C library function - signal()



The C library function void (*signal(int sig, void (*func)(int)))(int) sets a function to handle signal i.e. a signal handler with signal number sig.


Following is the declaration for signal() function.

void (*signal(int sig, void (*func)(int)))(int)


  • sig − This is the signal number to which a handling function is set. The following are few important standard signal numbers −

macro signal
SIGABRT (Signal Abort) Abnormal termination, such as is initiated by the function.
SIGFPE (Signal Floating-Point Exception) Erroneous arithmetic operation, such as zero divide or an operation resulting in overflow (not necessarily with a floating-point operation).
SIGILL (Signal Illegal Instruction) Invalid function image, such as an illegal instruction. This is generally due to a corruption in the code or to an attempt to execute data.
SIGINT (Signal Interrupt) Interactive attention signal. Generally generated by the application user.
SIGSEGV (Signal Segmentation Violation) Invalid access to storage − When a program tries to read or write outside the memory it is allocated for it.
SIGTERM (Signal Terminate) Termination request sent to program.
  • func − This is a pointer to a function. This can be a function defined by the programmer or one of the following predefined functions −

SIG_DFL Default handling − The signal is handled by the default action for that particular signal.
SIG_IGN Ignore Signal − The signal is ignored.

Return Value

This function returns the previous value of the signal handler, or SIG_ERR on error.


The following example shows the usage of signal() function.

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <signal.h>

void sighandler(int);

int main()
   signal(SIGINT, sighandler);

      printf("Going to sleep for a second...\n");

void sighandler(int signum)
   printf("Caught signal %d, coming out...\n", signum);

Let us compile and run the above program that will produce the following result and program will go in infinite loop. To come out of the program we used CTRL + C keys.

Going to sleep for a second...
Going to sleep for a second...
Going to sleep for a second...
Going to sleep for a second...
Going to sleep for a second...
Caught signal 2, coming out...