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| <h4 class="subsection">24.4.4 Signals Arriving While a Handler Runs</h4> |
| |
| <p><a name="index-race-conditions_002c-relating-to-signals-2932"></a> |
| What happens if another signal arrives while your signal handler |
| function is running? |
| |
| <p>When the handler for a particular signal is invoked, that signal is |
| automatically blocked until the handler returns. That means that if two |
| signals of the same kind arrive close together, the second one will be |
| held until the first has been handled. (The handler can explicitly |
| unblock the signal using <code>sigprocmask</code>, if you want to allow more |
| signals of this type to arrive; see <a href="Process-Signal-Mask.html#Process-Signal-Mask">Process Signal Mask</a>.) |
| |
| <p>However, your handler can still be interrupted by delivery of another |
| kind of signal. To avoid this, you can use the <code>sa_mask</code> member of |
| the action structure passed to <code>sigaction</code> to explicitly specify |
| which signals should be blocked while the signal handler runs. These |
| signals are in addition to the signal for which the handler was invoked, |
| and any other signals that are normally blocked by the process. |
| See <a href="Blocking-for-Handler.html#Blocking-for-Handler">Blocking for Handler</a>. |
| |
| <p>When the handler returns, the set of blocked signals is restored to the |
| value it had before the handler ran. So using <code>sigprocmask</code> inside |
| the handler only affects what signals can arrive during the execution of |
| the handler itself, not what signals can arrive once the handler returns. |
| |
| <p><strong>Portability Note:</strong> Always use <code>sigaction</code> to establish a |
| handler for a signal that you expect to receive asynchronously, if you |
| want your program to work properly on System V Unix. On this system, |
| the handling of a signal whose handler was established with |
| <code>signal</code> automatically sets the signal's action back to |
| <code>SIG_DFL</code>, and the handler must re-establish itself each time it |
| runs. This practice, while inconvenient, does work when signals cannot |
| arrive in succession. However, if another signal can arrive right away, |
| it may arrive before the handler can re-establish itself. Then the |
| second signal would receive the default handling, which could terminate |
| the process. |
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