func: int av_read_packet(AVFormatContext *s, AVPacket *pkt)
  {
      int ret, i;
      AVStream *st;
      for(;;){
          AVPacketList *pktl = s->raw_packet_buffer;
          if (pktl) {
              *pkt = pktl->pkt;
              if(s->streams[pkt->stream_index]->codec->codec_id != CODEC_ID_PROBE ||
                 !s->streams[pkt->stream_index]->probe_packets ||
                 s->raw_packet_buffer_remaining_size < pkt->size){
                  AVProbeData *pd = &s->streams[pkt->stream_index]->probe_data;
                  av_freep(&pd->buf);
                  pd->buf_size = 0;
                  s->raw_packet_buffer = pktl->next;
                  s->raw_packet_buffer_remaining_size += pkt->size;
                  av_free(pktl);
                  return 0;
          av_init_packet(pkt);
          ret= s->iformat->read_packet(s, pkt);
          if (ret < 0) {
              if (!pktl || ret == AVERROR(EAGAIN))
                  return ret;
              for (i = 0; i < s->nb_streams; i++)
                  s->streams[i]->probe_packets = 0;
          st= s->streams[pkt->stream_index];
          switch(st->codec->codec_type){
          case AVMEDIA_TYPE_VIDEO:
              if(s->video_codec_id)   st->codec->codec_id= s->video_codec_id;
              break;
          case AVMEDIA_TYPE_AUDIO:
              if(s->audio_codec_id)   st->codec->codec_id= s->audio_codec_id;
              break;
          case AVMEDIA_TYPE_SUBTITLE:
              if(s->subtitle_codec_id)st->codec->codec_id= s->subtitle_codec_id;
              break;
          if(!pktl && (st->codec->codec_id != CODEC_ID_PROBE ||
                       !st->probe_packets))
              return ret;
          add_to_pktbuf(&s->raw_packet_buffer, pkt, &s->raw_packet_buffer_end);
          s->raw_packet_buffer_remaining_size -= pkt->size;
          if(st->codec->codec_id == CODEC_ID_PROBE){
              AVProbeData *pd = &st->probe_data;
              av_log(s, AV_LOG_DEBUG, "probing stream %d\n", st->index);
              --st->probe_packets;
              pd->buf = av_realloc(pd->buf, pd->buf_size+pkt->size+AVPROBE_PADDING_SIZE);
              memcpy(pd->buf+pd->buf_size, pkt->data, pkt->size);
              pd->buf_size += pkt->size;
              memset(pd->buf+pd->buf_size, 0, AVPROBE_PADDING_SIZE);
              if(av_log2(pd->buf_size) != av_log2(pd->buf_size - pkt->size)){
                  //FIXME we dont reduce score to 0 for the case of running out of buffer space in bytes
                  set_codec_from_probe_data(s, st, pd, st->probe_packets > 0 ? AVPROBE_SCORE_MAX/4 : 0);
                  if(st->codec->codec_id != CODEC_ID_PROBE){
                      pd->buf_size=0;
                      av_freep(&pd->buf);
                      av_log(s, AV_LOG_DEBUG, "probed stream %d\n", st->index);