FFmpeg源码分析：avcodec_send_packet()与avcodec_receive_frame()音视频解码

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FFmpeg在libavcodec模块，旧版本提供avcodec_decode_video2()作为视频解码函数，avcodec_decode_audio4()作为音频解码函数。在FFmpeg 3.1版本新增avcodec_send_packet()与avcodec_receive_frame()作为音视频解码函数。后来，在3.4版本把avcodec_decode_video2()和avcodec_decode_audio4()标记为过时API。版本变更描述如下：

FFmpeg 3.12016-04-21 - 7fc329e - lavc 57.37.100 - avcodec.h
Add a new audio/video encoding and decoding API with decoupled input
and output -- avcodec_send_packet(), avcodec_receive_frame(),
avcodec_send_frame() and avcodec_receive_packet().FFmpeg 3.42017-09-26 - b1cf151c4d - lavc 57.106.102 - avcodec.h
Deprecate AVCodecContext.refcounted_frames. This was useful for deprecated
API only (avcodec_decode_video2/avcodec_decode_audio4). The new decode APIs
(avcodec_send_packet/avcodec_receive_frame) always work with reference
counted frames.

avcodec_send_packet()和avcodec_receive_frame()函数声明位于libavcodec/avcodec.h。我们来看看函数声明介绍：

/*** Supply raw packet data as input to a decoder.** @param avctx codec context* @param[in] avpkt The input AVPacket. Usually, this will be a single video*                  frame, or several complete audio frames.** @return 0 on success, otherwise negative error code:*      AVERROR(EAGAIN):   input is not accepted in the current state - user*                         must read output with avcodec_receive_frame()*      AVERROR_EOF:       the decoder has been flushed, and no new packets*      AVERROR(EINVAL):   codec not opened, it is an encoder, or requires flush*      AVERROR(ENOMEM):   failed to add packet to internal queue, or similar*/
int avcodec_send_packet(AVCodecContext *avctx, const AVPacket *avpkt);/*** Return decoded output data from a decoder.** @param avctx codec context* @param frame This will be set to a reference-counted video or audio*              frame (depending on the decoder type) allocated by the*              decoder. Note that the function will always call*              av_frame_unref(frame) before doing anything else.** @return*      0:                 success, a frame was returned*      AVERROR(EAGAIN):   output is not available in this state*      AVERROR_EOF:       the decoder has been fully flushed*      AVERROR(EINVAL):   codec not opened, or it is an encoder*      AVERROR_INPUT_CHANGED: current decoded frame has changed parameters*/
int avcodec_receive_frame(AVCodecContext *avctx, AVFrame *frame);

由描述可知，avcodec_send_packet()负责把AVpacket数据包发送给解码器，avcodec_receive_frame()则从解码器取出一帧AVFrame数据。返回0，代表解码成功；返回EAGAIN，代表当前状态无可输出的数据；返回EOF，代表到达文件流结尾；返回INVAL，代表解码器未打开或者当前打开的是编码器；返回INPUT_CHANGED，代表输入参数发生变化，比如width和height改变。

avcodec_send_packet()和avcodec_receive_frame()的解码流程图如下：

一、avcodec_send_packet发送AVPacket

1、avcodec_send_packet

avcodec_send_packet()函数位于libavcodec/decode.c，具体如下：

int avcodec_send_packet(AVCodecContext *avctx, const AVPacket *avpkt)
{AVCodecInternal *avci = avctx->internal;int ret;// 判断解码器是否打开，是否为解码器if (!avcodec_is_open(avctx) || !av_codec_is_decoder(avctx->codec))return AVERROR(EINVAL);if (avctx->internal->draining)return AVERROR_EOF;if (avpkt && !avpkt->size && avpkt->data)return AVERROR(EINVAL);av_packet_unref(avci->buffer_pkt);if (avpkt && (avpkt->data || avpkt->side_data_elems)) {ret = av_packet_ref(avci->buffer_pkt, avpkt);if (ret < 0)return ret;}// 发送packet到bitstream滤波器ret = av_bsf_send_packet(avci->bsf, avci->buffer_pkt);if (ret < 0) {av_packet_unref(avci->buffer_pkt);return ret;}if (!avci->buffer_frame->buf[0]) {ret = decode_receive_frame_internal(avctx, avci->buffer_frame);if (ret < 0 && ret != AVERROR(EAGAIN) && ret != AVERROR_EOF)return ret;}return 0;
}

