The sigprocmask() function is used to examine and/or change the caller's signal mask. If the value is SIG_BLOCK, the set pointed to by
the set argument is added to the current signal mask. If the value is SIG_UNBLOCK, the set pointed by the set argument
is removed from the current signal mask. If the value is SIG_SETMASK, the current signal mask is replaced by the set pointed to by the set
argument. If the oset argument is not NULL, the previous mask is stored in the space pointed to by oset. If the value of the
set argument is NULL, the value how is not significant and the caller's signal mask is unchanged; thus, the call can be used
to inquire about currently blocked signals.
If there are any pending unblocked signals after the call to sigprocmask(), at least one of those signals will be delivered before the call to sigprocmask() returns.
It is not possible to block those signals that cannot be ignored this restriction is silently imposed by the system. See sigaction(2).
If sigprocmask() fails, the caller's signal mask is not changed.