使用SIGALARM为connect设置超时

static void    connect_alarm(int);

int connect_timeo(int sockfd, const SA *saptr, socklen_t salen, int nsec)
{
    Sigfunc    *sigfunc;
    int        n;

    sigfunc = signal(SIGALRM, connect_alarm);
    if (alarm(nsec) != 0)
        err_msg("connect_timeo: alarm was already set");

    if ( (n = connect(sockfd, saptr, salen)) < 0) {
        close(sockfd);
        if (errno == EINTR)
            errno = ETIMEDOUT;
    }
    alarm(0);                    /* turn off the alarm */
    signal(SIGALRM, sigfunc);    /* restore previous signal handler */

    return(n);
}

static void connect_alarm(int signo)
{
    return;        /* just interrupt the connect() */
}