2、av_bsf_send_packet

由此可见，内部是调用av_bsf_send_packet()函数把packet发送给bitstream滤波器，代码位于libavcodec/bsf.c，具体如下：

int av_bsf_send_packet(AVBSFContext *ctx, AVPacket *pkt)
{AVBSFInternal *bsfi = ctx->internal;int ret;if (!pkt || IS_EMPTY(pkt)) {bsfi->eof = 1;return 0;}if (bsfi->eof) {return AVERROR(EINVAL);}if (!IS_EMPTY(bsfi->buffer_pkt))return AVERROR(EAGAIN);// 申请AVPacket内存，并拷贝数据ret = av_packet_make_refcounted(pkt);if (ret < 0)return ret;// 把pkt指针赋值给bsfi->buffer_pktav_packet_move_ref(bsfi->buffer_pkt, pkt);return 0;
}

av_bsf_send_packet()函数内部调用av_packet_make_refcounted()来申请AVPacket内存，并拷贝数据。然后调用av_packet_move_ref()函数把pkt指针赋值给bsfi->buffer_pkt。

二、avcodec_receive_frame接收AVFrame

1、avcodec_receive_frame

avcodec_receive_frame()函数同样位于libavcodec/decode.c，主要是调用内部函数decode_receive_frame_internal()实现解码，具体代码如下：

int avcodec_receive_frame(AVCodecContext *avctx, AVFrame *frame)
{AVCodecInternal *avci = avctx->internal;int ret, changed;av_frame_unref(frame);// 判断解码器是否打开，是否为解码器if (!avcodec_is_open(avctx) || !av_codec_is_decoder(avctx->codec))return AVERROR(EINVAL);if (avci->buffer_frame->buf[0]) {av_frame_move_ref(frame, avci->buffer_frame);} else {ret = decode_receive_frame_internal(avctx, frame);if (ret < 0)return ret;}if (avctx->codec_type == AVMEDIA_TYPE_VIDEO) {ret = apply_cropping(avctx, frame);if (ret < 0) {av_frame_unref(frame);return ret;}}......return 0;
}

2、decode_receive_frame_internal

decode_receive_frame_internal()函数判断是否支持avctx->codec->receive_frame，如果支持就调用avctx->codec->receive_frame()函数进行解码，否则调用decode_simple_receive_frame()函数进行解码。具体如下：

static int decode_receive_frame_internal(AVCodecContext *avctx, AVFrame *frame)
{AVCodecInternal *avci = avctx->internal;int ret;if (avctx->codec->receive_frame) {ret = avctx->codec->receive_frame(avctx, frame);if (ret != AVERROR(EAGAIN))av_packet_unref(avci->last_pkt_props);} else {ret = decode_simple_receive_frame(avctx, frame);}....../* free the per-frame decode data */av_buffer_unref(&frame->private_ref);return ret;
}

3、decode_simple_receive_frame

decode_simple_receive_frame()函数又调用decode_simple_internal()内部函数进行解码：

static int decode_simple_receive_frame(AVCodecContext *avctx, AVFrame *frame)
{int ret;int64_t discarded_samples = 0;while (!frame->buf[0]) {if (discarded_samples > avctx->max_samples)return AVERROR(EAGAIN);ret = decode_simple_internal(avctx, frame, &discarded_samples);if (ret < 0)return ret;}return 0;
}

4、decode_simple_internal

由描述可知，decode_simple_internal()函数是解码器的核心封装函数。我们应该循环调用avcodec_receive_frame()函数，直到返回EAGAIN。通过判断是否需要特定线程进行解码，如果需要就调用ff_thread_decode_frame()函数，否则调用avctx->codec->decode指向的解码函数。具体调用过程如下：

/** The core of the receive_frame_wrapper for the decoders implementing* the simple API. Certain decoders might consume partial packets without* returning any output, so this function needs to be called in a loop until it* returns EAGAIN.**/
static inline int decode_simple_internal(AVCodecContext *avctx, AVFrame *frame, int64_t *discarded_samples)
{AVCodecInternal   *avci = avctx->internal;DecodeSimpleContext *ds = &avci->ds;AVPacket           *pkt = ds->in_pkt;int got_frame, actual_got_frame;int ret;if (!pkt->data && !avci->draining) {av_packet_unref(pkt);ret = ff_decode_get_packet(avctx, pkt);if (ret < 0 && ret != AVERROR_EOF)return ret;}if (avci->draining_done)return AVERROR_EOF;if (!pkt->data &&!(avctx->codec->capabilities & AV_CODEC_CAP_DELAY ||avctx->active_thread_type & FF_THREAD_FRAME))return AVERROR_EOF;got_frame = 0;if (HAVE_THREADS && avctx->active_thread_type & FF_THREAD_FRAME) {ret = ff_thread_decode_frame(avctx, frame, &got_frame, pkt);} else {ret = avctx->codec->decode(avctx, frame, &got_frame, pkt);......}......return ret < 0 ? ret : 0;
}

