av_register_all
 avcodec_register_all    --code encode
  REGISTER_ENCDEC (AAC, aac);
    #define REGISTER_ENCDEC(X, x) REGISTER_ENCODER(X, x); REGISTER_DECODER(X, x)
      #define REGISTER_ENCODER(X, x)            \
      {                                         \
        extern AVCodec ff_##x##_encoder;        \  --extern AVCodec ff_aac_encoder;
        if (CONFIG_##X##_ENCODER)               \  --if (CONFIG_AAC_ENCODER)
            avcodec_register(&ff_##x##_encoder);\  --   avcodec_register(&ff_aac_encoder)
      }
 AVCodec ff_libfdk_aac_encoder = {
     .init                  = aac_encode_init,
     .encode2               = aac_encode_frame,
     .close                 = aac_encode_close,
 };
avformat_alloc_output_context2  --out AVFormatContext  AVFormatContext.AVOutputFormat
  avformat_alloc_context
  av_guess_format

avio_open2     --out AVIOContext
  ffurl_open
  ffio_fdopen

avformat_new_stream   --out AVStream
  av_mallocz
  avcodec_alloc_context3

avcodec_find_encoder --out AVCodec
  AVCodec *p = first_avcodec;   /* code info init encode close etc*/
  while(p){
    p = p->next;

    ...
  }

avcodec_open2
  ret = avctx->codec->init(avctx); --ff_libfdk_aac_encoder.init

avformat_write_header
  AVOutputFormat->write_header

avcodec_encode_audio2
  AVCodec.encode2 ----ff_libfdk_aac_encoder.encode2

for(){
  av_write_frame
}

av_write_trailer
  for(){
    write_packet
  }
  AVOutputFormat->write_trailer

avcodec_close
  AVCodec->close  ----ff_libfdk_aac_encoder.close

avformat_free_context
  ff_free_stream

avio_close
  avio_flush
  ffurl_close