1. 内存模型图

2. 分析流程

我们解复用后,媒体流数据就会被分离开来,分别生成对应AVPacketList,然后通过av_read_frame读取一个AVPacket来读取,然而如果有一个新的AVPacket也想要指向之前的AVPacket,那么内存模型应该是什么样子的呢?

通过思考应该有2种模型:
1.两个AVPacket同时指向同一块数据区域
2.先拷贝之前的数据区域,然后再给给新的AVPacket使用

由于我们数据区域是只读的,因此我们可以采用第一种方案比较稳妥,但是又迎来的新的问题,如果我们avpacket释放了数据,那么数据就会消失,那么avpacket2就会失效,那么该如何解决呢?
如果有朋友了解过C++的智能指针设计的话,那么就知道这种情况改如何设计了.
我们通过引入一个变量来标记该数据被多少AVPacket所使用,当该变量为1时释放就会真正的释放掉改数据.

3.追溯本源————源码分析

3.1 AVPacket队列 什么时候生成的？

我们先看一下上面这个流程.
我们仔细阅读一下avformat_open_input这个函数.

int avformat_open_input(AVFormatContext **ps, const char *filename,
                        ff_const59 AVInputFormat *fmt, AVDictionary **options)
{
    AVFormatContext *s = *ps;
    int i, ret = 0;
    AVDictionary *tmp = NULL;
    ID3v2ExtraMeta *id3v2_extra_meta = NULL;

    if (!s && !(s = avformat_alloc_context()))
        return AVERROR(ENOMEM);
    if (!s->av_class) {
        av_log(NULL, AV_LOG_ERROR, "Input context has not been properly allocated by avformat_alloc_context() and is not NULL either\n");
        return AVERROR(EINVAL);
    }
    if (fmt)
        s->iformat = fmt;

    if (options)
        av_dict_copy(&tmp, *options, 0);

    if (s->pb) // must be before any goto fail
        s->flags |= AVFMT_FLAG_CUSTOM_IO;

    if ((ret = av_opt_set_dict(s, &tmp)) < 0)
        goto fail;

    if (!(s->url = av_strdup(filename ? filename : ""))) {
        ret = AVERROR(ENOMEM);
        goto fail;
    }

#if FF_API_FORMAT_FILENAME
FF_DISABLE_DEPRECATION_WARNINGS
    av_strlcpy(s->filename, filename ? filename : "", sizeof(s->filename));
FF_ENABLE_DEPRECATION_WARNINGS
#endif
    if ((ret = init_input(s, filename, &tmp)) < 0)
        goto fail;
    s->probe_score = ret;

    if (!s->protocol_whitelist && s->pb && s->pb->protocol_whitelist) {
        s->protocol_whitelist = av_strdup(s->pb->protocol_whitelist);
        if (!s->protocol_whitelist) {
            ret = AVERROR(ENOMEM);
            goto fail;
        }
    }

    if (!s->protocol_blacklist && s->pb && s->pb->protocol_blacklist) {
        s->protocol_blacklist = av_strdup(s->pb->protocol_blacklist);
        if (!s->protocol_blacklist) {
            ret = AVERROR(ENOMEM);
            goto fail;
        }
    }

    if (s->format_whitelist && av_match_list(s->iformat->name, s->format_whitelist, ',') <= 0) {
        av_log(s, AV_LOG_ERROR, "Format not on whitelist \'%s\'\n", s->format_whitelist);
        ret = AVERROR(EINVAL);
        goto fail;
    }

    avio_skip(s->pb, s->skip_initial_bytes);

    /* Check filename in case an image number is expected. */
    if (s->iformat->flags & AVFMT_NEEDNUMBER) {
        if (!av_filename_number_test(filename)) {
            ret = AVERROR(EINVAL);
            goto fail;
        }
    }

    s->duration = s->start_time = AV_NOPTS_VALUE;

