#include <signal.h> 
 sigprocmask(how, set, oset);  int how;  sigset_t *set, *oset; 
 mask = sigmask(signum) 
 DESCRIPTION
The
 sigprocmask function examines and/or the current signal mask (those signals
that are blocked from delivery).
Signals are blocked if they are members of the current signal mask set.

If
 set is not null, the action of
 sigprocmask depends on the value of the parameter
 how . The signal mask is changed as a function of the specified
 set and the current mask.
The function is specified by
 how using one of the following values from <signal.h>:
 SIG_BLOCK \w'SIG_SETMASK\0\0'u
The new mask is the union of the current mask and the specified
 set . 
 SIG_UNBLOCK \w'SIG_SETMASK\0\0'u
The new mask is the intersection of the current mask
and the complement of the specified
 set . 
 SIG_SETMASK \w'SIG_SETMASK\0\0'u
The current mask is replaced by the specified
 set . 

If
 oset is not null,
the previous value of the signal mask.
When
 set is null, this provides a way to examine the signal mask without changing it.

The system
quietly disallows SIGKILL or SIGSTOP to be blocked.
 "RETURN VALUE
A 0 value indicated that the call succeeded. A -1 return value
indicates an error occurred and
 errno is set to indicated the reason.
 ERRORS
The
 sigprocmask call will fail and the signal mask will be unchanged if one
of the following occurs:
 15
[EINVAL]
 how has a value other than those listed here.
 "SEE ALSO"
kill(2), sigaction(2), sigsetops(2), sigsuspend(2)