av_dump_format()

此函数的作用：av_dump_format()是一个手工调试的函数，能使我们看到pFormatCtx->streams里面有什么内容。一般接下来我们使用av_find_stream_info()函数，它的作用是为pFormatCtx->streams填充上正确的信息。

/**
 * Print detailed information about the input or output format, such as
 * duration, bitrate, streams, container, programs, metadata, side data,
 * codec and time base.
 *
 * @param ic        the context to analyze
 * @param index     index of the stream to dump information about
 * @param url       the URL to print, such as source or destination file
 * @param is_output Select whether the specified context is an input(0) or output(1)
 */
void av_dump_format(AVFormatContext *ic,
                    int index,
                    const char *url,
                    int is_output);

av_compare_ts()

比较时间戳，决定写入视频还是写入音频。因为视频PTS总是0，而音频>0，函数判断视频进度落后该写入。

/**
 * Compare two timestamps each in its own time base.
 *
 * @return One of the following values:
 *         - -1 if `ts_a` is before `ts_b`
 *         - 1 if `ts_a` is after `ts_b`
 *         - 0 if they represent the same position
 *
 * @warning
 * The result of the function is undefined if one of the timestamps is outside
 * the `int64_t` range when represented in the other's timebase.
 */
int av_compare_ts(int64_t ts_a, AVRational tb_a, int64_t ts_b, AVRational tb_b);