The GNU C Library - Checking for Pending Signals

Checking for Pending Signals

You can find out which signals are pending at any time by calling sigpending . This function is declared in `signal.h'.

Function int sigpending (sigset_t *set)

The sigpending function stores information about pending signals in set. If there is a pending signal that is blocked from delivery, then that signal is a member of the returned set. (You can test whether a particular signal is a member of this set using sigismember ; see Signal Sets.)

The return value is 0 if successful, and -1 on failure.

Testing whether a signal is pending is not often useful. Testing when that signal is not blocked is almost certainly bad design.

Here is an example.

	#include <signal.h>
	#include <stddef.h>
	
	sigset_t base_mask, waiting_mask;
	
	sigemptyset (&base_mask);
	sigaddset (&base_mask, SIGINT);
	sigaddset (&base_mask, SIGTSTP);
	
	/* Block user interrupts while doing other processing. */
	sigprocmask (SIG_SETMASK, &base_mask, NULL); 
	...
	
	/* After a while, check to see whether any signals are pending. */
	sigpending (&waiting_mask);
	if (sigismember (&waiting_mask, SIGINT)) {
	  /* User has tried to kill the process. */
	}
	else if (sigismember (&waiting_mask, SIGTSTP)) {
	  /* User has tried to stop the process. */
	}

Remember that if there is a particular signal pending for your process, additional signals of that same type that arrive in the meantime might be discarded. For example, if a SIGINT signal is pending when another SIGINT signal arrives, your program will probably only see one of them when you unblock this signal.

Portability Note: The sigpending function is new in POSIX.1. Older systems have no equivalent facility.