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<html lang="en">
<head>
<title>Advanced Signal Handling - The GNU C Library</title>
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<p>
<a name="Advanced-Signal-Handling"></a>
Next: <a rel="next" accesskey="n" href="Signal-and-Sigaction.html#Signal-and-Sigaction">Signal and Sigaction</a>,
Previous: <a rel="previous" accesskey="p" href="Basic-Signal-Handling.html#Basic-Signal-Handling">Basic Signal Handling</a>,
Up: <a rel="up" accesskey="u" href="Signal-Actions.html#Signal-Actions">Signal Actions</a>
<hr>
</div>
<h4 class="subsection">24.3.2 Advanced Signal Handling</h4>
<p><a name="index-g_t_0040code_007bsigaction_007d-function-2917"></a>
The <code>sigaction</code> function has the same basic effect as
<code>signal</code>: to specify how a signal should be handled by the process.
However, <code>sigaction</code> offers more control, at the expense of more
complexity. In particular, <code>sigaction</code> allows you to specify
additional flags to control when the signal is generated and how the
handler is invoked.
<p>The <code>sigaction</code> function is declared in <samp><span class="file">signal.h</span></samp>.
<a name="index-signal_002eh-2918"></a>
<!-- signal.h -->
<!-- POSIX.1 -->
<div class="defun">
— Data Type: <b>struct sigaction</b><var><a name="index-struct-sigaction-2919"></a></var><br>
<blockquote><p>Structures of type <code>struct sigaction</code> are used in the
<code>sigaction</code> function to specify all the information about how to
handle a particular signal. This structure contains at least the
following members:
<dl>
<dt><code>sighandler_t sa_handler</code><dd>This is used in the same way as the <var>action</var> argument to the
<code>signal</code> function. The value can be <code>SIG_DFL</code>,
<code>SIG_IGN</code>, or a function pointer. See <a href="Basic-Signal-Handling.html#Basic-Signal-Handling">Basic Signal Handling</a>.
<br><dt><code>sigset_t sa_mask</code><dd>This specifies a set of signals to be blocked while the handler runs.
Blocking is explained in <a href="Blocking-for-Handler.html#Blocking-for-Handler">Blocking for Handler</a>. Note that the
signal that was delivered is automatically blocked by default before its
handler is started; this is true regardless of the value in
<code>sa_mask</code>. If you want that signal not to be blocked within its
handler, you must write code in the handler to unblock it.
<br><dt><code>int sa_flags</code><dd>This specifies various flags which can affect the behavior of
the signal. These are described in more detail in <a href="Flags-for-Sigaction.html#Flags-for-Sigaction">Flags for Sigaction</a>.
</dl>
</p></blockquote></div>
<!-- signal.h -->
<!-- POSIX.1 -->
<div class="defun">
— Function: int <b>sigaction</b> (<var>int signum, const struct sigaction *restrict action, struct sigaction *restrict old-action</var>)<var><a name="index-sigaction-2920"></a></var><br>
<blockquote><p>The <var>action</var> argument is used to set up a new action for the signal
<var>signum</var>, while the <var>old-action</var> argument is used to return
information about the action previously associated with this symbol.
(In other words, <var>old-action</var> has the same purpose as the
<code>signal</code> function's return value—you can check to see what the
old action in effect for the signal was, and restore it later if you
want.)
<p>Either <var>action</var> or <var>old-action</var> can be a null pointer. If
<var>old-action</var> is a null pointer, this simply suppresses the return
of information about the old action. If <var>action</var> is a null pointer,
the action associated with the signal <var>signum</var> is unchanged; this
allows you to inquire about how a signal is being handled without changing
that handling.
<p>The return value from <code>sigaction</code> is zero if it succeeds, and
<code>-1</code> on failure. The following <code>errno</code> error conditions are
defined for this function:
<dl>
<dt><code>EINVAL</code><dd>The <var>signum</var> argument is not valid, or you are trying to
trap or ignore <code>SIGKILL</code> or <code>SIGSTOP</code>.
</dl>
</p></blockquote></div>
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