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 <h4 class="subsection">24.6.2 Signaling Another Process</h4>

 <p><a name="index-killing-a-process-2951"></a>The <code>kill</code> function can be used to send a signal to another process.
 In spite of its name, it can be used for a lot of things other than
 causing a process to terminate.  Some examples of situations where you
 might want to send signals between processes are:

      <ul>
 <li>A parent process starts a child to perform a task&mdash;perhaps having the
 child running an infinite loop&mdash;and then terminates the child when the
 task is no longer needed.

      <li>A process executes as part of a group, and needs to terminate or notify
 the other processes in the group when an error or other event occurs.

      <li>Two processes need to synchronize while working together.
 </ul>

    <p>This section assumes that you know a little bit about how processes
 work.  For more information on this subject, see <a href="Processes.html#Processes">Processes</a>.

    <p>The <code>kill</code> function is declared in <samp><span class="file">signal.h</span></samp>.
 <a name="index-signal_002eh-2952"></a>
 <!-- signal.h -->
 <!-- POSIX.1 -->

 <div class="defun">
 &mdash; Function: int <b>kill</b> (<var>pid_t pid, int signum</var>)<var><a name="index-kill-2953"></a></var><br>
 <blockquote><p>The <code>kill</code> function sends the signal <var>signum</var> to the process
 or process group specified by <var>pid</var>.  Besides the signals listed in
 <a href="Standard-Signals.html#Standard-Signals">Standard Signals</a>, <var>signum</var> can also have a value of zero to
 check the validity of the <var>pid</var>.

         <p>The <var>pid</var> specifies the process or process group to receive the
 signal:

           <dl>
 <dt><var>pid</var><code> &gt; 0</code><dd>The process whose identifier is <var>pid</var>.

           <br><dt><var>pid</var><code> == 0</code><dd>All processes in the same process group as the sender.

           <br><dt><var>pid</var><code> &lt; -1</code><dd>The process group whose identifier is &minus;<var>pid</var>.

           <br><dt><var>pid</var><code> == -1</code><dd>If the process is privileged, send the signal to all processes except
 for some special system processes.  Otherwise, send the signal to all
 processes with the same effective user ID.
 </dl>

         <p>A process can send a signal to itself with a call like <code>kill&nbsp;(getpid(),&nbsp;</code><var>signum</var><code>)</code><!-- /@w -->.  If <code>kill</code> is used by a process to send
 a signal to itself, and the signal is not blocked, then <code>kill</code>
 delivers at least one signal (which might be some other pending
 unblocked signal instead of the signal <var>signum</var>) to that process
 before it returns.

         <p>The return value from <code>kill</code> is zero if the signal can be sent
 successfully.  Otherwise, no signal is sent, and a value of <code>-1</code> is
 returned.  If <var>pid</var> specifies sending a signal to several processes,
 <code>kill</code> succeeds if it can send the signal to at least one of them.
 There's no way you can tell which of the processes got the signal
 or whether all of them did.

         <p>The following <code>errno</code> error conditions are defined for this function:

           <dl>
 <dt><code>EINVAL</code><dd>The <var>signum</var> argument is an invalid or unsupported number.

           <br><dt><code>EPERM</code><dd>You do not have the privilege to send a signal to the process or any of
 the processes in the process group named by <var>pid</var>.

           <br><dt><code>ESRCH</code><dd>The <var>pid</var> argument does not refer to an existing process or group.
 </dl>
         </p></blockquote></div>

 <!-- signal.h -->
 <!-- BSD -->
 <div class="defun">
 &mdash; Function: int <b>killpg</b> (<var>int pgid, int signum</var>)<var><a name="index-killpg-2954"></a></var><br>
 <blockquote><p>This is similar to <code>kill</code>, but sends signal <var>signum</var> to the
 process group <var>pgid</var>.  This function is provided for compatibility
 with BSD; using <code>kill</code> to do this is more portable.
 </p></blockquote></div>

    <p>As a simple example of <code>kill</code>, the call <code>kill&nbsp;(getpid&nbsp;(),&nbsp;</code><var>sig</var><code>)</code><!-- /@w --> has the same effect as <code>raise&nbsp;(</code><var>sig</var><code>)</code><!-- /@w -->.

