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<h4 class="subsection">24.2.5 Job Control Signals</h4>

<p><a name="index-job-control-signals-2873"></a>
These signals are used to support job control.  If your system
doesn't support job control, then these macros are defined but the
signals themselves can't be raised or handled.

   <p>You should generally leave these signals alone unless you really
understand how job control works.  See <a href="Job-Control.html#Job-Control">Job Control</a>.

<!-- signal.h -->
<!-- POSIX.1 -->
<div class="defun">
&mdash; Macro: int <b>SIGCHLD</b><var><a name="index-SIGCHLD-2874"></a></var><br>
<blockquote><p><a name="index-child-process-signal-2875"></a>This signal is sent to a parent process whenever one of its child
processes terminates or stops.

        <p>The default action for this signal is to ignore it.  If you establish a
handler for this signal while there are child processes that have
terminated but not reported their status via <code>wait</code> or
<code>waitpid</code> (see <a href="Process-Completion.html#Process-Completion">Process Completion</a>), whether your new handler
applies to those processes or not depends on the particular operating
system. 
</p></blockquote></div>

<!-- signal.h -->
<!-- SVID -->
<div class="defun">
&mdash; Macro: int <b>SIGCLD</b><var><a name="index-SIGCLD-2876"></a></var><br>
<blockquote><p>This is an obsolete name for <code>SIGCHLD</code>. 
</p></blockquote></div>

<!-- signal.h -->
<!-- POSIX.1 -->
<div class="defun">
&mdash; Macro: int <b>SIGCONT</b><var><a name="index-SIGCONT-2877"></a></var><br>
<blockquote><p><a name="index-continue-signal-2878"></a>You can send a <code>SIGCONT</code> signal to a process to make it continue. 
This signal is special&mdash;it always makes the process continue if it is
stopped, before the signal is delivered.  The default behavior is to do
nothing else.  You cannot block this signal.  You can set a handler, but
<code>SIGCONT</code> always makes the process continue regardless.

        <p>Most programs have no reason to handle <code>SIGCONT</code>; they simply
resume execution without realizing they were ever stopped.  You can use
a handler for <code>SIGCONT</code> to make a program do something special when
it is stopped and continued&mdash;for example, to reprint a prompt when it
is suspended while waiting for input. 
</p></blockquote></div>

<!-- signal.h -->
<!-- POSIX.1 -->
<div class="defun">
&mdash; Macro: int <b>SIGSTOP</b><var><a name="index-SIGSTOP-2879"></a></var><br>
<blockquote><p>The <code>SIGSTOP</code> signal stops the process.  It cannot be handled,
ignored, or blocked. 
</p></blockquote></div>
   <a name="index-stop-signal-2880"></a>
<!-- signal.h -->
<!-- POSIX.1 -->

<div class="defun">
&mdash; Macro: int <b>SIGTSTP</b><var><a name="index-SIGTSTP-2881"></a></var><br>
<blockquote><p>The <code>SIGTSTP</code> signal is an interactive stop signal.  Unlike
<code>SIGSTOP</code>, this signal can be handled and ignored.

        <p>Your program should handle this signal if you have a special need to
leave files or system tables in a secure state when a process is
stopped.  For example, programs that turn off echoing should handle
<code>SIGTSTP</code> so they can turn echoing back on before stopping.

        <p>This signal is generated when the user types the SUSP character
(normally <kbd>C-z</kbd>).  For more information about terminal driver
support, see <a href="Special-Characters.html#Special-Characters">Special Characters</a>. 
</p></blockquote></div>
   <a name="index-interactive-stop-signal-2882"></a>
<!-- signal.h -->
<!-- POSIX.1 -->

<div class="defun">
&mdash; Macro: int <b>SIGTTIN</b><var><a name="index-SIGTTIN-2883"></a></var><br>
<blockquote><p>A process cannot read from the user's terminal while it is running
as a background job.  When any process in a background job tries to
read from the terminal, all of the processes in the job are sent a
<code>SIGTTIN</code> signal.  The default action for this signal is to
stop the process.  For more information about how this interacts with
the terminal driver, see <a href="Access-to-the-Terminal.html#Access-to-the-Terminal">Access to the Terminal</a>. 
</p></blockquote></div>
   <a name="index-terminal-input-signal-2884"></a>
<!-- signal.h -->
<!-- POSIX.1 -->

<div class="defun">
&mdash; Macro: int <b>SIGTTOU</b><var><a name="index-SIGTTOU-2885"></a></var><br>
<blockquote><p>This is similar to <code>SIGTTIN</code>, but is generated when a process in a
background job attempts to write to the terminal or set its modes. 
Again, the default action is to stop the process.  <code>SIGTTOU</code> is
only generated for an attempt to write to the terminal if the
<code>TOSTOP</code> output mode is set; see <a href="Output-Modes.html#Output-Modes">Output Modes</a>. 
</p></blockquote></div>
   <a name="index-terminal-output-signal-2886"></a>
While a process is stopped, no more signals can be delivered to it until
it is continued, except <code>SIGKILL</code> signals and (obviously)
<code>SIGCONT</code> signals.  The signals are marked as pending, but not
delivered until the process is continued.  The <code>SIGKILL</code> signal
always causes termination of the process and can't be blocked, handled
or ignored.  You can ignore <code>SIGCONT</code>, but it always causes the
process to be continued anyway if it is stopped.  Sending a
<code>SIGCONT</code> signal to a process causes any pending stop signals for
that process to be discarded.  Likewise, any pending <code>SIGCONT</code>
signals for a process are discarded when it receives a stop signal.

   <p>When a process in an orphaned process group (see <a href="Orphaned-Process-Groups.html#Orphaned-Process-Groups">Orphaned Process Groups</a>) receives a <code>SIGTSTP</code>, <code>SIGTTIN</code>, or <code>SIGTTOU</code>
signal and does not handle it, the process does not stop.  Stopping the
process would probably not be very useful, since there is no shell
program that will notice it stop and allow the user to continue it. 
What happens instead depends on the operating system you are using. 
Some systems may do nothing; others may deliver another signal instead,
such as <code>SIGKILL</code> or <code>SIGHUP</code>.  In the GNU system, the process
dies with <code>SIGKILL</code>; this avoids the problem of many stopped,
orphaned processes lying around the system.

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