非阻塞connect

int connect_nonb(int sockfd, const SA *saptr, socklen_t salen, int nsec)
{
    int              flags, n, error;
    socklen_t        len;
    fd_set           rset, wset;
    struct timeval   tval;

    flags = fcntl(sockfd, F_GETFL, 0);
    fcntl(sockfd, F_SETFL, flags | O_NONBLOCK);

    error = 0;
    if ( (n = connect(sockfd, saptr, salen)) < 0)
        if (errno != EINPROGRESS)
            return(-1);

    /* Do whatever we want while the connect is taking place. */

    if (n == 0)
        goto done;    /* connect completed immediately */

    FD_ZERO(&rset);
    FD_SET(sockfd, &rset);
    wset = rset;
    tval.tv_sec = nsec;
    tval.tv_usec = 0;

    if ( (n = select(sockfd+1, &rset, &wset, NULL,
                     nsec ? &tval : NULL)) == 0) {
        close(sockfd);        /* timeout */
        errno = ETIMEDOUT;
        return(-1);
    }

    if (FD_ISSET(sockfd, &rset) || FD_ISSET(sockfd, &wset)) {
        len = sizeof(error);
        if (getsockopt(sockfd, SOL_SOCKET, SO_ERROR, &error, &len) < 0)
            return(-1);            /* Solaris pending error */
    } else
        err_quit("select error: sockfd not set");

    done:
    fcntl(sockfd, F_SETFL, flags); /* restore file status flags */

    if (error) {
        close(sockfd);             /* just in case */
        errno = error;
        return(-1);
    }
    return(0);
}