seek是播放器的常用操作，也是衡量播放器好坏的重要标志之一。
一.效果
下面是对正在播放的4K视频执行seek操作
二.原理
1.av_seek_frame
FFmpeg提供了av_seek_frame函数来执行seek操作，函数定义如下所示：
/**
* 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 a