ffmpeg源码跟踪笔记之av_write_frame 与 av_interleaved_write_frame

1、函数介绍

av_interleaved_write_frame函数介绍：

/**
 * Write a packet to an output media file ensuring correct interleaving.
 *
 * This function will buffer the packets internally as needed to make sure the
 * packets in the output file are properly interleaved in the order of
 * increasing dts. Callers doing their own interleaving should call
 * av_write_frame() instead of this function.
 *
 * @param s media file handle
 * @param pkt The packet containing the data to be written.
 *            <br>
 *            If the packet is reference-counted, this function will take
 *            ownership of this reference and unreference it later when it sees
 *            fit.
 *            The caller must not access the data through this reference after
 *            this function returns. If the packet is not reference-counted,
 *            libavformat will make a copy.
 *            <br>
 *            This parameter can be NULL (at any time, not just at the end), to
 *            flush the interleaving queues.
 *            <br>
 *            Packet's @ref AVPacket.stream_index "stream_index" field must be
 *            set to the index of the corresponding stream in @ref
 *            AVFormatContext.streams "s->streams". It is very strongly
 *            recommended that timing information (@ref AVPacket.pts "pts", @ref
 *            AVPacket.dts "dts", @ref AVPacket.duration "duration") is set to
 *            correct values.
 *
 * @return 0 on success, a negative AVERROR on error. Libavformat will always
 *         take care of freeing the packet, even if this function fails.
 *
 * @see av_write_frame(), AVFormatContext.max_interleave_delta
 */
int av_interleaved_write_frame(AVFormatContext *s, AVPacket *pkt);

av_write_frame函数介绍：

/**
 * Write a packet to an output media file.
 *
 * This function passes the packet directly to the muxer, without any buffering
 * or reordering. The caller is responsible for correctly interleaving the
 * packets if the format requires it. Callers that want libavformat to handle
 * the interleaving should call av_interleaved_write_frame() instead of this
 * function.
 *
 * @param s media file handle
 * @param pkt The packet containing the data to be written. Note that unlike
 *            av_interleaved_write_frame(), this function does not take
 *            ownership of the packet passed to it (though some muxers may make
 *            an internal reference to the input packet).
 *            <br>
 *            This parameter can be NULL (at any time, not just at the end), in
 *            order to immediately flush data buffered within the muxer, for
 *            muxers that buffer up data internally before writing it to the
 *            output.
 *            <br>
 *            Packet's @ref AVPacket.stream_index "stream_index" field must be
 *            set to the index of the corresponding stream in @ref
 *            AVFormatContext.streams "s->streams". It is very strongly
 *            recommended that timing information (@ref AVPacket.pts "pts", @ref
 *            AVPacket.dts "dts", @ref AVPacket.duration "duration") is set to
 *            correct values.
 * @return < 0 on error, = 0 if OK, 1 if flushed and there is no more data to flush
 *
 * @see av_interleaved_write_frame()
 */
int av_write_frame(AVFormatContext *s, AVPacket *pkt);

2、函数调用图

av_interleaved_write_frame函数调用图：

av_write_frame函数调用图：

3、函数代码

av_interleaved_write_frame函数代码：

int av_interleaved_write_frame(AVFormatContext *s, AVPacket *pkt)
{
    int ret, flush = 0;
    //检查pkt是否合法
    ret = check_packet(s, pkt);
    if (ret < 0)
        goto fail;

    if (pkt) {
        AVStream *st = s->streams[pkt->stream_index];

        av_dlog(s, "av_interleaved_write_frame size:%d dts:%s pts:%s\n",
                pkt->size, av_ts2str(pkt->dts), av_ts2str(pkt->pts));
        if ((ret = compute_pkt_fields2(s, st, pkt)) < 0 && !(s->oformat->flags & AVFMT_NOTIMESTAMPS))//计算和检查pts、dts、duration
            goto fail;

