[MPlayer-users] some question about libmpdemux

[Zongyao QU]

This is not a bug report, 
but some barrier I met when I want to do some hack to mplayer.

My goal is to get the info of all the a/v streams, 
such as how many audio/video streams in the file, 
and fps, sample rate, codec... of each stream.

I found the demuxer may be the right place start to understand the overall.

in the definition of demuxer_t(which is in demuxer.h)
there are members of 
 void* a_streams[MAX_A_STREAMS]; // audio streams (sh_audio_t)
 void* v_streams[MAX_V_STREAMS]; // video sterams (sh_video_t)
each of them is an array of the sh_audio_t/sh_video_t pointers.

so just before the playback, I added my code,
which are for-loops to look into these arrays.

sh_audio_t *a;
int num =0;
for (int i=0;i < MAX_A_STREAMS; i++) {
  if (a = demuxer[i]) {
    num++;
    printf("%d, aid:%d",i,a->aid);
  }
}

My questions are:
1. is there any way more direct to get the number of a/v streams 
   and their infos, rather than a for loop?

2. if there is none...
   In the for-loop above, the loop will always run MAX_A_STREAMS times,
   is there any condition to judge so that I could exit the loop earlier? 
   which means how I could make sure there is no sh_audio_t* anymore 
   in the array.

3. I have tested my code at some level,
   I found that in the array, 
   the valid sh_audio_t* does not always start at index 0 in the array.
   and the aid is also not always start from 0. for example:
------------------------
a mp4 file, which has only one audio stream:
1, aid:1
------------------------
------------------------
a mkv file, which has two audio streams:
2, aid:0
3, aid:1
------------------------
so how really does the demuxer work with these ids?