sigaddset - add signal to signal set

Syntax	#include <signal.h> int sigaddset(sigset_t *set, int sig);
Description	sigaddset() adds the signal sig to the signal set pointed to by set.
Return val.	0	if successful.
	-1	if an error occurs. errno is set to indicate the error.
Errors	sigaddset() will fail if:
	EINVAL	The value of sig is an invalid or unsupported signal number.
Notes	Applications should call sigemptyset() or sigfillset() for each object of type sigset_t prior to any other use of that object. If such an object is not initialized in this way, but is nonetheless supplied as an argument to any of sigaction(), sigaddset(), sigdelset(), sigismember(), sigpending() or sigprocmask(), the behavior is undefined.
See also	sigdelset(), sigemptyset(), sigfillset(), sigismember(), signal.h.