av_seek_frame的调用

ffmpeg中当av_seek_frame的时候需要读文件的开始位置，调用方式

ret = av_read_frame(pFormatCtx, packet);
if (ret < 0)
{
        ret = av_seek_frame(pFormatCtx, videoindex, pFormatCtx->streams[videoindex]->start_time, 0);  //seek到视频的开始位置
	if (ret<0) {
		printf("av_seek_frame ==video== error\n");
	}
	avcodec_flush_buffers(pFormatCtx->streams[videoindex]->codec);    //清空解码器的缓存
	printf("av_seek_frame ==video== error\n");
			
	continue;
}

代码注释：

/**
 * 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);

/**
 * Reset the internal decoder state / flush internal buffers. Should be called        
 * e.g. when seeking or when switching to a different stream.              //当seek的时候调用或者转到另一个流的时候调用
 *
 * @note when refcounted frames are not used (i.e. avctx->refcounted_frames is 0),
 * this invalidates the frames previously returned from the decoder. When
 * refcounted frames are used, the decoder just releases any references it might
 * keep internally, but the caller's reference remains valid.
 */
void avcodec_flush_buffers(AVCodecContext *avctx);    

ffmpeg.c中的seek_to_start 调用

还是需要将ffmpeg中各功能的代码 了解透彻，各功能在ffmpeg中应该都能了解到

static int seek_to_start(InputFile *ifile, AVFormatContext *is)
{
    InputStream *ist;
    AVCodecContext *avctx;
    int i, ret, has_audio = 0;
    int64_t duration = 0;

    ret = av_seek_frame(is, -1, is->start_time, 0);    //=================================*****************************          

    if (ret < 0)
        return ret;

    for (i = 0; i < ifile->nb_streams; i++) {
        ist   = input_streams[ifile->ist_index + i];
        avctx = ist->dec_ctx;

        // flush decoders
        if (ist->decoding_needed) {
            process_input_packet(ist, NULL, 1);
            avcodec_flush_buffers(avctx); //=================================*****************************          
        }

        /* duration is the length of the last frame in a stream
         * when audio stream is present we don't care about
         * last video frame length because it's not defined exactly */
        if (avctx->codec_type == AVMEDIA_TYPE_AUDIO && ist->nb_samples)
            has_audio = 1;
    }

    for (i = 0; i < ifile->nb_streams; i++) {
        ist   = input_streams[ifile->ist_index + i];
        avctx = ist->dec_ctx;

        if (has_audio) {
            if (avctx->codec_type == AVMEDIA_TYPE_AUDIO && ist->nb_samples) {
                AVRational sample_rate = {1, avctx->sample_rate};

                duration = av_rescale_q(ist->nb_samples, sample_rate, ist->st->time_base);
            } else
                continue;
        } else {
            if (ist->framerate.num) {
                duration = av_rescale_q(1, ist->framerate, ist->st->time_base);
            } else if (ist->st->avg_frame_rate.num) {
                duration = av_rescale_q(1, ist->st->avg_frame_rate, ist->st->time_base);
            } else duration = 1;
        }
        if (!ifile->duration)
            ifile->time_base = ist->st->time_base;
        /* the total duration of the stream, max_pts - min_pts is
         * the duration of the stream without the last frame */
        duration += ist->max_pts - ist->min_pts;
        ifile->time_base = duration_max(duration, &ifile->duration, ist->st->time_base,
                                        ifile->time_base);
    }

    if (ifile->loop > 0)
        ifile->loop--;

    return ret;
}