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SIGSETMASK(3) Library Functions Manual SIGSETMASK(3)

sigsetmask
set current signal mask

#include <signal.h>

int
sigsetmask(int mask);

This interface is made obsolete 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(3) 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), sigaddset(3), sigblock(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.
August 1, 2017 OpenBSD-current