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| <h4 class="subsection">24.4.3 Nonlocal Control Transfer in Handlers</h4> |
| |
| <p><a name="index-non_002dlocal-exit_002c-from-signal-handler-2931"></a> |
| You can do a nonlocal transfer of control out of a signal handler using |
| the <code>setjmp</code> and <code>longjmp</code> facilities (see <a href="Non_002dLocal-Exits.html#Non_002dLocal-Exits">Non-Local Exits</a>). |
| |
| <p>When the handler does a nonlocal control transfer, the part of the |
| program that was running will not continue. If this part of the program |
| was in the middle of updating an important data structure, the data |
| structure will remain inconsistent. Since the program does not |
| terminate, the inconsistency is likely to be noticed later on. |
| |
| <p>There are two ways to avoid this problem. One is to block the signal |
| for the parts of the program that update important data structures. |
| Blocking the signal delays its delivery until it is unblocked, once the |
| critical updating is finished. See <a href="Blocking-Signals.html#Blocking-Signals">Blocking Signals</a>. |
| |
| <p>The other way to re-initialize the crucial data structures in the signal |
| handler, or make their values consistent. |
| |
| <p>Here is a rather schematic example showing the reinitialization of one |
| global variable. |
| |
| <pre class="smallexample"> #include <signal.h> |
| #include <setjmp.h> |
| |
| jmp_buf return_to_top_level; |
| |
| volatile sig_atomic_t waiting_for_input; |
| |
| void |
| handle_sigint (int signum) |
| { |
| /* <span class="roman">We may have been waiting for input when the signal arrived,</span> |
| <span class="roman">but we are no longer waiting once we transfer control.</span> */ |
| waiting_for_input = 0; |
| longjmp (return_to_top_level, 1); |
| } |
| |
| int |
| main (void) |
| { |
| ... |
| signal (SIGINT, sigint_handler); |
| ... |
| while (1) { |
| prepare_for_command (); |
| if (setjmp (return_to_top_level) == 0) |
| read_and_execute_command (); |
| } |
| } |
| |
| /* <span class="roman">Imagine this is a subroutine used by various commands.</span> */ |
| char * |
| read_data () |
| { |
| if (input_from_terminal) { |
| waiting_for_input = 1; |
| ... |
| waiting_for_input = 0; |
| } else { |
| ... |
| } |
| } |
| </pre> |
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