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| <h4 class="subsection">24.8.2 Problems with <code>pause</code></h4> |
| |
| <p>The simplicity of <code>pause</code> can conceal serious timing errors that |
| can make a program hang mysteriously. |
| |
| <p>It is safe to use <code>pause</code> if the real work of your program is done |
| by the signal handlers themselves, and the “main program” does nothing |
| but call <code>pause</code>. Each time a signal is delivered, the handler |
| will do the next batch of work that is to be done, and then return, so |
| that the main loop of the program can call <code>pause</code> again. |
| |
| <p>You can't safely use <code>pause</code> to wait until one more signal arrives, |
| and then resume real work. Even if you arrange for the signal handler |
| to cooperate by setting a flag, you still can't use <code>pause</code> |
| reliably. Here is an example of this problem: |
| |
| <pre class="smallexample"> /* <code>usr_interrupt</code><span class="roman"> is set by the signal handler.</span> */ |
| if (!usr_interrupt) |
| pause (); |
| |
| /* <span class="roman">Do work once the signal arrives.</span> */ |
| ... |
| </pre> |
| <p class="noindent">This has a bug: the signal could arrive after the variable |
| <code>usr_interrupt</code> is checked, but before the call to <code>pause</code>. |
| If no further signals arrive, the process would never wake up again. |
| |
| <p>You can put an upper limit on the excess waiting by using <code>sleep</code> |
| in a loop, instead of using <code>pause</code>. (See <a href="Sleeping.html#Sleeping">Sleeping</a>, for more |
| about <code>sleep</code>.) Here is what this looks like: |
| |
| <pre class="smallexample"> /* <code>usr_interrupt</code><span class="roman"> is set by the signal handler.</span> |
| while (!usr_interrupt) |
| sleep (1); |
| |
| /* <span class="roman">Do work once the signal arrives.</span> */ |
| ... |
| </pre> |
| <p>For some purposes, that is good enough. But with a little more |
| complexity, you can wait reliably until a particular signal handler is |
| run, using <code>sigsuspend</code>. |
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