设置定时器(timer), 捕捉SIGALRM信号
void handler(int signo){return;}
struct sigaction act;
sigaction(SIGALRM, NULL, &act);
act.sa_handler = handler;
act.sa_flags &= ~SA_RESTART;//清除掉SIGALRM信号的SA_RESTART
sigaction(SIGALRM, &act, NULL);
alarm(5);
if (recv(,,,) < 0) ……