signal

Defined in header `<signal.h>`
void (signal( int sig, void (handler) (int))) (int);

Sets the error handler for signal sig. The signal handler can be set so that default handling will occur, signal is ignored, or a user-defined function is called.

When signal handler is set to a function and a signal occurs, it is implementation defined whether signal(sig, SIG_DFL) will be executed immediately before the start of signal handler. Also, the implementation can prevent some implementation-defined set of signals from occurring while the signal handler runs.

Parameters

sig

-

the signal to set the signal handler to. It can be an implementation-defined value or one of the following values:

SIGABRTSIGFPESIGILLSIGINTSIGSEGVSIGTERM

defines signal types
(macro constant)

handler

-

the signal handler. This must be one of the following:

SIG_DFL macro. The signal handler is set to default signal handler.
SIG_IGN macro. The signal is ignored.
pointer to a function. The signature of the function must be equivalent to the following:

void fun(int sig);

Return value

Previous signal handler on success or SIG_ERR on failure (setting a signal handler can be disabled on some implementations).

Signal handler

The following limitations are imposed on the user-defined function that is installed as a signal handler.

If the user defined function returns when handling SIGFPE, SIGILL or SIGSEGV, the behavior is undefined.

If the signal handler is called as a result of abort or raise, the behavior is undefined if the signal handler calls raise.

If the signal handler is called NOT as a result of abort or raise (in other words, the signal handler is asynchronous), the behavior is undefined if

the signal handler calls any function within the standard library, except

abort
_Exit
quick_exit
signal with the first argument being the number of the signal currently handled (async handler can re-register itself, but not other signals).
atomic functions from <stdatomic.h> if the atomic arguments are lock-free
atomic_is_lock_free (with any kind of atomic arguments)

the signal handler refers to any object with static or thread-local(since C11) storage duration that is not a lock-free atomic(since C11) other than by assigning to a static volatile sig_atomic_t.

On entry to the signal handler, the state of the floating-point environment and the values of all objects is unspecified, except for

objects of type volatile sig_atomic_t
objects of lock-free atomic types (since C11)
side effects made visible through atomic_signal_fence (since C11)

On return from a signal handler, the value of any object modified by the signal handler that is not volatile sig_atomic_t or lock-free atomic(since C11) is undefined.

The behavior is undefined if signal is used in a multithreaded program. It is not required to be thread-safe.

Notes

POSIX requires that signal is thread-safe, and specifies a list of async-signal-safe library functions that may be called from any signal handler.

Besides abort and raise, POSIX specifies that kill, pthread_kill, and sigqueue generate synchronous signals.

POSIX recommends sigaction instead of signal, due to its underspecified behavior and significant implementation variations, regarding signal delivery while a signal handler is executed.

Example

Run this code

#include <signal.h>
#include <stdio.h>
 
volatile sig_atomic_t gSignalStatus;
 
void signal_handler(int signal)
{
  gSignalStatus = signal;
}
 
int main(void)
{
  signal(SIGINT, signal_handler);
 
  printf("SignalValue: %d\n", gSignalStatus);
  printf("Sending signal: %d\n", SIGINT);
  raise(SIGINT);
  printf("SignalValue: %d\n", gSignalStatus);
}

Output:

SignalValue: 0
Sending signal: 2
SignalValue: 2

References

C17 standard (ISO/IEC 9899:2018):

7.14.1.1 The signal function (p: 193-194)

C11 standard (ISO/IEC 9899:2011):

7.14.1.1 The signal function (p: 266-267)

C99 standard (ISO/IEC 9899:1999):

7.14.1.1 The signal function (p: 247-248)

C89/C90 standard (ISO/IEC 9899:1990):

4.7.1.1 The signal function

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