C signal handling

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In the C Standard Library, signal processing defines how a program handles various 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 interactive attention key on a keyboard).

Contents

Standard signals

The C standard defines only 6 signals. They are all defined in signal.h header (csignal header in C++): [1]

Additional signals may be specified in the signal.h header by the implementation. For example, Unix and Unix-like operating systems (such as Linux) define more than 15 additional signals; see Unix signal. [2]

Debugging

Handling

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

A signal handler is a 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 longjmp().

Signal handlers can be set with signal() or sigaction(). The behavior of signal() has been changed multiple times across history and its use is discouraged. [3] 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 abort(), exit(), or longjmp().

Functions

FunctionDescription
raise artificially sends a signal to the calling process
killartificially sends a signal to a specified process
signal sets the action taken when the program receives a specific signal

Example usage

#include<signal.h>#include<stdio.h>#include<stdlib.h>volatilesig_atomic_tstatus=0;staticvoidcatch_function(intsigno){status=signo;}intmain(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);returnEXIT_FAILURE;}puts("Raising the interactive attention signal.");if(raise(SIGINT)){fputs("Error raising the signal.\n",stderr);returnEXIT_FAILURE;}if(status==SIGINT)puts("Interactive attention signal caught.");puts("Exiting.");returnEXIT_SUCCESS;// exiting after raising signal}

See also

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References

  1. ↑ ISO/IEC 9899:1999 specification (PDF). p. 258, § 7.14 Signal handling.
  2. ↑ "The Open Group Base Specifications Issue 6 – signal.h – signals" . Retrieved 10 January 2012.
  3. ↑ https://man7.org/linux/man-pages/man2/signal.2.html Signal(2) manpage
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