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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)

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.