    /* Allocate private data. */
    if (s->iformat->priv_data_size > 0) {
        if (!(s->priv_data = av_mallocz(s->iformat->priv_data_size))) {
            ret = AVERROR(ENOMEM);
            goto fail;
        }
        if (s->iformat->priv_class) {
            *(const AVClass **) s->priv_data = s->iformat->priv_class;
            av_opt_set_defaults(s->priv_data);
            if ((ret = av_opt_set_dict(s->priv_data, &tmp)) < 0)
                goto fail;
        }
    }

    /* e.g. AVFMT_NOFILE formats will not have a AVIOContext */
    if (s->pb)
        ff_id3v2_read_dict(s->pb, &s->internal->id3v2_meta, ID3v2_DEFAULT_MAGIC, &id3v2_extra_meta);


    if (!(s->flags&AVFMT_FLAG_PRIV_OPT) && s->iformat->read_header)
        if ((ret = s->iformat->read_header(s)) < 0)
            goto fail;

    if (!s->metadata) {
        s->metadata = s->internal->id3v2_meta;
        s->internal->id3v2_meta = NULL;
    } else if (s->internal->id3v2_meta) {
        int level = AV_LOG_WARNING;
        if (s->error_recognition & AV_EF_COMPLIANT)
            level = AV_LOG_ERROR;
        av_log(s, level, "Discarding ID3 tags because more suitable tags were found.\n");
        av_dict_free(&s->internal->id3v2_meta);
        if (s->error_recognition & AV_EF_EXPLODE) {
            ret = AVERROR_INVALIDDATA;
            goto close;
        }
    }

    if (id3v2_extra_meta) {
        if (!strcmp(s->iformat->name, "mp3") || !strcmp(s->iformat->name, "aac") ||
            !strcmp(s->iformat->name, "tta") || !strcmp(s->iformat->name, "wav")) {
            if ((ret = ff_id3v2_parse_apic(s, &id3v2_extra_meta)) < 0)
                goto close;
            if ((ret = ff_id3v2_parse_chapters(s, &id3v2_extra_meta)) < 0)
                goto close;
            if ((ret = ff_id3v2_parse_priv(s, &id3v2_extra_meta)) < 0)
                goto close;
        } else
            av_log(s, AV_LOG_DEBUG, "demuxer does not support additional id3 data, skipping\n");
    }
    ff_id3v2_free_extra_meta(&id3v2_extra_meta);

    if ((ret = avformat_queue_attached_pictures(s)) < 0)
        goto close;

    if (!(s->flags&AVFMT_FLAG_PRIV_OPT) && s->pb && !s->internal->data_offset)
        s->internal->data_offset = avio_tell(s->pb);

    s->internal->raw_packet_buffer_remaining_size = RAW_PACKET_BUFFER_SIZE;

    update_stream_avctx(s);

    for (i = 0; i < s->nb_streams; i++)
        s->streams[i]->internal->orig_codec_id = s->streams[i]->codecpar->codec_id;

    if (options) {
        av_dict_free(options);
        *options = tmp;
    }
    *ps = s;
    return 0;

close:
    if (s->iformat->read_close)
        s->iformat->read_close(s);
fail:
    ff_id3v2_free_extra_meta(&id3v2_extra_meta);
    av_dict_free(&tmp);
    if (s->pb && !(s->flags & AVFMT_FLAG_CUSTOM_IO))
        avio_closep(&s->pb);
    avformat_free_context(s);
    *ps = NULL;
    return ret;
}

其中关键的是：

    if ((ret = avformat_queue_attached_pictures(s)) < 0)
        goto close;

avformat_open_input调用了avformat_queue_attached_pictures这个函数,那么我看一下这个函数是干嘛的?

