Description

   The sigprocmask function is used to examine or change the signal
   mask of the calling process.

   Typically, use the sigprocmask SIG_BLOCK value to block signals
   during a critical section of code, then use the sigprocmask SIG_
   SETMASK value to restore the mask to the previous value returned
   by the sigprocmask SIG_BLOCK value.

   If there are any unblocked signals pending after the call to the
   sigprocmask function, at least one of those signals is delivered
   before the sigprocmask function returns.

   You cannot block SIGKILL or SIGSTOP signals with the sigprocmask
   function. If a program attempts to block one of these signals,
   the sigprocmask function gives no indication of the error.