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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN"><html><head><!-- Copyright 1997 The Open Group, All Rights Reserved --><title>signal</title></head><body bgcolor=white><center><font size=2>The Single UNIX &reg; Specification, Version 2<br>Copyright &copy; 1997 The Open Group</font></center><hr size=2 noshade><h4><a name = "tag_000_008_714">&nbsp;</a>NAME</h4><blockquote>signal, sigset, sighold, sigrelse, sigignore, sigpause - signal management</blockquote><h4><a name = "tag_000_008_715">&nbsp;</a>SYNOPSIS</h4><blockquote><pre><code>#include &lt;<a href="signal.h.html">signal.h</a>&gt;void (*signal(int <i>sig</i>, void (*<i>func</i>)(int)))(int);int sighold(int <i>sig</i>);int sigignore(int <i>sig</i>);int sigpause(int <i>sig</i>);int sigrelse(int <i>sig</i>);void (*sigset(int <i>sig</i>, void (*<i>disp</i>)(int)))(int);</code></pre></blockquote><h4><a name = "tag_000_008_716">&nbsp;</a>DESCRIPTION</h4><blockquote>Use of any of these functions is unspecified in a multi-threaded process.<p>The<i>signal()</i>function chooses one of three ways in which receipt of the signal number<i>sig</i>is to be subsequently handled.  If the value of<i>func</i>is SIG_DFL, default handling for that signal will occur.If the value of<i>func</i>is SIG_IGN, the signal will be ignored.Otherwise,<i>func</i>must point to a function to be called when that signal occurs.Such a function is called a<i>signal handler</i>.<p>When a signal occurs, if<i>func</i>points to a function, first the equivalent of a:<pre><code>signal(<i>sig</i>, SIG_DFL);</code></pre><p>is executed or an implementation-dependent blocking of the signal isperformed.  (If the value of<i>sig</i>is SIGILL, whether the reset to SIG_DFL occurs is implementation-dependent.)Next the equivalent of:<pre><code>(*func)(sig);</code></pre>is executed.  The<i>func</i>function may terminate by executing a<b>return</b>statement or by calling<i><a href="abort.html">abort()</a></i>,<i><a href="exit.html">exit()</a></i>,or<i><a href="longjmp.html">longjmp()</a></i>.If<i>func</i>executes a<b>return</b>statement and the value of<i>sig</i>was SIGFPE or any other implementation-dependent value corresponding to acomputational exception, the behaviour is undefined.  Otherwise, the programwill resume execution at the point it was interrupted.<p>If the signal occurs other than as the result of calling<i><a href="abort.html">abort()</a></i>,<i><a href="kill.html">kill()</a></i>or<i><a href="raise.html">raise()</a></i>,the behaviour is undefined if the signal handler callsany function in the standard library other than one of thefunctions listed on the<i><a href="sigaction.html">sigaction()</a></i>pageor refers to any object with static storage durationother than by assigning a value to a static storage durationvariable of type<b>volatile</b><b>sig_atomic_t</b>.Furthermore, if such a call fails,the value of<i>errno</i>is indeterminate.<p>At program startup, the equivalent of:<pre><code>signal(<i>sig</i>, SIG_IGN);</code></pre><p>is executed for some signals, and the equivalent of:<pre><code>signal(<i>sig</i>, SIG_DFL);</code></pre><p>is executed for all other signals (see<i>exec</i>).<p>The<i>sigset()</i>,<i>sighold()</i>,<i>sigignore()</i>,<i>sigpause()</i>and<i>sigrelse()</i>functions provide simplified signal management.<p>The<i>sigset()</i>function is used to modify signal dispositions.  The <i>sig</i> argumentspecifies the signal, which may be any signal except SIGKILL and SIGSTOP.  The<i>disp</i> argument specifies the signal's disposition, which may be SIG_DFL,SIG_IGN or the address of a signal handler.  If<i>sigset()</i>is used, and <i>disp</i> is the address of a signal handler, the system willadd <i>sig</i> to the calling process' signal mask before executing the signalhandler; when the signal handler returns, the system will restore the callingprocess' signal mask to its state prior the delivery of the signal.  Inaddition, if<i>sigset()</i>is used, and <i>disp</i> is equal to SIG_HOLD, <i>sig</i> will be added to thecalling process' signal mask and <i>sig</i>'s disposition will remainunchanged.  If<i>sigset()</i>is used, and disp is not equal to SIG_HOLD, sig will be removed from thecalling process' signal mask.<p>The<i>sighold()</i>function adds <i>sig</i> to the calling process' signal mask.<p>The<i>sigrelse()</i>function removes <i>sig</i> from the calling process' signal mask.