C signal handling: Difference between revisions

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

{{Use dmy dates|date=MayJuly 20202022}}

{{C_Standard Library}}

In the [[C Standard Library]], '''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==

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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=https://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> -– [[Floatingfloating point exception|'''f'''loating '''p'''oint '''e'''xception]].

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

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

*<code>SIGSEGV</code> -– "[[segmentation violation|'''seg'''mentation '''v'''iolation]]", 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]]) define more than 15 additional signals; see [[Unix signal]].<ref name="sus">{{cite web | url=https://pubs.opengroup.org/onlinepubs/007904975/ | title=The Open Group Base Specifications Issue 6 -– signal.h -– signals | accessdate=10 January 2012}}</ref>

=== Debugging ===

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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>.

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 isits nowuse consideredis deprecateddiscouraged.<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).

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

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#include <stdio.h>

#include <stdlib.h>

volatile sig_atomic_t status = 0;

static void catch_function(int signo) {

status = signo;

puts("Interactive attention signal caught.");

}

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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 EXIT_SUCCESS;