static int my_read_packet(void *opaque, uint8_t* buf, int buf_size)
{
  FILE *in_file = (FILE*)opaque;
  int read_size = fread(buf, 1, buf_size, in_file);
  printf("my_read_packet read_size:%d, buf_size:%d\n", read_size, buf_size);
  if(read_size <=0)
  {
    return AVERROR_EOF;
  }
  return read_size;
}

FILE *in_file = NULL;
in_file = fopen(in_file_name, "rb");
if(!in_file){return;}

uint8_t *io_buffer = av_malloc(BUF_SIZE);
AVIOContext *avio_ctx = avio_alloc_context(io_buffer, BUF_SIZE, 0, (void*)in_file,
                        my_read_packet, NULL, NULL);
AVFormatContext *format_ctx = avformat_alloc_context();
format_ctx->pb = avio_ctx; // 关联
int ret = avformat_open_input(&format_ctx, NULL/*不再需要文件地址*/, NULL, NULL);
if(ret<0){return;}

// ....
// ！！！这里再调用av_read_frame可读取裸流

fclose(in_file);
av_free(io_buffer);
avformat_close_input(&format_ctx);