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| <h4 class="subsection">24.3.1 Basic Signal Handling</h4> |
| |
| <p><a name="index-g_t_0040code_007bsignal_007d-function-2907"></a> |
| The <code>signal</code> function provides a simple interface for establishing |
| an action for a particular signal. The function and associated macros |
| are declared in the header file <samp><span class="file">signal.h</span></samp>. |
| <a name="index-signal_002eh-2908"></a> |
| <!-- signal.h --> |
| <!-- GNU --> |
| |
| <div class="defun"> |
| — Data Type: <b>sighandler_t</b><var><a name="index-sighandler_005ft-2909"></a></var><br> |
| <blockquote><p>This is the type of signal handler functions. Signal handlers take one |
| integer argument specifying the signal number, and have return type |
| <code>void</code>. So, you should define handler functions like this: |
| |
| <pre class="smallexample"> void <var>handler</var> (int <code>signum</code>) { ... } |
| </pre> |
| <p>The name <code>sighandler_t</code> for this data type is a GNU extension. |
| </p></blockquote></div> |
| |
| <!-- signal.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Function: sighandler_t <b>signal</b> (<var>int signum, sighandler_t action</var>)<var><a name="index-signal-2910"></a></var><br> |
| <blockquote><p>The <code>signal</code> function establishes <var>action</var> as the action for |
| the signal <var>signum</var>. |
| |
| <p>The first argument, <var>signum</var>, identifies the signal whose behavior |
| you want to control, and should be a signal number. The proper way to |
| specify a signal number is with one of the symbolic signal names |
| (see <a href="Standard-Signals.html#Standard-Signals">Standard Signals</a>)—don't use an explicit number, because |
| the numerical code for a given kind of signal may vary from operating |
| system to operating system. |
| |
| <p>The second argument, <var>action</var>, specifies the action to use for the |
| signal <var>signum</var>. This can be one of the following: |
| |
| <dl> |
| <dt><code>SIG_DFL</code><dd><a name="index-SIG_005fDFL-2911"></a><a name="index-default-action-for-a-signal-2912"></a><code>SIG_DFL</code> specifies the default action for the particular signal. |
| The default actions for various kinds of signals are stated in |
| <a href="Standard-Signals.html#Standard-Signals">Standard Signals</a>. |
| |
| <br><dt><code>SIG_IGN</code><dd><a name="index-SIG_005fIGN-2913"></a><a name="index-ignore-action-for-a-signal-2914"></a><code>SIG_IGN</code> specifies that the signal should be ignored. |
| |
| <p>Your program generally should not ignore signals that represent serious |
| events or that are normally used to request termination. You cannot |
| ignore the <code>SIGKILL</code> or <code>SIGSTOP</code> signals at all. You can |
| ignore program error signals like <code>SIGSEGV</code>, but ignoring the error |
| won't enable the program to continue executing meaningfully. Ignoring |
| user requests such as <code>SIGINT</code>, <code>SIGQUIT</code>, and <code>SIGTSTP</code> |
| is unfriendly. |
| |
| <p>When you do not wish signals to be delivered during a certain part of |
| the program, the thing to do is to block them, not ignore them. |
| See <a href="Blocking-Signals.html#Blocking-Signals">Blocking Signals</a>. |
| |
| <br><dt><var>handler</var><dd>Supply the address of a handler function in your program, to specify |
| running this handler as the way to deliver the signal. |
| |
| <p>For more information about defining signal handler functions, |
| see <a href="Defining-Handlers.html#Defining-Handlers">Defining Handlers</a>. |
| </dl> |
| |
| <p>If you set the action for a signal to <code>SIG_IGN</code>, or if you set it |
| to <code>SIG_DFL</code> and the default action is to ignore that signal, then |
| any pending signals of that type are discarded (even if they are |
| blocked). Discarding the pending signals means that they will never be |
| delivered, not even if you subsequently specify another action and |
| unblock this kind of signal. |
| |