        if (pkt->dts == AV_NOPTS_VALUE && !(s->oformat->flags & AVFMT_NOTIMESTAMPS)) {
            ret = AVERROR(EINVAL);
            goto fail;
        }
    } else {
        av_dlog(s, "av_interleaved_write_frame FLUSH\n");
        flush = 1;
    }

    for (;; ) {
        AVPacket opkt;
        int ret = interleave_packet(s, &opkt, pkt, flush);
        if (pkt) {
            memset(pkt, 0, sizeof(*pkt));
            av_init_packet(pkt);
            pkt = NULL;
        }
        if (ret <= 0) //FIXME cleanup needed for ret<0 ?
            return ret;

        ret = write_packet(s, &opkt);
        if (ret >= 0)
            s->streams[opkt.stream_index]->nb_frames++;

        av_free_packet(&opkt);

        if (ret < 0)
            return ret;
        if(s->pb && s->pb->error)
            return s->pb->error;
    }
fail:
    av_packet_unref(pkt);
    return ret;
}

av_write_frame函数代码：

int av_write_frame(AVFormatContext *s, AVPacket *pkt)
{
    int ret;

    ret = check_packet(s, pkt);
    if (ret < 0)
        return ret;

    if (!pkt) {
        if (s->oformat->flags & AVFMT_ALLOW_FLUSH) {
            ret = s->oformat->write_packet(s, NULL);
            if (s->flush_packets && s->pb && s->pb->error >= 0 && s->flags & AVFMT_FLAG_FLUSH_PACKETS)
                avio_flush(s->pb);
            if (ret >= 0 && s->pb && s->pb->error < 0)
                ret = s->pb->error;
            return ret;
        }
        return 1;
    }

    ret = compute_pkt_fields2(s, s->streams[pkt->stream_index], pkt);

    if (ret < 0 && !(s->oformat->flags & AVFMT_NOTIMESTAMPS))
        return ret;

    ret = write_packet(s, pkt);
    if (ret >= 0 && s->pb && s->pb->error < 0)
        ret = s->pb->error;

    if (ret >= 0)
        s->streams[pkt->stream_index]->nb_frames++;
    return ret;
}

4、解释

可以看出2个函数的区别在于av_interleaved_write_frame调用了interleave_packet，而av_write_frame没有调用，看看interleave_packet的代码：

static int interleave_packet(AVFormatContext *s, AVPacket *out, AVPacket *in, int flush)
{
    if (s->oformat->interleave_packet) {
        int ret = s->oformat->interleave_packet(s, out, in, flush);
        if (in)
            av_free_packet(in);
        return ret;
    } else
        return ff_interleave_packet_per_dts(s, out, in, flush);
}

代码非常简单，如果AVOutputFormat有interleave_packet函数指针，则调用，如果没有则调用ff_interleave_packet_per_dts，看看ff_interleave_packet_per_dts的代码：

int ff_interleave_packet_per_dts(AVFormatContext *s, AVPacket *out,
                                 AVPacket *pkt, int flush)
{
    AVPacketList *pktl;
    int stream_count = 0;
    int noninterleaved_count = 0;
    int i, ret;

    if (pkt) {//把pkt加入到AVPacketList中去
        if ((ret = ff_interleave_add_packet(s, pkt, interleave_compare_dts)) < 0)
            return ret;
    }

    for (i = 0; i < s->nb_streams; i++) {
        if (s->streams[i]->last_in_packet_buffer) {
            ++stream_count;
        } else if (s->streams[i]->codec->codec_type != AVMEDIA_TYPE_ATTACHMENT &&
                   s->streams[i]->codec->codec_id != AV_CODEC_ID_VP8 &&
                   s->streams[i]->codec->codec_id != AV_CODEC_ID_VP9) {
            ++noninterleaved_count;
        }
    }

    if (s->internal->nb_interleaved_streams == stream_count)
        flush = 1;

