sigblock - block signals
Sigblock causes the signals specified in mask to be added to the set of
signals currently being blocked from delivery. Signal i is blocked if
the i-th bit in mask is a 1.
It is not possible to block SIGKILL, SIGSTOP, or SIGCONT; this
restriction is silently imposed by the system.
The previous set of masked signals is returned.
kill(2), sigvec(2), sigsetmask(2),
4th Berkeley Distribution 15 June 1983 SIGBLOCK(2)