5、ff_thread_decode_frame

ff_thread_decode_frame()函数位于libavcodec/pthread_frame.c。首先是调用submit_packet()函数提交packet到下一个解码线程，然后调用数组中最前面线程进行解码。相关代码如下：

int ff_thread_decode_frame(AVCodecContext *avctx,AVFrame *picture, int *got_picture_ptr,AVPacket *avpkt)
{FrameThreadContext *fctx = avctx->internal->thread_ctx;int finished = fctx->next_finished;PerThreadContext *p;int err;async_unlock(fctx);/** Submit a packet to the next decoding thread.*/p = &fctx->threads[fctx->next_decoding];err = submit_packet(p, avctx, avpkt);if (err)goto finish;....../* Return the next available frame from the oldest thread.*/do {p = &fctx->threads[finished++];if (atomic_load(&p->state) != STATE_INPUT_READY) {pthread_mutex_lock(&p->progress_mutex);while (atomic_load_explicit(&p->state, memory_order_relaxed) != STATE_INPUT_READY)pthread_cond_wait(&p->output_cond, &p->progress_mutex);pthread_mutex_unlock(&p->progress_mutex);}av_frame_move_ref(picture, p->frame);*got_picture_ptr = p->got_frame;picture->pkt_dts = p->avpkt->dts;err = p->result;p->got_frame = 0;p->result = 0;if (finished >= avctx->thread_count) finished = 0;} while (!avpkt->size && !*got_picture_ptr && err >= 0 && finished != fctx->next_finished);update_context_from_thread(avctx, p->avctx, 1);if (fctx->next_decoding >= avctx->thread_count)fctx->next_decoding = 0;fctx->next_finished = finished;/* return the size of the consumed packet if no error occurred */if (err >= 0)err = avpkt->size;
finish:async_lock(fctx);return err;
}

6、h264_decode_frame

以h264解码器为例，位于libavcodec/h264dec.c，对应的AVCodec如下：

AVCodec ff_h264_decoder = {.name                  = "h264",.long_name             = NULL_IF_CONFIG_SMALL("H.264/AVC/MPEG-4 AVC/MPEG-4 part10"),.type                  = AVMEDIA_TYPE_VIDEO,.id                    = AV_CODEC_ID_H264,.priv_data_size        = sizeof(H264Context),.init                  = h264_decode_init,.close                 = h264_decode_end,.decode                = h264_decode_frame,.capabilities          = AV_CODEC_CAP_DR1 |AV_CODEC_CAP_DELAY | AV_CODEC_CAP_SLICE_THREADS |AV_CODEC_CAP_FRAME_THREADS,.flush                 = h264_decode_flush,.update_thread_context = ONLY_IF_THREADS_ENABLED(ff_h264_update_thread_context),.profiles              = NULL_IF_CONFIG_SMALL(ff_h264_profiles),.priv_class            = &h264_class,
};

此时， avctx->codec->decode函数指针指向的是h264_decode_frame。如果存在extradata就调用ff_h264_decode_extradata()函数解析，最关键是调用decode_nal_units()函数来解码NAL单元。相关代码如下：

