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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 “unspecified”. |
| Instead, use <code>pthread_sigmask</code>. |
| |
| <!-- signal.h --> |
| <!-- POSIX.1 --> |
| <div class="defun"> |
| — 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>—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>—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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