* Open an input stream and read the header. The codecs are not opened.
 * The stream must be closed with avformat_close_input().
 *
 * @param ps       Pointer to user-supplied AVFormatContext (allocated by
 *                 avformat_alloc_context). May be a pointer to NULL, in
 *                 which case an AVFormatContext is allocated by this
 *                 function and written into ps.
 *                 Note that a user-supplied AVFormatContext will be freed
 *                 on failure.
 * @param url      URL of the stream to open.
 * @param fmt      If non-NULL, this parameter forces a specific input format.
 *                 Otherwise the format is autodetected.
 * @param options  A dictionary filled with AVFormatContext and demuxer-private
 *                 options.
 *                 On return this parameter will be destroyed and replaced with
 *                 a dict containing options that were not found. May be NULL.
 *
 * @return 0 on success, a negative AVERROR on failure.
 *
 * @note If you want to use custom IO, preallocate the format context and set its pb field.
 */
int avformat_open_input(AVFormatContext **ps, const char *filename,
                        const AVInputFormat *fmt, AVDictionary **options)

1.avformat_alloc_context 初始化ps

2.init_input 初始化ps->pb和ps->iformat

3.s->iformat->read_header 获取nb_streams信息