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    <p>Use <a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/sigaction.html" rel="noreferrer"><code>sigaction()</code></a> unless you've got very compelling reasons not to do so.</p> <p>The <a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/signal.html" rel="noreferrer"><code>signal()</code></a> interface has antiquity (and hence availability) in its favour, and it is defined in the C standard. Nevertheless, it has a number of undesirable characteristics that <code>sigaction()</code> avoids - unless you use the flags explicitly added to <code>sigaction()</code> to allow it to faithfully simulate the old <code>signal()</code> behaviour.</p> <ol> <li>The <code>signal()</code> function does not (necessarily) block other signals from arriving while the current handler is executing; <code>sigaction()</code> can block other signals until the current handler returns.</li> <li>The <code>signal()</code> function (usually) resets the signal action back to <code>SIG_DFL</code> (default) for almost all signals. This means that the <code>signal()</code> handler must reinstall itself as its first action. It also opens up a window of vulnerability between the time when the signal is detected and the handler is reinstalled during which if a second instance of the signal arrives, the default behaviour (usually terminate, sometimes with prejudice - aka core dump) occurs.</li> <li>The exact behaviour of <code>signal()</code> varies between systems — and the standards permit those variations.</li> </ol> <p>These are generally good reasons for using <code>sigaction()</code> instead of <code>signal()</code>. However, the interface of <code>sigaction()</code> is undeniably more fiddly.</p> <p>Whichever of the two you use, do not be tempted by the alternative signal interfaces such as <a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/sighold.html" rel="noreferrer"><code>sighold()</code></a>, <a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/sigignore.html" rel="noreferrer"><code>sigignore()</code></a>, <a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/sigpause.html" rel="noreferrer"><code>sigpause()</code></a> and <a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/sigrelse.html" rel="noreferrer"><code>sigrelse()</code></a>. They are nominally alternatives to <code>sigaction()</code>, but they are only barely standardized and are present in POSIX for backwards compatibility rather than for serious use. Note that the POSIX standard says their behaviour in multi-threaded programs is undefined.</p> <p>Multi-threaded programs and signals is a whole other complicated story. <s>AFAIK, both <code>signal()</code> and <code>sigaction()</code> are OK in multi-threaded applications.</s></p> <p><a href="https://stackoverflow.com/users/1287251/cornstalks">Cornstalks</a> <a href="https://stackoverflow.com/questions/231912/what-is-the-difference-between-sigaction-and-signal/232711?noredirect=1#comment43835195_232711">observes</a>:</p> <blockquote> <p>The Linux man page for <a href="http://man7.org/linux/man-pages/man2/signal.2.html" rel="noreferrer"><code>signal()</code></a> says:</p> <p><code>&nbsp;&nbsp;</code><em>The effects of <strong><code>signal()</code></strong> in a multi-threaded process are unspecified.</em></p> <p>Thus, I think <code>sigaction()</code> is the only that can be used safely in a multi-threaded process. </p> </blockquote> <p>That's interesting. The Linux manual page is more restrictive than POSIX in this case. POSIX specifies for <a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/signal.html" rel="noreferrer"><code>signal()</code></a>:</p> <blockquote> <p>If the process is multi-threaded, or if the process is single-threaded and a signal handler is executed other than as the result of:</p> <ul> <li>The process calling <code>abort()</code>, <code>raise()</code>, <code>kill()</code>, <code>pthread_kill()</code>, or <code>sigqueue()</code> to generate a signal that is not blocked</li> <li>A pending signal being unblocked and being delivered before the call that unblocked it returns</li> </ul> <p>the behavior is undefined if the signal handler refers to any object other than <code>errno</code> with static storage duration other than by assigning a value to an object declared as <code>volatile sig_atomic_t</code>, or if the signal handler calls any function defined in this standard other than one of the functions listed in <a href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_04" rel="noreferrer">Signal Concepts</a>.</p> </blockquote> <p>So POSIX clearly specifies the behaviour of <code>signal()</code> in a multi-threaded application.</p> <p>Nevertheless, <code>sigaction()</code> is to be preferred in essentially all circumstances — and portable multi-threaded code should use <code>sigaction()</code> unless there's an overwhelming reason why it can't (such as "only use functions defined by Standard C" — and yes, C11 code can be multi-threaded). Which is basically what the opening paragraph of this answer also says.</p>
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    1. COThis description of `signal` is actually of the Unix System V behavior. POSIX allows either this behavior or the much more sane BSD behavior, but since you can't be sure which one you'll get, it's still best to use `sigaction`.
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