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| <h4 class="subsection">24.4.1 Signal Handlers that Return</h4> |
| |
| <p>Handlers which return normally are usually used for signals such as |
| <code>SIGALRM</code> and the I/O and interprocess communication signals. But |
| a handler for <code>SIGINT</code> might also return normally after setting a |
| flag that tells the program to exit at a convenient time. |
| |
| <p>It is not safe to return normally from the handler for a program error |
| signal, because the behavior of the program when the handler function |
| returns is not defined after a program error. See <a href="Program-Error-Signals.html#Program-Error-Signals">Program Error Signals</a>. |
| |
| <p>Handlers that return normally must modify some global variable in order |
| to have any effect. Typically, the variable is one that is examined |
| periodically by the program during normal operation. Its data type |
| should be <code>sig_atomic_t</code> for reasons described in <a href="Atomic-Data-Access.html#Atomic-Data-Access">Atomic Data Access</a>. |
| |
| <p>Here is a simple example of such a program. It executes the body of |
| the loop until it has noticed that a <code>SIGALRM</code> signal has arrived. |
| This technique is useful because it allows the iteration in progress |
| when the signal arrives to complete before the loop exits. |
| |
| <pre class="smallexample"> #include <signal.h> |
| #include <stdio.h> |
| #include <stdlib.h> |
| |
| /* <span class="roman">This flag controls termination of the main loop.</span> */ |
| volatile sig_atomic_t keep_going = 1; |
| |
| /* <span class="roman">The signal handler just clears the flag and re-enables itself.</span> */ |
| void |
| catch_alarm (int sig) |
| { |
| keep_going = 0; |
| signal (sig, catch_alarm); |
| } |
| |
| void |
| do_stuff (void) |
| { |
| puts ("Doing stuff while waiting for alarm...."); |
| } |
| |
| int |
| main (void) |
| { |
| /* <span class="roman">Establish a handler for SIGALRM signals.</span> */ |
| signal (SIGALRM, catch_alarm); |
| |
| /* <span class="roman">Set an alarm to go off in a little while.</span> */ |
| alarm (2); |
| |
| /* <span class="roman">Check the flag once in a while to see when to quit.</span> */ |
| while (keep_going) |
| do_stuff (); |
| |
| return EXIT_SUCCESS; |
| } |
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