This interface is made obsolete by sigprocmask(2).
The sigblock system call adds the signals specified in mask to the
set of signals currently being blocked from delivery.
The sigsetmask system call replaces the set of blocked signals totally
with a new set specified in mask. Signals are blocked if the
corresponding bit in mask is a 1.
The current set of blocked signals can be obtained using siggetmask.
The sigmask macro is provided to construct the mask for a given
Prototypes for these functions are only available if _BSD_SOURCE is
defined before the inclusion of any system header file.
It is not possible to block SIGKILL or SIGSTOP - this restriction
is silently imposed by the system.