static int rtsp_write_packet(AVFormatContext *s, AVPacket *pkt)
{
    RTSPState *rt = s->priv_data;
    RTSPStream *rtsp_st;
    fd_set rfds;
    int n, tcp_fd;
    struct timeval tv;
    AVFormatContext *rtpctx;
    int ret;

    tcp_fd = url_get_file_handle(rt->rtsp_hd);

    while (1) {
        FD_ZERO(&rfds);
        FD_SET(tcp_fd, &rfds);
        tv.tv_sec = 0;
        tv.tv_usec = 0;
        n = select(tcp_fd + 1, &rfds, NULL, NULL, &tv);
        if (n <= 0)
            break;
        if (FD_ISSET(tcp_fd, &rfds)) {
            RTSPMessageHeader reply;

            /* Don't let ff_rtsp_read_reply handle interleaved packets,
             * since it would block and wait for an RTSP reply on the socket
             * (which may not be coming any time soon) if it handles
             * interleaved packets internally. */
            ret = ff_rtsp_read_reply(s, &reply, NULL, 1, NULL);
            if (ret < 0)
                return AVERROR(EPIPE);
            if (ret == 1)
                ff_rtsp_skip_packet(s);
            /* XXX: parse message */
            if (rt->state != RTSP_STATE_STREAMING)
                return AVERROR(EPIPE);
        }
    }

    if (pkt->stream_index < 0 || pkt->stream_index >= rt->nb_rtsp_streams)
        return AVERROR_INVALIDDATA;
    rtsp_st = rt->rtsp_streams[pkt->stream_index];
    rtpctx = rtsp_st->transport_priv;

    ret = ff_write_chained(rtpctx, 0, pkt, s);
    /* ff_write_chained does all the RTP packetization. If using TCP as
     * transport, rtpctx->pb is only a dyn_packet_buf that queues up the
     * packets, so we need to send them out on the TCP connection separately.
     */
    if (!ret && rt->lower_transport == RTSP_LOWER_TRANSPORT_TCP)
        ret = tcp_write_packet(s, rtsp_st);
    return ret;
}