24.7.6 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)

Preliminary: | MT-Safe | AS-Unsafe lock/hurd | AC-Unsafe lock/hurd | See POSIX Safety Concepts.

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.