C signal handling: Difference between revisions

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{{Short description|Handling of signals in the C programming language}}

{{Use dmy dates|date=JanuaryJuly 20122022}}

{{C_Standard Library}}

In the [[C Standard Library]], a '''signal processing''' defines how a program handles various [[Signal (computing)|signals]] while it executes. A signal can report some exceptional behavior within the program (''such as [[division by zero]]''), or a signal can report some asynchronous event outside the program (''such as someone striking an [[SIGINT (POSIX)|interactive attention key]] on a keyboard'').

==Standard signals==

{{see also|Unix signal}}

The C standard defines only 6 signals. They are all defined in <code>signal.h</code> header (<code>csignal</code> header in [[C++]]):<ref name=c99>{{cite book | url=httphttps://www.open-std.org/jtc1/sc22/wg14/www/docs/n1124.pdf | title=ISO/IEC 9899:1999 specification | at=p. 258, § 7.14 ''Signal handling''}}</ref>

*<code>SIGABRT</code> -– "abort", abnormal termination.

*<code>SIGFPE</code> -– [[floating point exception]].

*<code>SIGILL</code> -– "illegal", invalid instruction.

*<code>SIGINT</code> -– "interrupt", interactive attention request sent to the program.

*<code>SIGSEGV</code> -– "[[segmentation violation]]", invalid memory access.

*<code>SIGTERM</code> -– "terminate", termination request sent to the program.

Additional signals may be specified in the <code>signal.h</code> header by the implementation. For example, Unix and [[Unix-like]] operating systems (such as [[Linux]]) definesdefine more than 15 additional signals; see [[Unix signal]].<ref name="sus">{{cite web | url=httphttps://pubs.opengroup.org/onlinepubs/007904975/ | title=The Open Group Base Specifications Issue 6 -– signal.h -– signals | accessdate=10 January 2012}}</ref>

=== Debugging ===

*<code>SIGTRAP</code> for debugging purposes. It's platform-dependent and may be used on [[Unix]]-like operating systems.

==Handling==

A signal can be generated by calling <code>raise()</code> or <code>kill()</code> system calls. <code>raise()</code> sends a signal to the current process, <code>kill()</code> sends a signal to a specific process.

A signal handler can be specified for all but two signals ([[SIGKILL]] and [[SIGSTOP]] cannot be caught, blocked or ignored). A signal handler is a [[Function (computer science)|function]] which is called by the target environment when the corresponding signal occurs. The target environment suspends execution of the program until the signal handler returns or calls <code>longjmp()</code>. For maximum portability, an asynchronous signal handler should only:

Signal handlers can be set with <code>signal()</code> or <code>sigaction()</code>. The behavior of <code>signal()</code> has been changed multiple times across history and its use is discouraged.<ref>https://man7.org/linux/man-pages/man2/signal.2.html Signal(2) manpage</ref> It is only portable when used to set a signal's disposition to SIG_DFL or SIG_IGN. Signal handlers can be specified for all but two signals ([[SIGKILL]] and [[SIGSTOP]] cannot be caught, blocked or ignored).

A signal handler can be specified for all but two signals ([[SIGKILL]] and [[SIGSTOP]] cannot be caught, blocked or ignored). A signal handler is a [[Function (computer science)|function]] which is called by the target environment when the corresponding signal occurs. The target environment suspends execution of the program until the signal handler returns or calls <code>longjmp()</code>. For maximum portability, an asynchronous signal handler should only:

*make successful calls to the function <code>signal()</code>

*assign values to objects of type [[volatile variable|volatile]] <code>sig_atomic_t</code>

*return control to its caller

If the signal reports an error within the program (and the signal is not asynchronous), the signal handler can terminate by calling <code>abort()</code>, <code>exit()</code>, or <code>longjmp()</code>.

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==Functions==

{| class="wikitable" style="font-size:0.85em"

! Function

! Description

|-

| {{anchor|raise}}<code>[httphttps://en.cppreference.com/w/c/program/raise raise]</code>

| artificially raisessends a signal to the calling process

|-

| {{anchor|signalraise}}<code>[http://en.cppreference.com/w/c/program/signal signal]kill</code>

|artificially sends a signal to a specified process

|-

| {{anchor|signal}}<code>[https://en.cppreference.com/w/c/program/signal signal]</code>

| sets the action taken when the program receives a specific signal

|}

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==Example usage==

<sourcesyntaxhighlight lang="c">

#include <signal.h>

#include <stdio.h>

#include <stdlib.h>

volatile sig_atomic_t status = 0;

static void catch_function(int signal) {

puts("Interactive attention signal caught.");

static void catch_function(int signalsigno) {

status = signo;

}

int main(void) {

// Set above function as signal handler for the SIGINT signal:

if (signal(SIGINT, catch_function) == SIG_ERR) {

fputs("An error occurred while setting a signal handler.\n", stderr);

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}

puts("Raising the interactive attention signal.");

if (raise(SIGINT) != 0) {

fputs("Error raising the signal.\n", stderr);

return EXIT_FAILURE;

}

if (status == SIGINT) puts("Interactive attention signal caught.");

puts("Exiting.");

return 0EXIT_SUCCESS;

// exiting after raising signal

}

</syntaxhighlight>

</source>

==See also==

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==References==

{{Reflist}}

==External links==

* [http://www.xgc.com/manuals/xgclib/c1632.html XGC's manual for signal.h]

{{Use dmy dates|date=January 2012}}

[[Category:C standard library]]

[[es:Signal.h]]

[[pt:Signal.h]]

[[ru:Signal.h]]

[[uk:Signal.h]]