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<h4 class="subsection">24.7.3 Process Signal Mask</h4>

<p><a name="index-signal-mask-2965"></a><a name="index-process-signal-mask-2966"></a>
The collection of signals that are currently blocked is called the
<dfn>signal mask</dfn>.  Each process has its own signal mask.  When you
create a new process (see <a href="Creating-a-Process.html#Creating-a-Process">Creating a Process</a>), it inherits its
parent's mask.  You can block or unblock signals with total flexibility
by modifying the signal mask.

   <p>The prototype for the <code>sigprocmask</code> function is in <samp><span class="file">signal.h</span></samp>. 
<a name="index-signal_002eh-2967"></a>
Note that you must not use <code>sigprocmask</code> in multi-threaded processes,
because each thread has its own signal mask and there is no single process
signal mask. According to POSIX, the behavior of <code>sigprocmask</code> in a
multi-threaded process is &ldquo;unspecified&rdquo;. 
Instead, use <code>pthread_sigmask</code>.

<!-- signal.h -->
<!-- POSIX.1 -->
<div class="defun">
&mdash; Function: int <b>sigprocmask</b> (<var>int how, const sigset_t *restrict set, sigset_t *restrict oldset</var>)<var><a name="index-sigprocmask-2968"></a></var><br>
<blockquote><p>The <code>sigprocmask</code> function is used to examine or change the calling
process's signal mask.  The <var>how</var> argument determines how the signal
mask is changed, and must be one of the following values:

          <dl>
<!-- signal.h -->
<!-- POSIX.1 -->
<a name="index-SIG_005fBLOCK-2969"></a><dt><code>SIG_BLOCK</code><dd>Block the signals in <code>set</code>&mdash;add them to the existing mask.  In
other words, the new mask is the union of the existing mask and
<var>set</var>.

          <!-- signal.h -->
          <!-- POSIX.1 -->
          <p><a name="index-SIG_005fUNBLOCK-2970"></a><br><dt><code>SIG_UNBLOCK</code><dd>Unblock the signals in <var>set</var>&mdash;remove them from the existing mask.

          <!-- signal.h -->
          <!-- POSIX.1 -->
          <p><a name="index-SIG_005fSETMASK-2971"></a><br><dt><code>SIG_SETMASK</code><dd>Use <var>set</var> for the mask; ignore the previous value of the mask. 
</dl>

        <p>The last argument, <var>oldset</var>, is used to return information about the
old process signal mask.  If you just want to change the mask without
looking at it, pass a null pointer as the <var>oldset</var> argument. 
Similarly, if you want to know what's in the mask without changing it,
pass a null pointer for <var>set</var> (in this case the <var>how</var> argument
is not significant).  The <var>oldset</var> argument is often used to
remember the previous signal mask in order to restore it later.  (Since
the signal mask is inherited over <code>fork</code> and <code>exec</code> calls, you
can't predict what its contents are when your program starts running.)

        <p>If invoking <code>sigprocmask</code> causes any pending signals to be
unblocked, at least one of those signals is delivered to the process
before <code>sigprocmask</code> returns.  The order in which pending signals
are delivered is not specified, but you can control the order explicitly
by making multiple <code>sigprocmask</code> calls to unblock various signals
one at a time.

        <p>The <code>sigprocmask</code> function returns <code>0</code> if successful, and <code>-1</code>
to indicate an error.  The following <code>errno</code> error conditions are
defined for this function:

          <dl>
<dt><code>EINVAL</code><dd>The <var>how</var> argument is invalid. 
</dl>

        <p>You can't block the <code>SIGKILL</code> and <code>SIGSTOP</code> signals, but
if the signal set includes these, <code>sigprocmask</code> just ignores
them instead of returning an error status.

        <p>Remember, too, that blocking program error signals such as <code>SIGFPE</code>
leads to undesirable results for signals generated by an actual program
error (as opposed to signals sent with <code>raise</code> or <code>kill</code>). 
This is because your program may be too broken to be able to continue
executing to a point where the signal is unblocked again. 
See <a href="Program-Error-Signals.html#Program-Error-Signals">Program Error Signals</a>. 
</p></blockquote></div>

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