    if (s->max_interleave_delta > 0 &&
        s->internal->packet_buffer &&
        !flush &&
        s->internal->nb_interleaved_streams == stream_count+noninterleaved_count
    ) {
        AVPacket *top_pkt = &s->internal->packet_buffer->pkt;
        int64_t delta_dts = INT64_MIN;
        int64_t top_dts = av_rescale_q(top_pkt->dts,
                                       s->streams[top_pkt->stream_index]->time_base,
                                       AV_TIME_BASE_Q);

        for (i = 0; i < s->nb_streams; i++) {
            int64_t last_dts;
            const AVPacketList *last = s->streams[i]->last_in_packet_buffer;

            if (!last)
                continue;

            last_dts = av_rescale_q(last->pkt.dts,
                                    s->streams[i]->time_base,
                                    AV_TIME_BASE_Q);
            delta_dts = FFMAX(delta_dts, last_dts - top_dts);
        }

        if (delta_dts > s->max_interleave_delta) {
            av_log(s, AV_LOG_DEBUG,
                   "Delay between the first packet and last packet in the "
                   "muxing queue is %"PRId64" > %"PRId64": forcing output\n",
                   delta_dts, s->max_interleave_delta);
            flush = 1;
        }
    }

    if (stream_count && flush) {
        AVStream *st;
        pktl = s->internal->packet_buffer;
        *out = pktl->pkt;
        st   = s->streams[out->stream_index];

        s->internal->packet_buffer = pktl->next;
        if (!s->internal->packet_buffer)
            s->internal->packet_buffer_end = NULL;

        if (st->last_in_packet_buffer == pktl)
            st->last_in_packet_buffer = NULL;
        av_freep(&pktl);

        return 1;
    } else {
        av_init_packet(out);
        return 0;
    }
}

从代码中看到，函数ff_interleave_packet_per_dts调用ff_interleave_add_packet把pkt加入缓存，然后再从缓存中取出第一个pkt返回。
看看ff_interleave_add_packet的代码：

int ff_interleave_add_packet(AVFormatContext *s, AVPacket *pkt,
                             int (*compare)(AVFormatContext *, AVPacket *, AVPacket *))
{
    int ret;
    AVPacketList **next_point, *this_pktl;
    AVStream *st   = s->streams[pkt->stream_index];
    int chunked    = s->max_chunk_size || s->max_chunk_duration;

    this_pktl      = av_mallocz(sizeof(AVPacketList));
    if (!this_pktl)
        return AVERROR(ENOMEM);
    this_pktl->pkt = *pkt;
#if FF_API_DESTRUCT_PACKET
FF_DISABLE_DEPRECATION_WARNINGS
    pkt->destruct  = NULL;           // do not free original but only the copy
FF_ENABLE_DEPRECATION_WARNINGS
#endif
    pkt->buf       = NULL;
    pkt->side_data = NULL;
    pkt->side_data_elems = 0;
    if ((pkt->flags & AV_PKT_FLAG_UNCODED_FRAME)) {
        av_assert0(pkt->size == UNCODED_FRAME_PACKET_SIZE);
        av_assert0(((AVFrame *)pkt->data)->buf);
    } else {
        // Duplicate the packet if it uses non-allocated memory
        if ((ret = av_dup_packet(&this_pktl->pkt)) < 0) {
            av_free(this_pktl);
            return ret;
        }
    }

    if (s->streams[pkt->stream_index]->last_in_packet_buffer) {
        next_point = &(st->last_in_packet_buffer->next);
    } else {
        next_point = &s->internal->packet_buffer;
    }

