"></a>concurrency for
applications.</p>
<div class="mdbook-alerts mdbook-alerts-note">
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</p>
<p>Threads in <em>NNG</em> are built upon platform support, which may be based upon operating
system supplied threads, process, or coroutines. The appearance of concurrency does
not guarantee true concurrency, and scheduling between threads may not necessarily be
pre-emptive. While this will not adversely impact applications that use this facility
for I/O bound concurrency, it may not provide good results for purely <a name="a003"></a>CPU-bound operations.</p>
</div>
<div class="mdbook-alerts mdbook-alerts-important">
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<span class="mdbook-alerts-icon"></span>
important
</p>
<p>Thread objects created by this function may not be real system-level
threads capable of performing blocking I/O operations using normal blocking system calls.
If use of blocking system calls is required (not including APIs provided
by the <em>NNG</em> library itself of course), then real OS-specific threads
should be created instead (such as with <code>pthread_create</code> or similar functions.)
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<p>The system may impose limits on the number of threads that can be created.
Typically applications should not create more than a dozen of these.
If greater concurrency or scalability is needed, consider instead using
an asynchronous model using <a href="/api/aio.html#asynchronous-io-handle"><code>nng_aio</code></a> structures.</p>
</div>
<h2 id="thread-structure"><a class="header" href="#thread-structure">Thread Structure</a></h2>
<pre><code class="language-c">typedef struct nng_thread nng_thread;
</code></pre>
<p>The <a name="a004"></a><code>nng_thread</code> structure represents a thread, which is a single execution context.
A given thread will have its own stack, and CPU registers. However global state, as well
as values allocated on the heap, will be shared and accessible to all threads in the system
(See the <a href="/api/synch.html">Synchronization</a> chapter for functions to help with data sharing between different threads.)</p>
<p>Multiple threads can be thought of as running concurrently, even though
they might not actually do so.</p>
<p>I/O operations that block (i.e. wait for completion) will block the
thread, while allowing other threads to proceed.</p>
<h2 id="creating-a-thread"><a class="header" href="#creating-a-thread">Creating a Thread</a></h2>
<pre><code class="language-c">int nng_thread_create(nng_thread **thrp, void (*func)(void *), void *arg);
</code></pre>
<p>The <a name="a005"></a><code>nng_thread_create</code> function creates a thread, which will execute <em>func</em>, with
the given argument <em>arg</em>, and returns a pointer to it in <em>thrp</em>.</p>
<p>The thread may begin execution immediately.</p>
<p>The thread will persist until <em>func</em> returns.</p>
<p>This function returns zero on success, but may return <code>NNG_ENOMEM</code> if insufficient
resources to create a thread are available.</p>
<h2 id="destroying-a-thread"><a class="header" href="#destroying-a-thread">Destroying a Thread</a></h2>
<pre><code class="language-c">void nng_thread_destroy(nng_thread *thr);
</code></pre>
<p>The <a name="a006"></a><code>nng_thread_destroy</code> function waits for the thread <em>thr</em> to finish execution.
This function should be called to reclaim any resources associated with the thread when done.
It also has the effect of blocking execution in the caller until <em>thr</em> has completed executing.</p>
<h2 id="thread-names"><a class="header" href="#thread-names">Thread Names</a></h2>
<pre><code class="language-c">void nng_thread_set_name(nng_thread *thr, const char *name);
</code></pre>
<p>In order to facilitate debugging, <a name="a007"></a><code>nng_thread_set_name</code> may be called
to provide a name for the thread. This may change how the thread is represented
in debuggers. Not all platforms support setting the thread name.</p>
<h2 id="see-also"><a class="header" href="#see-also">See Also</a></h2>
<p><a href="/api/synch.html">Synchronization</a>,
<a href="/api/aio.html">Asynchronous Operations</a></p>
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