sig_handler
sa.sa_handler = sig_handler;
sa.sa_handler = sig_handler;
sig_handler(0); /* exit */
sig_handler(0);
.sa_sigaction = sig_handler,
.sa_sigaction = sig_handler,
signal(SIGUSR1, sig_handler);
signal(sig, sig_handler);
static void sig_handler(int);
sa.sa_sigaction = sig_handler;
sa.sa_handler = sig_handler;
signal(SIGINFO, sig_handler);
sa.sa_sigaction = &sig_handler;