static int h264_decode_frame(AVCodecContext *avctx, void *data,int *got_frame, AVPacket *avpkt)
{......ff_h264_unref_picture(h, &h->last_pic_for_ec);/* end of stream, output what is still in the buffers */if (buf_size == 0)return send_next_delayed_frame(h, pict, got_frame, 0);if (av_packet_get_side_data(avpkt, AV_PKT_DATA_NEW_EXTRADATA, NULL)) {buffer_size_t side_size;uint8_t *side = av_packet_get_side_data(avpkt, AV_PKT_DATA_NEW_EXTRADATA, &side_size);ff_h264_decode_extradata(side, side_size,&h->ps, &h->is_avc, &h->nal_length_size,avctx->err_recognition, avctx);}if (h->is_avc && buf_size >= 9 && buf[0]==1 && buf[2]==0 && (buf[4]&0xFC)==0xFC) {// 解析avcc对应的extradataif (is_avcc_extradata(buf, buf_size))return ff_h264_decode_extradata(buf, buf_size,&h->ps, &h->is_avc, &h->nal_length_size,avctx->err_recognition, avctx);}// 解码NAL单元buf_index = decode_nal_units(h, buf, buf_size);if (buf_index < 0)return AVERROR_INVALIDDATA;if (!h->cur_pic_ptr && h->nal_unit_type == H264_NAL_END_SEQUENCE) {av_assert0(buf_index <= buf_size);return send_next_delayed_frame(h, pict, got_frame, buf_index);}if (!(avctx->flags2 & AV_CODEC_FLAG2_CHUNKS) && (!h->cur_pic_ptr || !h->has_slice)) {if (avctx->skip_frame >= AVDISCARD_NONREF ||buf_size >= 4 && !memcmp("Q264", buf, 4))return buf_size;return AVERROR_INVALIDDATA;}if (!(avctx->flags2 & AV_CODEC_FLAG2_CHUNKS) ||(h->mb_y >= h->mb_height && h->mb_height)) {if ((ret = ff_h264_field_end(h, &h->slice_ctx[0], 0)) < 0)return ret;/* Wait for second field. */if (h->next_output_pic) {ret = finalize_frame(h, pict, h->next_output_pic, got_frame);if (ret < 0)return ret;}}ff_h264_unref_picture(h, &h->last_pic_for_ec);return get_consumed_bytes(buf_index, buf_size);
}

三、avcodec_decode_video2视频解码

由于avcodec_decode_video2()函数已经过时，所以内部提供compat_decode()来兼容旧版本，具体代码如下：

int avcodec_decode_video2(AVCodecContext *avctx, AVFrame *picture,int *got_picture_ptr,const AVPacket *avpkt)
{return compat_decode(avctx, picture, got_picture_ptr, avpkt);
}

我们来看看compat_decode()函数源码，其实内部也是调用avcodec_send_packet()和avcodec_receive_frame()进行解码，具体如下：

static int compat_decode(AVCodecContext *avctx, AVFrame *frame,int *got_frame, const AVPacket *pkt)
{AVCodecInternal *avci = avctx->internal;int ret = 0;*got_frame = 0;if (avci->draining_done && pkt && pkt->size != 0) {avcodec_flush_buffers(avctx);}if (avci->compat_decode_partial_size > 0 &&avci->compat_decode_partial_size != pkt->size) {ret = AVERROR(EINVAL);goto finish;}if (!avci->compat_decode_partial_size) {// 调用avcodec_send_packet发送packetret = avcodec_send_packet(avctx, pkt);if (ret == AVERROR_EOF)ret = 0;else if (ret == AVERROR(EAGAIN)) {/* we fully drain all the output in each decode call, so this should not* ever happen */ret = AVERROR_BUG;goto finish;} else if (ret < 0)goto finish;}while (ret >= 0) {// 循环调用avcodec_receive_frame接收frameret = avcodec_receive_frame(avctx, frame);if (ret < 0) {if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF)ret = 0;goto finish;}if (frame != avci->compat_decode_frame) {if (!avctx->refcounted_frames) {ret = unrefcount_frame(avci, frame);if (ret < 0)goto finish;}*got_frame = 1;frame = avci->compat_decode_frame;} else {if (!avci->compat_decode_warned) {avci->compat_decode_warned = 1;}}if (avci->draining || (!avctx->codec->bsfs && avci->compat_decode_consumed < pkt->size)) {break;}}finish:if (ret == 0) {/* if there are any bsfs then assume full packet is always consumed */if (avctx->codec->bsfs)ret = pkt->size;elseret = FFMIN(avci->compat_decode_consumed, pkt->size);}avci->compat_decode_consumed = 0;avci->compat_decode_partial_size = (ret >= 0) ? pkt->size - ret : 0;return ret;
}

四、avcodec_decode_audio4音频解码

和avcodec_decode_video2()函数一样，avcodec_decode_audio4()函数已经过时，内部也是提供compat_decode()来兼容旧版本，具体代码如下：

int avcodec_decode_audio4(AVCodecContext *avctx,AVFrame *frame,int *got_frame_ptr,const AVPacket *avpkt)
{return compat_decode(avctx, frame, got_frame_ptr, avpkt);
}

至此，avcodec_send_packet()和avcodec_receive_frame()组成的解码函数已经分析完毕。

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