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<title>GnuTLS 3.4.0: Asynchronous operation</title>
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<a name="Asynchronous-operation"></a>
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<p>
Next: <a href="DTLS-sessions.html#DTLS-sessions" accesskey="n" rel="next">DTLS sessions</a>, Up: <a href="Setting-up-the-transport-layer.html#Setting-up-the-transport-layer" accesskey="u" rel="up">Setting up the transport layer</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Function-and-Data-Index.html#Function-and-Data-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="Asynchronous-operation-1"></a>
<h4 class="subsection">6.5.1 Asynchronous operation</h4>
<p><acronym>GnuTLS</acronym> can be used with asynchronous socket or event-driven programming.
The approach is similar to using Berkeley sockets under such an environment.
The blocking, due to network interaction, calls such as
<a href="Core-TLS-API.html#gnutls_005fhandshake">gnutls_handshake</a>, <a href="Core-TLS-API.html#gnutls_005frecord_005frecv">gnutls_record_recv</a>,
can be set to non-blocking by setting the underlying sockets to non-blocking.
If other push and pull functions are setup, then they should behave the same
way as <code>recv</code> and <code>send</code> when used in a non-blocking
way, i.e., set errno to <code>EAGAIN</code>. Since, during a TLS protocol session
<acronym>GnuTLS</acronym> does not block except for network interaction, the non blocking
<code>EAGAIN</code> errno will be propagated and <acronym>GnuTLS</acronym> functions
will return the <code>GNUTLS_E_AGAIN</code> error code. Such calls can be resumed the
same way as a system call would.
The only exception is <a href="Core-TLS-API.html#gnutls_005frecord_005fsend">gnutls_record_send</a>,
which if interrupted subsequent calls need not to include the data to be
sent (can be called with NULL argument).
</p>
<p>The <code>select</code> system call can also be used in combination with the
<acronym>GnuTLS</acronym> functions. <code>select</code> allows monitoring of sockets
and notifies on them being ready for reading or writing data. Note however
that this system call cannot notify on data present in <acronym>GnuTLS</acronym>
read buffers, it is only applicable to the kernel sockets API. Thus if
you are using it for reading from a <acronym>GnuTLS</acronym> session, make sure
that any cached data are read completely. That can be achieved by checking there
are no data waiting to be read (using <a href="Core-TLS-API.html#gnutls_005frecord_005fcheck_005fpending">gnutls_record_check_pending</a>),
either before the <code>select</code> system call, or after a call to
<a href="Core-TLS-API.html#gnutls_005frecord_005frecv">gnutls_record_recv</a>. <acronym>GnuTLS</acronym> does not keep a write buffer,
thus when writing no additional actions are required.
</p>
<p>Although in the TLS protocol implementation each call to receive or send
function implies to restoring the same function that was interrupted, in
the DTLS protocol this requirement isn’t true.
There are cases where a retransmission is required, which are indicated by
a received message and thus <a href="Core-TLS-API.html#gnutls_005frecord_005fget_005fdirection">gnutls_record_get_direction</a> must be called
to decide which direction to check prior to restoring a function call.
</p>
<dl>
<dt><a name="index-gnutls_005frecord_005fget_005fdirection"></a>Function: <em>int</em> <strong>gnutls_record_get_direction</strong> <em>(gnutls_session_t <var>session</var>)</em></dt>
<dd><p><var>session</var>: is a <code>gnutls_session_t</code> type.
</p>
<p>This function provides information about the internals of the
record protocol and is only useful if a prior gnutls function call
(e.g. <code>gnutls_handshake()</code> ) was interrupted for some reason, that
is, if a function returned <code>GNUTLS_E_INTERRUPTED</code> or
<code>GNUTLS_E_AGAIN</code> . In such a case, you might want to call <code>select()</code>
or <code>poll()</code> before calling the interrupted gnutls function again. To
tell you whether a file descriptor should be selected for either
reading or writing, <code>gnutls_record_get_direction()</code> returns 0 if the
interrupted function was trying to read data, and 1 if it was
trying to write data.
</p>
<p>This function’s output is unreliable if you are using the
<code>session</code> in different threads, for sending and receiving.
</p>
<p><strong>Returns:</strong> 0 if trying to read data, 1 if trying to write data.
</p></dd></dl>
<p>Moreover, to prevent blocking from DTLS’ retransmission timers to block a
handshake, the <a href="Core-TLS-API.html#gnutls_005finit">gnutls_init</a> function should be called with the
<code>GNUTLS_NONBLOCK</code> flag set (see <a href="Session-initialization.html#Session-initialization">Session initialization</a>). In that
case, in order to be able to use the DTLS handshake timers, the function
<a href="Datagram-TLS-API.html#gnutls_005fdtls_005fget_005ftimeout">gnutls_dtls_get_timeout</a> should be used to estimate when to call
<a href="Core-TLS-API.html#gnutls_005fhandshake">gnutls_handshake</a> if no packets have been received.
</p>
<hr>
<div class="header">
<p>
Next: <a href="DTLS-sessions.html#DTLS-sessions" accesskey="n" rel="next">DTLS sessions</a>, Up: <a href="Setting-up-the-transport-layer.html#Setting-up-the-transport-layer" accesskey="u" rel="up">Setting up the transport layer</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Function-and-Data-Index.html#Function-and-Data-Index" title="Index" rel="index">Index</a>]</p>
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