| <p>The <code>signal</code> function returns the action that was previously in |
| effect for the specified <var>signum</var>. You can save this value and |
| restore it later by calling <code>signal</code> again. |
| |
| <p>If <code>signal</code> can't honor the request, it returns <code>SIG_ERR</code> |
| instead. The following <code>errno</code> error conditions are defined for |
| this function: |
| |
| <dl> |
| <dt><code>EINVAL</code><dd>You specified an invalid <var>signum</var>; or you tried to ignore or provide |
| a handler for <code>SIGKILL</code> or <code>SIGSTOP</code>. |
| </dl> |
| </p></blockquote></div> |
| |
| <p><strong>Compatibility Note:</strong> A problem encountered when working with the |
| <code>signal</code> function is that it has different semantics on BSD and |
| SVID systems. The difference is that on SVID systems the signal handler |
| is deinstalled after signal delivery. On BSD systems the |
| handler must be explicitly deinstalled. In the GNU C Library we use the |
| BSD version by default. To use the SVID version you can either use the |
| function <code>sysv_signal</code> (see below) or use the <code>_XOPEN_SOURCE</code> |
| feature select macro (see <a href="Feature-Test-Macros.html#Feature-Test-Macros">Feature Test Macros</a>). In general, use of these |
| functions should be avoided because of compatibility problems. It |
| is better to use <code>sigaction</code> if it is available since the results |
| are much more reliable. |
| |
| <p>Here is a simple example of setting up a handler to delete temporary |
| files when certain fatal signals happen: |
| |
| <pre class="smallexample"> #include <signal.h> |
| |
| void |
| termination_handler (int signum) |
| { |
| struct temp_file *p; |
| |
| for (p = temp_file_list; p; p = p->next) |
| unlink (p->name); |
| } |
| |
| int |
| main (void) |
| { |
| ... |
| if (signal (SIGINT, termination_handler) == SIG_IGN) |
| signal (SIGINT, SIG_IGN); |
| if (signal (SIGHUP, termination_handler) == SIG_IGN) |
| signal (SIGHUP, SIG_IGN); |
| if (signal (SIGTERM, termination_handler) == SIG_IGN) |
| signal (SIGTERM, SIG_IGN); |
| ... |
| } |
| </pre> |
| <p class="noindent">Note that if a given signal was previously set to be ignored, this code |
| avoids altering that setting. This is because non-job-control shells |
| often ignore certain signals when starting children, and it is important |
| for the children to respect this. |
| |
| <p>We do not handle <code>SIGQUIT</code> or the program error signals in this |
| example because these are designed to provide information for debugging |
| (a core dump), and the temporary files may give useful information. |
| |
| <!-- signal.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: sighandler_t <b>sysv_signal</b> (<var>int signum, sighandler_t action</var>)<var><a name="index-sysv_005fsignal-2915"></a></var><br> |
| <blockquote><p>The <code>sysv_signal</code> implements the behavior of the standard |
| <code>signal</code> function as found on SVID systems. The difference to BSD |
| systems is that the handler is deinstalled after a delivery of a signal. |
| |
| <p><strong>Compatibility Note:</strong> As said above for <code>signal</code>, this |
| function should be avoided when possible. <code>sigaction</code> is the |
| preferred method. |
| </p></blockquote></div> |
| |
| <!-- signal.h --> |
| <!-- SVID --> |
| <div class="defun"> |
| — Function: sighandler_t <b>ssignal</b> (<var>int signum, sighandler_t action</var>)<var><a name="index-ssignal-2916"></a></var><br> |
| <blockquote><p>The <code>ssignal</code> function does the same thing as <code>signal</code>; it is |
| provided only for compatibility with SVID. |
| </p></blockquote></div> |
| |
| <!-- signal.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Macro: sighandler_t <b>SIG_ERR</b><var><a name="index-SIG_005fERR-2917"></a></var><br> |
| <blockquote><p>The value of this macro is used as the return value from <code>signal</code> |
| to indicate an error. |
| </p></blockquote></div> |
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