    if (chunked) {
        uint64_t max= av_rescale_q_rnd(s->max_chunk_duration, AV_TIME_BASE_Q, st->time_base, AV_ROUND_UP);
        st->interleaver_chunk_size     += pkt->size;
        st->interleaver_chunk_duration += pkt->duration;
        if (   (s->max_chunk_size && st->interleaver_chunk_size > s->max_chunk_size)
            || (max && st->interleaver_chunk_duration           > max)) {
            st->interleaver_chunk_size      = 0;
            this_pktl->pkt.flags |= CHUNK_START;
            if (max && st->interleaver_chunk_duration > max) {
                int64_t syncoffset = (st->codec->codec_type == AVMEDIA_TYPE_VIDEO)*max/2;
                int64_t syncto = av_rescale(pkt->dts + syncoffset, 1, max)*max - syncoffset;

                st->interleaver_chunk_duration += (pkt->dts - syncto)/8 - max;
            } else
                st->interleaver_chunk_duration = 0;
        }
    }
    if (*next_point) {
        if (chunked && !(this_pktl->pkt.flags & CHUNK_START))
            goto next_non_null;

        if (compare(s, &s->internal->packet_buffer_end->pkt, pkt)) {
            while (   *next_point
                   && ((chunked && !((*next_point)->pkt.flags&CHUNK_START))
                       || !compare(s, &(*next_point)->pkt, pkt)))
                next_point = &(*next_point)->next;
            if (*next_point)
                goto next_non_null;
        } else {
            next_point = &(s->internal->packet_buffer_end->next);
        }
    }
    av_assert1(!*next_point);

    s->internal->packet_buffer_end = this_pktl;
next_non_null:

    this_pktl->next = *next_point;

    s->streams[pkt->stream_index]->last_in_packet_buffer =
        *next_point                                      = this_pktl;

    return 0;
}

代码中可以看到，函数ff_interleave_add_packet把pkt经过排序（如果缓存中有pkt的话）加入到缓存中，这个排序是根据ff_interleave_packet_per_dts函数传入的compare（其实就是函数interleave_compare_dts）指针做的，来看看interleave_compare_dts函数的代码：

static int interleave_compare_dts(AVFormatContext *s, AVPacket *next,
                                  AVPacket *pkt)
{
    AVStream *st  = s->streams[pkt->stream_index];
    AVStream *st2 = s->streams[next->stream_index];
    int comp      = av_compare_ts(next->dts, st2->time_base, pkt->dts,
                                  st->time_base);
    if (s->audio_preload && ((st->codec->codec_type == AVMEDIA_TYPE_AUDIO) != (st2->codec->codec_type == AVMEDIA_TYPE_AUDIO))) {
        int64_t ts = av_rescale_q(pkt ->dts, st ->time_base, AV_TIME_BASE_Q) - s->audio_preload*(st ->codec->codec_type == AVMEDIA_TYPE_AUDIO);
        int64_t ts2= av_rescale_q(next->dts, st2->time_base, AV_TIME_BASE_Q) - s->audio_preload*(st2->codec->codec_type == AVMEDIA_TYPE_AUDIO);
        if (ts == ts2) {
            ts= ( pkt ->dts* st->time_base.num*AV_TIME_BASE - s->audio_preload*(int64_t)(st ->codec->codec_type == AVMEDIA_TYPE_AUDIO)* st->time_base.den)*st2->time_base.den
               -( next->dts*st2->time_base.num*AV_TIME_BASE - s->audio_preload*(int64_t)(st2->codec->codec_type == AVMEDIA_TYPE_AUDIO)*st2->time_base.den)* st->time_base.den;
            ts2=0;
        }
        comp= (ts>ts2) - (ts<ts2);
    }

    if (comp == 0)
        return pkt->stream_index < next->stream_index;
    return comp > 0;
}

从代码中可以看到interleave_compare_dts函数其实就是在比较传入的2个pkt的dts值，其实就是比那个的dts更早，如果有音频还有audio_preload（音频预加载时间）的话，还需要把音频包的预加载时间算进去。

好了，现在整体逻辑就清晰了，得出的结论是：在有多个流的情况下要用av_interleaved_write_frame，只有单一流2个函数都可以用。