I need to implement an timeout socket which the send()/recv() system calls should return after a timeout. And I think the standard method should be, call select()/pool() before send()/recv(). Below is the example code I copy from ffmpeg source code:

    int ff_network_wait_fd(int fd, int write)
    {
        int ev = write ? POLLOUT : POLLIN;
        struct pollfd p = { .fd = fd, .events = ev, .revents = 0 };
        int ret;
        ret = poll(&p, 1, 100);                                                      

        return ret < 0 ? ff_neterrno() : p.revents &
               (ev | POLLERR | POLLHUP) ? 0 :     AVERROR(EAGAIN);
    }                                                                                

    int ff_network_wait_fd_timeout(int fd, int write, int64_t timeout, AVIOInterruptCB *int_cb)
    {
        int ret;
        int64_t wait_start = 0;                                                 

        while (1) {
            if (ff_check_interrupt(int_cb))
                return AVERROR_EXIT;
            ret = ff_network_wait_fd(fd, write);
            if (ret != AVERROR(EAGAIN))
                return ret;
            if (timeout > 0) {
                if (!wait_start)
                    wait_start = av_gettime();
                else if (av_gettime() - wait_start > timeout)
                    return AVERROR(ETIMEDOUT);
            }
        }
    }

    static int tcp_write(URLContext *h, const uint8_t *buf, int size)
    {
        TCPContext *s = h->priv_data;
        int ret;                                                                

        if (!(h->flags & AVIO_FLAG_NONBLOCK)) {
            ret = ff_network_wait_fd_timeout(s->fd, 1, h->rw_timeout, &h->interrupt_callback);
            if (ret)
                return ret;
        }
        ret = send(s->fd, buf, size, 0);
        return ret < 0 ? ff_neterrno() : ret;
    }

Seems that tcp_write() call ff_network_wait_fd_timeout() to prepare the environment. But it makes me so confusion that, if network cable unplugged after ff_network_wait_fd()(this should call poll() system call), should send() return -1, or still blocked? Thanks very much.

1. You don't need poll() to implement a timeout for recv(). Just call setsockopt() with the option SO_RCVTIMEO and deal with the EWOULDBLOCK errno as a timeout.

2. You do need to use poll() for a send() timeout.

3. A broken TCP connection can only be detected by trying to write to it. There is no network event that triggers a poll() event.