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| <h4 class="subsection">24.8.3 Using <code>sigsuspend</code></h4> |
| |
| <p>The clean and reliable way to wait for a signal to arrive is to block it |
| and then use <code>sigsuspend</code>. By using <code>sigsuspend</code> in a loop, |
| you can wait for certain kinds of signals, while letting other kinds of |
| signals be handled by their handlers. |
| |
| <!-- signal.h --> |
| <!-- POSIX.1 --> |
| <div class="defun"> |
| — Function: int <b>sigsuspend</b> (<var>const sigset_t *set</var>)<var><a name="index-sigsuspend-2982"></a></var><br> |
| <blockquote><p>This function replaces the process's signal mask with <var>set</var> and then |
| suspends the process until a signal is delivered whose action is either |
| to terminate the process or invoke a signal handling function. In other |
| words, the program is effectively suspended until one of the signals that |
| is not a member of <var>set</var> arrives. |
| |
| <p>If the process is woken up by delivery of a signal that invokes a handler |
| function, and the handler function returns, then <code>sigsuspend</code> also |
| returns. |
| |
| <p>The mask remains <var>set</var> only as long as <code>sigsuspend</code> is waiting. |
| The function <code>sigsuspend</code> always restores the previous signal mask |
| when it returns. |
| |
| <p>The return value and error conditions are the same as for <code>pause</code>. |
| </p></blockquote></div> |
| |
| <p>With <code>sigsuspend</code>, you can replace the <code>pause</code> or <code>sleep</code> |
| loop in the previous section with something completely reliable: |
| |
| <pre class="smallexample"> sigset_t mask, oldmask; |
| |
| ... |
| |
| /* <span class="roman">Set up the mask of signals to temporarily block.</span> */ |
| sigemptyset (&mask); |
| sigaddset (&mask, SIGUSR1); |
| |
| ... |
| |
| /* <span class="roman">Wait for a signal to arrive.</span> */ |
| sigprocmask (SIG_BLOCK, &mask, &oldmask); |
| while (!usr_interrupt) |
| sigsuspend (&oldmask); |
| sigprocmask (SIG_UNBLOCK, &mask, NULL); |
| </pre> |
| <p>This last piece of code is a little tricky. The key point to remember |
| here is that when <code>sigsuspend</code> returns, it resets the process's |
| signal mask to the original value, the value from before the call to |
| <code>sigsuspend</code>—in this case, the <code>SIGUSR1</code> signal is once |
| again blocked. The second call to <code>sigprocmask</code> is |
| necessary to explicitly unblock this signal. |
| |
| <p>One other point: you may be wondering why the <code>while</code> loop is |
| necessary at all, since the program is apparently only waiting for one |
| <code>SIGUSR1</code> signal. The answer is that the mask passed to |
| <code>sigsuspend</code> permits the process to be woken up by the delivery of |
| other kinds of signals, as well—for example, job control signals. If |
| the process is woken up by a signal that doesn't set |
| <code>usr_interrupt</code>, it just suspends itself again until the “right” |
| kind of signal eventually arrives. |
| |
| <p>This technique takes a few more lines of preparation, but that is needed |
| just once for each kind of wait criterion you want to use. The code |
| that actually waits is just four lines. |
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