musl/src/signal/sigaddset.c

#include <signal.h>
#include <errno.h>

int sigaddset(sigset_t *set, int sig)
{
	unsigned s = sig-1;
	if (s >= 8*sizeof(sigset_t) || sig-32U<3) {
		errno = EINVAL;
		return -1;
	}
	set->__bits[s/8/sizeof *set->__bits] |= 1UL<<(s&8*sizeof *set->__bits-1);
	return 0;
}