ultrix man page for sigprocmask

sigprocmask(3)						     Library Functions Manual						    sigprocmask(3)

Name
       sigprocmask - examine and change blocked signals (POSIX)

Syntax
       #include <signal.h>

       int sigprocmask(how, set, oset)
       int how;
       sigset_t *set, *oset;

Description
       The system call is used to examine and/or change the calling process's signal mask. If the value of the argument set is not NULL, it points
       to a set of signals that will be used to change the currently blocked set.

       The value of the argument how indicates the manner in which the set is changed as defined by the following values, defined in <signal.h>:

       SIG_BLOCK
	      The resulting signal set is the union of the current set and the signal set pointed to by the argument set.

       SIG_UNBLOCK
	      The resulting signal set is the intersection of the current set and the complement of the signal set pointed to by the argument set.

       SIG_SETMASK
	      The resulting signal set is the signal set pointed to by the argument set.

       If the argument oset is not NULL, the previous mask is stored in the space pointed to by oset.  If the value of the argument set  is  NULL,
       the process's signal mask is unchanged; thus, the function can be used to enquire about currently blocked signals.

       The signal masks used as arguments to this function are manipulated using the functions.

       As a system restriction, SIGKILL and SIGSTOP cannot be blocked.

Return Values
       A 0 return value indicates a successful call.  A -1 return value indicates an error and errno is set to indicated the reason.

Diagnostics
       The function fails and the signal mask remains unchanged if the follow occurs:

       [EINVAL]       The value of the how argument is not equal to one of the defined values.

See Also
       kill(2), sigsetmask(2), sigvec(2), sigblock(2), sigsetops(3)

																    sigprocmask(3)