avcodec_decode_video2()的函数实现如下所示：

int attribute_align_arg avcodec_decode_video2(AVCodecContext *avctx, AVFrame *picture,int *got_picture_ptr,const AVPacket *avpkt)
{
    AVCodecInternal *avci = avctx->internal;
    int ret;
    // copy to ensure we do not change avpkt
    AVPacket tmp = *avpkt;

    if (!avctx->codec)
        return AVERROR(EINVAL);
    if (avctx->codec->type != AVMEDIA_TYPE_VIDEO) 
	{
        av_log(avctx, AV_LOG_ERROR, "Invalid media type for video\n");
        return AVERROR(EINVAL);
    }

    *got_picture_ptr = 0;
    if ((avctx->coded_width || avctx->coded_height) && av_image_check_size(avctx->coded_width, avctx->coded_height, 0, avctx))
        return AVERROR(EINVAL);

    avcodec_get_frame_defaults(picture);

    if (!avctx->refcounted_frames)
        av_frame_unref(&avci->to_free);

    if ((avctx->codec->capabilities & CODEC_CAP_DELAY) || avpkt->size || (avctx->active_thread_type & FF_THREAD_FRAME)) 
	{
        int did_split = av_packet_split_side_data(&tmp);
        apply_param_change(avctx, &tmp);
        avctx->pkt = &tmp;
        if (HAVE_THREADS && avctx->active_thread_type & FF_THREAD_FRAME)
            ret = ff_thread_decode_frame(avctx, picture, got_picture_ptr,
                                         &tmp);
        else 
		{
            ret = avctx->codec->decode(avctx, picture, got_picture_ptr,
                                       &tmp);
            picture->pkt_dts = avpkt->dts;

            if(!avctx->has_b_frames)
			{
                av_frame_set_pkt_pos(picture, avpkt->pos);
            }
            //FIXME these should be under if(!avctx->has_b_frames)
            /* get_buffer is supposed to set frame parameters */
            if (!(avctx->codec->capabilities & CODEC_CAP_DR1)) 
			{
                if (!picture->sample_aspect_ratio.num)    picture->sample_aspect_ratio = avctx->sample_aspect_ratio;
                if (!picture->width)                      picture->width               = avctx->width;
                if (!picture->height)                     picture->height              = avctx->height;
                if (picture->format == AV_PIX_FMT_NONE)   picture->format              = avctx->pix_fmt;
            }
        }
        add_metadata_from_side_data(avctx, picture);

        emms_c(); //needed to avoid an emms_c() call before every return;

        avctx->pkt = NULL;
        if (did_split) {
            av_packet_free_side_data(&tmp);
            if(ret == tmp.size)
                ret = avpkt->size;
        }

        if (ret < 0 && picture->data[0])
            av_frame_unref(picture);

        if (*got_picture_ptr)
		{
            if (!avctx->refcounted_frames) 
			{
                avci->to_free = *picture;
                avci->to_free.extended_data = avci->to_free.data;
                memset(picture->buf, 0, sizeof(picture->buf));
            }

            avctx->frame_number++;
            av_frame_set_best_effort_timestamp(picture,
                                               guess_correct_pts(avctx,
                                                                 picture->pkt_pts,
                                                                 picture->pkt_dts));
        }
    } else
        ret = 0;

    /* many decoders assign whole AVFrames, thus overwriting extended_data;
     * make sure it's set correctly */
    picture->extended_data = picture->data;

    return ret;
}

该函数的输入和输出参数分别为const AVPacket *avpkt和AVFrame *picture，分别表示待解码的码流和解码完成后的音视频信号，函数的核心是ret = avctx->codec->decode(avctx, picture, got_picture_ptr, &tmp)，调用预定义好的解码器进行解码。