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---
version: tip
layout: manpage_v2
title: nng_aio_alloc(3)
---
<h1>nng_aio_alloc(3)</h1>
<div class="sect1">
<h2 id="_name">NAME</h2>
<div class="sectionbody">
<div class="paragraph">
<p>nng_aio_alloc - allocate asynchronous I/O handle</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_synopsis">SYNOPSIS</h2>
<div class="sectionbody">
<div class="listingblock">
<div class="content">
<pre class="pygments highlight"><code data-lang="c"><span></span><span class="tok-cp">#include</span><span class="tok-w"> </span><span class="tok-cpf"><nng/nng.h></span>
<span class="tok-kt">int</span><span class="tok-w"> </span><span class="tok-nf">nng_aio_alloc</span><span class="tok-p">(</span><span class="tok-n">nng_aio</span><span class="tok-w"> </span><span class="tok-o">**</span><span class="tok-n">aiop</span><span class="tok-p">,</span><span class="tok-w"> </span><span class="tok-kt">void</span><span class="tok-w"> </span><span class="tok-p">(</span><span class="tok-o">*</span><span class="tok-n">callb</span><span class="tok-p">)(</span><span class="tok-kt">void</span><span class="tok-w"> </span><span class="tok-o">*</span><span class="tok-p">),</span><span class="tok-w"> </span><span class="tok-kt">void</span><span class="tok-w"> </span><span class="tok-o">*</span><span class="tok-n">arg</span><span class="tok-p">);</span></code></pre>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_description">DESCRIPTION</h2>
<div class="sectionbody">
<div class="paragraph">
<p>The <code>nng_aio_alloc()</code> function allocates a handle for asynchronous I/O
operations, and stores a pointer to it in <em>aiop</em>.
The handle is initialized with a completion callback of <em>callb</em>,
which will be executed when an associated asynchronous operation finishes.
It will be called with the argument <em>arg</em>.</p>
</div>
<div class="admonitionblock note">
<table>
<tr>
<td class="icon">
<i class="fa icon-note" title="Note"></i>
</td>
<td class="content">
The callback <em>callb</em> must not perform any blocking operations, and
must complete its execution quickly. If <em>callb</em> does block, this can
lead ultimately to an apparent "hang" or deadlock in the application.
This also means you should avoid operations such as allocating new objects,
which also means opening or closing sockets, dialers, and so forth.
</td>
</tr>
</table>
</div>
<div class="admonitionblock tip">
<table>
<tr>
<td class="icon">
<i class="fa icon-tip" title="Tip"></i>
</td>
<td class="content">
If more complex or blocking work needs to be performed by <em>callb</em>, a separate
thread can be used, along with a <a href="nng_cv_alloc.3supp.html">condition variable</a>
which can be signaled by the callback.
</td>
</tr>
</table>
</div>
<div class="paragraph">
<p>Asynchronous I/O operations all take an <a href="nng_aio.5.html"><code>nng_aio</code></a>
handle such as allocated by this function.
Such operations are usually started by a function that returns immediately.
The operation is then run asynchronously, and completes sometime later.
When that operation is complete, the callback supplied here is called,
and that callback is able to determine the result of the operation using
<a href="nng_aio_result.3.html"><code>nng_aio_result()</code></a>,
<a href="nng_aio_count.3.html"><code>nng_aio_count()</code></a>,
and <a href="nng_aio_get_output.3.html"><code>nng_aio_get_output()</code></a>.</p>
</div>
<div class="paragraph">
<p>It is possible to wait synchronously for an otherwise asynchronous operation
by using the function <a href="nng_aio_wait.3.html"><code>nng_aio_wait()</code></a>.
In that case, it is permissible for <em>callb</em> and <em>arg</em> to both be <code>NULL</code>.
Note that if these are <code>NULL</code>, then it will not be possible to determine when the
operation is complete except by calling the aforementioned
<a href="nng_aio_wait.3.html"><code>nng_aio_wait()</code></a>.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_return_values">RETURN VALUES</h2>
<div class="sectionbody">
<div class="paragraph">
<p>This function returns 0 on success, and non-zero otherwise.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_errors">ERRORS</h2>
<div class="sectionbody">
<div class="hdlist">
<table>
<tr>
<td class="hdlist1">
<code>NNG_ENOMEM</code>
</td>
<td class="hdlist2">
<p>Insufficient free memory to perform the operation.</p>
</td>
</tr>
</table>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_see_also">SEE ALSO</h2>
<div class="sectionbody">
<div class="paragraph text-left">
<p><a href="nng_aio_abort.3.html">nng_aio_abort(3)</a>,
<a href="nng_aio_cancel.3.html">nng_aio_cancel(3)</a>,
<a href="nng_aio_count.3.html">nng_aio_count(3)</a>,
<a href="nng_aio_free.3.html">nng_aio_free(3)</a>,
<a href="nng_aio_get_input.3.html">nng_aio_get_input(3)</a>,
<a href="nng_aio_get_msg.3.html">nng_aio_get_msg(3)</a>,
<a href="nng_aio_get_output.3.html">nng_aio_get_output(3)</a>,
<a href="nng_aio_result.3.html">nng_aio_result(3)</a>,
<a href="nng_aio_set_input.3.html">nng_aio_set_input(3)</a>,
<a href="nng_aio_set_iov.3.html">nng_aio_set_iov(3)</a>,
<a href="nng_aio_set_msg.3.html">nng_aio_set_msg(3)</a>,
<a href="nng_aio_set_timeout.3.html">nng_aio_set_timeout(3)</a>,
<a href="nng_aio_stop.3.html">nng_aio_stop(3)</a>,
<a href="nng_aio_wait.3.html">nng_aio_wait(3)</a>,
<a href="nng_strerror.3.html">nng_strerror(3)</a>,
<a href="nng_aio.5.html">nng_aio(5)</a>,
<a href="nng.7.html">nng(7)</a></p>
</div>
</div>
</div>
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