Signal Handling with signal
suggest changeSignal numbers can be synchronous (like SIGSEGV
– segmentation fault) when they are triggered by a malfunctioning of the program itself or asynchronous (like SIGINT
- interactive attention) when they are initiated from outside the program, e.g by a keypress as Cntrl-C
.
The signal()
function is part of the ISO C standard and can be used to assign a function to handle a specific signal
#include <stdio.h> /* printf() */
#include <stdlib.h> /* abort() */
#include <signal.h> /* signal() */
void handler_nonportable(int sig)
{
/* undefined behavior, maybe fine on specific platform */
printf("Catched: %d\n", sig);
/* abort is safe to call */
abort();
}
sig_atomic_t volatile finished = 0;
void handler(int sig)
{
switch (sig) {
/* hardware interrupts should not return */
case SIGSEGV:
case SIGFPE:
case SIGILL:
/* quick_exit is safe to call */
quick_exit(EXIT_FAILURE);
/* use _Exit in pre-C11 */
_Exit(EXIT_FAILURE);
default:
/* Reset the signal to the default handler,
so we will not be called again if things go
wrong on return. */
signal(sig, SIG_DFL);
/* let everybody know that we are finished */
finished = sig;
return;
}
}
int main(void)
{
/* Catch the SIGSEGV signal, raised on segmentation faults (i.e NULL ptr access */
if (signal(SIGSEGV, &handler) == SIG_ERR) {
perror("could not establish handler for SIGSEGV");
return EXIT_FAILURE;
}
/* Catch the SIGTERM signal, termination request */
if (signal(SIGTERM, &handler) == SIG_ERR) {
perror("could not establish handler for SIGTERM");
return EXIT_FAILURE;
}
/* Ignore the SIGINT signal, by setting the handler to `SIG_IGN`. */
signal(SIGINT, SIG_IGN);
/* Do something that takes some time here, and leaves
the time to terminate the program from the keyboard. */
/* Then: */
if (finished) {
fprintf(stderr, "we have been terminated by signal %d\n", (int)finished);
return EXIT_FAILURE;
}
/* Try to force a segmentation fault, and raise a SIGSEGV */
{
char* ptr = 0;
*ptr = 0;
}
/* This should never be executed */
return EXIT_SUCCESS;
}
Using signal()
imposes important limitations what you are allowed to do inside the signal handlers, see the remarks for further information.
POSIX recommends the usage of sigaction()
instead of signal()
, due to its underspecified behavior and significant implementation variations. POSIX also defines many more signals than ISO C standard, including SIGUSR1
and SIGUSR2
, which can be used freely by the programmer for any purpose.