< c‎ | program
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.


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

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.


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.


#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);
  printf("SignalValue: %d\n", gSignalStatus);


SignalValue: 0
Sending signal: 2
SignalValue: 2


  • C11 standard (ISO/IEC 9899:2011):
  • The signal function (p: 266-267)
  • C99 standard (ISO/IEC 9899:1999):
  • The signal function (p: 247-248)
  • C89/C90 standard (ISO/IEC 9899:1990):
  • The signal function

See also

runs the signal handler for particular signal