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SIGSETMASK(3) Library Functions Manual SIGSETMASK(3)

sigsetmask
set current signal mask

#include <signal.h>
int
sigsetmask(int mask);
sigmask(int signum);

This interface is made obsoleted by:
sigprocmask(2).
sigsetmask() sets the current signal mask. Signals are blocked from delivery if the corresponding bit in mask is a 1; the macro sigmask() is provided to construct the mask for a given signum.
The system quietly disallows SIGKILL or SIGSTOP to be blocked.

The previous set of masked signals is returned.

The following example utilizing sigsetmask():
int omask; 
 
omask = sigblock(sigmask(SIGINT) | sigmask(SIGHUP)); 
 
... 
 
sigsetmask(omask & ~(sigmask(SIGINT) | sigmask(SIGHUP)));
Could be converted literally to:
sigset_t set, oset; 
 
sigemptyset(&set); 
sigaddset(&set, SIGINT); 
sigaddset(&set, SIGHUP); 
sigprocmask(SIG_BLOCK, &set, &oset); 
 
... 
 
sigdelset(&oset, SIGINT); 
sigdelset(&oset, SIGHUP); 
sigprocmask(SIG_SETMASK, &oset, NULL);
Another, clearer, alternative is:
sigset_t set; 
 
sigemptyset(&set); 
sigaddset(&set, SIGINT); 
sigaddset(&set, SIGHUP); 
sigprocmask(SIG_BLOCK, &set, NULL); 
 
... 
 
sigprocmask(SIG_UNBLOCK, &set, NULL);
To completely clear the signal mask using sigsetmask() one can do:
(void) sigsetmask(0);
Which can be expressed via sigprocmask(2) as:
sigset_t eset; 
 
sigemptyset(&eset); 
(void) sigprocmask(SIG_SETMASK, &eset, NULL);

kill(2), sigaction(2), sigprocmask(2), sigsuspend(2), sigblock(3), sigsetops(3), sigvec(3)

A sigsetmask() system call first appeared in 4.2BSD. In 4.3BSD-Reno, it was reimplemented as a wrapper around sigprocmask(2). The old system call was kept for compatibility until OpenBSD 4.9.
July 17, 2013 OpenBSD-6.1