寫開源項目的時候需要做seek功能，使用了av_seek_frame的功能，記錄如下。先看段文檔

int av_seek_frame   (   AVFormatContext * s,

int stream_index,

int64_t timestamp,

int flags

)

Seek to the keyframe at timestamp.

'timestamp' in 'stream_index'.

Parameters

s   media file handle

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.

timestamp   Timestamp in AVStream.time_base units or, if no stream is specified, in AV_TIME_BASE units.

flags   flags which select direction and seeking mode

#define AVSEEK_FLAG_BACKWARD 1 ///< seek backward

#define AVSEEK_FLAG_BYTE    2 ///< seeking based on position in bytes

#define AVSEEK_FLAG_ANY      4 ///< seek to any frame, even non-keyframes

#define AVSEEK_FLAG_FRAME    8 ///< seeking based on frame number

Returns

>= 0 on success

解釋###

timebase指的是時間戳，對應(yīng)pts時間戳，如果index是-1，則使用AV_TIMEBASE作為timebase并由ffmpeg自動轉(zhuǎn)換成默認時間戳， 如果指定了stream那么就要使用相應(yīng)的stream的timebase來計算pts了。這里注意的是比如seek到32s不能簡單的直接32*AV_TIMEBASE來計算時間戳，因為pts不一定是從0開始的，所以要加上起始的pts。

stream_index是選擇針對哪一條媒體流來做seek

flag用來指定尋找尋找的I幀和指定點之間的位置關(guān)系，因為seek過去的時間點不一定就處在I幀的地方，解碼需要依賴于I幀，所以這時候就得選擇一個附近的I幀，flag表明要seek到當前幀的前面一個I幀還是后面一個I幀

seek到32s的例子###

int ret = av_seek_frame(inputFormatContext, -1, ((double)ms/(double)1000)*AV_TIME_BASE + (double)inputFormatContext->start_time, AVSEEK_FLAG_BACKWARD);//不指定stream進行seek

int ret = av_seek_frame(inputFormatContext, videoIndex, (double)videoTimeBase.den/(double)videoTimeBase.num * ((double)ms/(double)1000)*AV_TIME_BASE + (double)inputFormatContext->start_time/(double)AV_TIME_BASE*((double)videoTimeBase.den/(double)videoTimeBase.num)*/, AVSEEK_FLAG_BACKWARD);//指定stream進行seek