Checking for Pending Signals - The GNU C Library

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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.