func: static int get_packetheader(NUTContext *nut, ByteIOContext *bc, int prefix_length, int calculate_checksum)
  
  {
  
      int64_t start, size, last_size;
  
      start= url_ftell(bc) - prefix_length;
  
  
  
      if(start != nut->packet_start + nut->written_packet_size){
  
          av_log(nut->avf, AV_LOG_ERROR, "get_packetheader called at weird position\n");
  
          return -1;
  
      }
  
      
  
      if(calculate_checksum)
  
          init_checksum(bc, update_adler32, 0);
  
  
  
      size= get_v(bc);
  
      last_size= get_v(bc);
  
      if(nut->written_packet_size != last_size){
  
          av_log(nut->avf, AV_LOG_ERROR, "packet size missmatch %d != %lld at %lld\n", nut->written_packet_size, last_size, start);
  
          return -1;
  
      }
  
  
  
      nut->last_packet_start = nut->packet_start;
  
      nut->packet_start = start;
  
      nut->written_packet_size= size;
  
  
  
      return size;
  
  }
  