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| <h4 class="subsection">24.7.1 Why Blocking Signals is Useful</h4> |
| |
| <p>Temporary blocking of signals with <code>sigprocmask</code> gives you a way to |
| prevent interrupts during critical parts of your code. If signals |
| arrive in that part of the program, they are delivered later, after you |
| unblock them. |
| |
| <p>One example where this is useful is for sharing data between a signal |
| handler and the rest of the program. If the type of the data is not |
| <code>sig_atomic_t</code> (see <a href="Atomic-Data-Access.html#Atomic-Data-Access">Atomic Data Access</a>), then the signal |
| handler could run when the rest of the program has only half finished |
| reading or writing the data. This would lead to confusing consequences. |
| |
| <p>To make the program reliable, you can prevent the signal handler from |
| running while the rest of the program is examining or modifying that |
| data—by blocking the appropriate signal around the parts of the |
| program that touch the data. |
| |
| <p>Blocking signals is also necessary when you want to perform a certain |
| action only if a signal has not arrived. Suppose that the handler for |
| the signal sets a flag of type <code>sig_atomic_t</code>; you would like to |
| test the flag and perform the action if the flag is not set. This is |
| unreliable. Suppose the signal is delivered immediately after you test |
| the flag, but before the consequent action: then the program will |
| perform the action even though the signal has arrived. |
| |
| <p>The only way to test reliably for whether a signal has yet arrived is to |
| test while the signal is blocked. |
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