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PTHREAD_SIGMASK(3) Library Functions Manual PTHREAD_SIGMASK(3)

examine and/or change a thread's signal mask

#include <signal.h>
pthread_sigmask(int how, const sigset_t *set, sigset_t *oset);

The pthread_sigmask() function examines and/or changes the calling thread's signal mask.
If set is not NULL, it specifies a set of signals to be modified, and how specifies what to set the signal mask to:
Union of the current mask and set.
Intersection of the current mask and the complement of set.
If oset is not NULL, the previous signal mask is stored in the location pointed to by oset.
SIGKILL and SIGSTOP cannot be blocked, and will be silently ignored if included in the signal mask.

If successful, pthread_sigmask() returns 0. Otherwise, an error is returned.

pthread_sigmask() will fail if:
how is not one of the defined values.

sigaction(2), sigpending(2), sigprocmask(2), sigsuspend(2), pthreads(3), sigaddset(3)

pthread_sigmask() conforms to ISO/IEC 9945-1:1996 (“POSIX.1”).
May 29, 2017 OpenBSD-current