av_seek_frame
函数原型：
/**

• Seek to the keyframe at timestamp.
• ‘timestamp’ in ‘stream_index’.
• @param s media file handle
• @param stream_index If stream_index is (-1), a default
• stream is selected, and timestamp is automatically converted
• from AV_TIME_BASE units to the stream specific time_base.
• @param timestamp Timestamp in AVStream.time_base units
• or, if no stream is specified, in AV_TIME_BASE units.
• @param flags flags which select direction and seeking mode
• @return >= 0 on success
  */
  int av_seek_frame(AVFormatContext *s, int stream_index, int64_t timestamp, int flags); //-1 default

分析:上面函数的第一个参数是我们封装格式的上下文，第二个参数是对应的我们的索引。就是移动到这个帧的位置。第三个参数是时间戳，移动到哪个时间位置。第四个参数是标识位，标识移动的方法。

#define AVSEEK_FLAG_BACKWARD -1 //<seek backward
#define AVSEEK_FLAG_BYTE 2 //<seeking base on position in bytes
如下图