<p>The<i>sigignore()</i>function sets the disposition of <i>sig</i> to SIG_IGN.<p>The<i>sigpause()</i>function removes <i>sig</i> from the calling process' signal mask and suspendsthe calling process until a signal is received.The<i>sigpause()</i>function restores the process' signal mask to its originalstate before returning.   <p>If the action for the SIGCHLD signal is set to SIG_IGN, child processes of thecalling processes will not be transformed into zombie processes when theyterminate.  If the calling process subsequently waits for its children, andthe process has no unwaited for children that were transformed into zombieprocesses, it will block until all of its children terminate, and<i><a href="wait.html">wait()</a></i>,<i><a href="wait3.html">wait3()</a></i>,<i><a href="waitid.html">waitid()</a></i>and<i><a href="waitpid.html">waitpid()</a></i>will fail and set<i>errno</i>to [ECHILD].</blockquote><h4><a name = "tag_000_008_717">&nbsp;</a>RETURN VALUE</h4><blockquote>If the request can be honoured,<i>signal()</i>returns the value of<i>func</i>for the most recent call to<i>signal()</i>for the specified signal<i>sig</i>.Otherwise, SIG_ERR is returned and a positive value is stored in<i>errno</i>.<p>Upon successful completion,<i>sigset()</i>returns SIG_HOLD if the signal had been blocked and the signal's previousdisposition if it had not been blocked.  Otherwise, SIG_ERRis returned and <i>errno</i> is set to indicate the error.<p>The <i>sigpause()</i>function suspends execution of the thread until asignal is received, whereupon it returns -1 and sets <i>errno</i> to [EINTR].<p>For all other functions, upon successful completion, 0 is returned.Otherwise, -1 is returned and <i>errno</i> is set to indicate the error.</blockquote><h4><a name = "tag_000_008_718">&nbsp;</a>ERRORS</h4><blockquote>The<i>signal()</i>function will fail if:<dl compact><dt>[EINVAL]<dd>The<i>sig</i>argument is not a valid signal number or an attempt is made tocatch a signal that cannot be caught or ignore a signal thatcannot be ignored.</dl><p>The<i>signal()</i>function may fail if:<dl compact><dt>[EINVAL]<dd>An attempt was made to set the action to SIG_DFL for a signalthat cannot be caught or ignored (or both).</dl><p>The<i>sigset()</i>,<i>sighold()</i>,<i>sigrelse()</i>,<i>sigignore()</i>and<i>sigpause()</i>functions will fail if:<dl compact><dt>[EINVAL]<dd>The <i>sig</i> argument is an illegal signal number.</dl><p>The<i>sigset()</i>,and<i>sigignore()</i>functions will fail if:<dl compact><dt>[EINVAL]<dd>An attempt is made to catch a signal that cannot be caught, or to ignore asignal that cannot be ignored.</dl></blockquote><h4><a name = "tag_000_008_719">&nbsp;</a>EXAMPLES</h4><blockquote>None.</blockquote><h4><a name = "tag_000_008_720">&nbsp;</a>APPLICATION USAGE</h4><blockquote>The<i><a href="sigaction.html">sigaction()</a></i>function provides a more comprehensive and reliable mechanism forcontrolling signals; new applications should use<i><a href="sigaction.html">sigaction()</a></i>rather than<i>signal()</i>.<p>The<i>sighold()</i>function, in conjunction with<i>sigrelse()</i>or<i>sigpause()</i>,may be used to establish critical regions of code that requirethe delivery of a signal to be temporarily deferred.<p>The<i><a href="sigsuspend.html">sigsuspend()</a></i>function should be used in preference to<i>sigpause()</i>for broader portability.</blockquote><h4><a name = "tag_000_008_721">&nbsp;</a>FUTURE DIRECTIONS</h4><blockquote>None.</blockquote><h4><a name = "tag_000_008_722">&nbsp;</a>SEE ALSO</h4><blockquote><i><a href="exec.html">exec</a></i>,<i><a href="pause.html">pause()</a></i>,<i><a href="sigaction.html">sigaction()</a></i>,<i><a href="sigsuspend.html">sigsuspend()</a></i>,<i><a href="waitid.html">waitid()</a></i>,<i><a href="signal.h.html">&lt;signal.h&gt;</a></i>.</blockquote><h4>DERIVATION</h4><blockquote>Derived from Issue 1 of the SVID.</blockquote><hr size=2 noshade><center><font size=2>UNIX &reg; is a registered Trademark of The Open Group.<br>Copyright &copy; 1997 The Open Group<br> [ <a href="../index.html">Main Index</a> | <a href="../xshix.html">XSH</a> | <a href="../xcuix.html">XCU</a> | <a href="../xbdix.html">XBD</a> | <a href="../cursesix.html">XCURSES</a> | <a href="../xnsix.html">XNS</a> ]</font></center><hr size=2 noshade></body></html>
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