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<a name="Signal-Handling-_0028signal_002eh_0029"></a>
<h2 class="chapter">7 Signal Handling (<samp>signal.h</samp>)</h2>

<p>A <em>signal</em> is an event that interrupts the normal flow of control
in your program.  Your operating environment normally defines the full
set of signals available (see <samp>sys/signal.h</samp>), as well as the
default means of dealing with them&mdash;typically, either printing an
error message and aborting your program, or ignoring the signal.
</p>
<p>All systems support at least the following signals:
</p><dl compact="compact">
<dt><code>SIGABRT</code></dt>
<dd><p>Abnormal termination of a program; raised by the <code>abort</code> function.
</p>
</dd>
<dt><code>SIGFPE</code></dt>
<dd><p>A domain error in arithmetic, such as overflow, or division by zero.
</p>
</dd>
<dt><code>SIGILL</code></dt>
<dd><p>Attempt to execute as a function data that is not executable.
</p>
</dd>
<dt><code>SIGINT</code></dt>
<dd><p>Interrupt; an interactive attention signal.
</p>
</dd>
<dt><code>SIGSEGV</code></dt>
<dd><p>An attempt to access a memory location that is not available.
</p>
</dd>
<dt><code>SIGTERM</code></dt>
<dd><p>A request that your program end execution.
</p></dd>
</dl>

<p>Two functions are available for dealing with asynchronous
signals&mdash;one to allow your program to send signals to itself (this is
called <em>raising</em> a signal), and one to specify subroutines (called
<em>handlers</em> to handle particular signals that you anticipate may
occur&mdash;whether raised by your own program or the operating environment.
</p>
<p>To support these functions, <samp>signal.h</samp> defines three macros:
</p>
<dl compact="compact">
<dt><code>SIG_DFL</code></dt>
<dd><p>Used with the <code>signal</code> function in place of a pointer to a
handler subroutine, to select the operating environment&rsquo;s default
handling of a signal.
</p>
</dd>
<dt><code>SIG_IGN</code></dt>
<dd><p>Used with the <code>signal</code> function in place of a pointer to a
handler, to ignore a particular signal.
</p>
</dd>
<dt><code>SIG_ERR</code></dt>
<dd><p>Returned by the <code>signal</code> function in place of a pointer to a
handler, to indicate that your request to set up a handler could not
be honored for some reason.
</p></dd>
</dl>

<p><samp>signal.h</samp> also defines an integral type, <code>sig_atomic_t</code>.
This type is not used in any function declarations; it exists only to
allow your signal handlers to declare a static storage location where
they may store a signal value.  (Static storage is not otherwise
reliable from signal handlers.)
</p>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top">&bull; <a href="psignal.html#psignal" accesskey="1">psignal</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Print a signal message to standard error
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="raise.html#raise" accesskey="2">raise</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Send a signal
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="signal.html#signal" accesskey="3">signal</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Specify handler subroutine for a signal
</td></tr>
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