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	Next: <a rel="next" accesskey="n" href="Permission-for-kill.html#Permission-for-kill">Permission for kill</a>,
	Previous: <a rel="previous" accesskey="p" href="Signaling-Yourself.html#Signaling-Yourself">Signaling Yourself</a>,
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	<h4 class="subsection">24.6.2 Signaling Another Process</h4>

	<p><a name="index-killing-a-process-2951"></a>The <code>kill</code> function can be used to send a signal to another process.
	In spite of its name, it can be used for a lot of things other than
	causing a process to terminate. Some examples of situations where you
	might want to send signals between processes are:

	<ul>
	<li>A parent process starts a child to perform a task—perhaps having the
	child running an infinite loop—and then terminates the child when the
	task is no longer needed.

	<li>A process executes as part of a group, and needs to terminate or notify
	the other processes in the group when an error or other event occurs.

	<li>Two processes need to synchronize while working together.
	</ul>

	<p>This section assumes that you know a little bit about how processes
	work. For more information on this subject, see <a href="Processes.html#Processes">Processes</a>.

	<p>The <code>kill</code> function is declared in <samp><span class="file">signal.h</span></samp>.
	<a name="index-signal_002eh-2952"></a>
	<!-- signal.h -->
	<!-- POSIX.1 -->

	<div class="defun">
	— Function: int <b>kill</b> (<var>pid_t pid, int signum</var>)<var><a name="index-kill-2953"></a></var><br>
	<blockquote><p>The <code>kill</code> function sends the signal <var>signum</var> to the process
	or process group specified by <var>pid</var>. Besides the signals listed in
	<a href="Standard-Signals.html#Standard-Signals">Standard Signals</a>, <var>signum</var> can also have a value of zero to
	check the validity of the <var>pid</var>.

	<p>The <var>pid</var> specifies the process or process group to receive the
	signal:

	<dl>
	<dt><var>pid</var><code> > 0</code><dd>The process whose identifier is <var>pid</var>.

	<br><dt><var>pid</var><code> == 0</code><dd>All processes in the same process group as the sender.

	<br><dt><var>pid</var><code> < -1</code><dd>The process group whose identifier is −<var>pid</var>.

	<br><dt><var>pid</var><code> == -1</code><dd>If the process is privileged, send the signal to all processes except
	for some special system processes. Otherwise, send the signal to all
	processes with the same effective user ID.
	</dl>

	<p>A process can send a signal to itself with a call like <code>kill (getpid(), </code><var>signum</var><code>)</code><!-- /@w -->. If <code>kill</code> is used by a process to send
	a signal to itself, and the signal is not blocked, then <code>kill</code>
	delivers at least one signal (which might be some other pending
	unblocked signal instead of the signal <var>signum</var>) to that process
	before it returns.

	<p>The return value from <code>kill</code> is zero if the signal can be sent
	successfully. Otherwise, no signal is sent, and a value of <code>-1</code> is
	returned. If <var>pid</var> specifies sending a signal to several processes,
	<code>kill</code> succeeds if it can send the signal to at least one of them.
	There's no way you can tell which of the processes got the signal
	or whether all of them did.

	<p>The following <code>errno</code> error conditions are defined for this function:

	<dl>
	<dt><code>EINVAL</code><dd>The <var>signum</var> argument is an invalid or unsupported number.

	<br><dt><code>EPERM</code><dd>You do not have the privilege to send a signal to the process or any of
	the processes in the process group named by <var>pid</var>.

	<br><dt><code>ESRCH</code><dd>The <var>pid</var> argument does not refer to an existing process or group.
	</dl>
	</p></blockquote></div>

	<!-- signal.h -->
	<!-- BSD -->
	<div class="defun">
	— Function: int <b>killpg</b> (<var>int pgid, int signum</var>)<var><a name="index-killpg-2954"></a></var><br>
	<blockquote><p>This is similar to <code>kill</code>, but sends signal <var>signum</var> to the
	process group <var>pgid</var>. This function is provided for compatibility
	with BSD; using <code>kill</code> to do this is more portable.
	</p></blockquote></div>

	<p>As a simple example of <code>kill</code>, the call <code>kill (getpid (), </code><var>sig</var><code>)</code><!-- /@w --> has the same effect as <code>raise (</code><var>sig</var><code>)</code><!-- /@w -->.

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