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| <h4 class="subsection">24.7.5 Blocking Signals for a Handler</h4> |
| |
| <p><a name="index-blocking-signals_002c-in-a-handler-2972"></a> |
| When a signal handler is invoked, you usually want it to be able to |
| finish without being interrupted by another signal. From the moment the |
| handler starts until the moment it finishes, you must block signals that |
| might confuse it or corrupt its data. |
| |
| <p>When a handler function is invoked on a signal, that signal is |
| automatically blocked (in addition to any other signals that are already |
| in the process's signal mask) during the time the handler is running. |
| If you set up a handler for <code>SIGTSTP</code>, for instance, then the |
| arrival of that signal forces further <code>SIGTSTP</code> signals to wait |
| during the execution of the handler. |
| |
| <p>However, by default, other kinds of signals are not blocked; they can |
| arrive during handler execution. |
| |
| <p>The reliable way to block other kinds of signals during the execution of |
| the handler is to use the <code>sa_mask</code> member of the <code>sigaction</code> |
| structure. |
| |
| <p>Here is an example: |
| |
| <pre class="smallexample"> #include <signal.h> |
| #include <stddef.h> |
| |
| void catch_stop (); |
| |
| void |
| install_handler (void) |
| { |
| struct sigaction setup_action; |
| sigset_t block_mask; |
| |
| sigemptyset (&block_mask); |
| /* <span class="roman">Block other terminal-generated signals while handler runs.</span> */ |
| sigaddset (&block_mask, SIGINT); |
| sigaddset (&block_mask, SIGQUIT); |
| setup_action.sa_handler = catch_stop; |
| setup_action.sa_mask = block_mask; |
| setup_action.sa_flags = 0; |
| sigaction (SIGTSTP, &setup_action, NULL); |
| } |
| </pre> |
| <p>This is more reliable than blocking the other signals explicitly in the |
| code for the handler. If you block signals explicitly in the handler, |
| you can't avoid at least a short interval at the beginning of the |
| handler where they are not yet blocked. |
| |
| <p>You cannot remove signals from the process's current mask using this |
| mechanism. However, you can make calls to <code>sigprocmask</code> within |
| your handler to block or unblock signals as you wish. |
| |
| <p>In any case, when the handler returns, the system restores the mask that |
| was in place before the handler was entered. If any signals that become |
| unblocked by this restoration are pending, the process will receive |
| those signals immediately, before returning to the code that was |
| interrupted. |
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