Description

   The sigsuspend function replaces the signal mask of the process
   with the set of signals pointed to by the signal_mask argument.
   Then it suspends execution of the process until delivery of
   a signal whose action is either to execute a signal catching
   function or to terminate the process. You cannot block the
   SIGKILL or SIGSTOP signals with the sigsuspend function. If a
   program attempts to block either of these signals, sigsuspend
   gives no indication of the error.

   If delivery of a signal causes the process to terminate,
   sigsuspend does not return. If delivery of a signal causes a
   signal catching function to execute, sigsuspend returns after the
   signal catching function returns, with the signal mask restored
   to the set that existed prior to the call to sigsuspend.

   The sigsuspend function sets the signal mask and waits for
   an unblocked signal as one atomic operation. This means that
   signals cannot occur between the operations of setting the mask
   and waiting for a signal. If a program invokes sigprocmask SIG_
   SETMASK and sigsuspend separately, a signal that occurs between
   these functions is often not noticed by sigsuspend.

   In normal usage, a signal is blocked by using the sigprocmask
   function at the beginning of a critical section. The process then
   determines whether there is work for it to do. If there is no
   work, the process waits for work by calling sigsuspend with the
   mask previously returned by sigprocmask.

   If a signal is intercepted by the calling process and control
   is returned from the signal handler, the calling process resumes
   execution after sigsuspend, which always returns a value of -1
   and sets errno to EINTR.

   See also sigpause and sigprocmask.