int avformat_queue_attached_pictures(AVFormatContext *s)
{
    int i, ret;
    for (i = 0; i < s->nb_streams; i++)
        if (s->streams[i]->disposition & AV_DISPOSITION_ATTACHED_PIC &&
            s->streams[i]->discard < AVDISCARD_ALL) {
            if (s->streams[i]->attached_pic.size <= 0) {
                av_log(s, AV_LOG_WARNING,
                    "Attached picture on stream %d has invalid size, "
                    "ignoring\n", i);
                continue;
            }

            ret = ff_packet_list_put(&s->internal->raw_packet_buffer,
                                     &s->internal->raw_packet_buffer_end,
                                     &s->streams[i]->attached_pic,
                                     FF_PACKETLIST_FLAG_REF_PACKET);
            if (ret < 0)
                return ret;
        }
    return 0;
}

根据函数很明显可以看出这个函数是对每一个流进行调用ff_packet_list_put构建AVPacketList链表.
我们仔细看一下ff_packet_list_put这个函数:

int ff_packet_list_put(AVPacketList **packet_buffer,
                       AVPacketList **plast_pktl,
                       AVPacket      *pkt, int flags)
{
    AVPacketList *pktl = av_mallocz(sizeof(AVPacketList));
    int ret;

    if (!pktl)
        return AVERROR(ENOMEM);

    if (flags & FF_PACKETLIST_FLAG_REF_PACKET) {
        if ((ret = av_packet_ref(&pktl->pkt, pkt)) < 0) {
            av_free(pktl);
            return ret;
        }
    } else {
        // TODO: Adapt callers in this file so the line below can use
        //       av_packet_move_ref() to effectively move the reference
        //       to the list.
        pktl->pkt = *pkt;
    }

    if (*packet_buffer)
        (*plast_pktl)->next = pktl;
    else
        *packet_buffer = pktl;

    /* Add the packet in the buffered packet list. */
    *plast_pktl = pktl;
    return 0;
}

这个函数其实就是一个链表的尾插函数,每次都创建一个AVPacketList,而AVPacketList结构体中封装的是:AVPacket

typedef struct AVPacketList {
    AVPacket pkt;
    struct AVPacketList *next;
} AVPacketList;

那么到这里我们就回答了一开始标题的问题了.

4 .AVPacket和AVFrame相关操作API

5. av_read_frame源码分析

int av_read_frame(AVFormatContext *s, AVPacket *pkt)
{
    const int genpts = s->flags & AVFMT_FLAG_GENPTS;
    int eof = 0;
    int ret;
    AVStream *st;

    if (!genpts) {
        ret = s->internal->packet_buffer
              ? ff_packet_list_get(&s->internal->packet_buffer,
                                        &s->internal->packet_buffer_end, pkt)
              : read_frame_internal(s, pkt);
        if (ret < 0)
            return ret;
        goto return_packet;
    }

    for (;;) {
        AVPacketList *pktl = s->internal->packet_buffer;

        if (pktl) {
            AVPacket *next_pkt = &pktl->pkt;

            if (next_pkt->dts != AV_NOPTS_VALUE) {
                int wrap_bits = s->streams[next_pkt->stream_index]->pts_wrap_bits;
                // last dts seen for this stream. if any of packets following
                // current one had no dts, we will set this to AV_NOPTS_VALUE.
                int64_t last_dts = next_pkt->dts;
                av_assert2(wrap_bits <= 64);
                while (pktl && next_pkt->pts == AV_NOPTS_VALUE) {
                    if (pktl->pkt.stream_index == next_pkt->stream_index &&
                        av_compare_mod(next_pkt->dts, pktl->pkt.dts, 2ULL << (wrap_bits - 1)) < 0) {
                        if (av_compare_mod(pktl->pkt.pts, pktl->pkt.dts, 2ULL << (wrap_bits - 1))) {
                            // not B-frame
                            next_pkt->pts = pktl->pkt.dts;
                        }
                        if (last_dts != AV_NOPTS_VALUE) {
                            // Once last dts was set to AV_NOPTS_VALUE, we don't change it.
                            last_dts = pktl->pkt.dts;
                        }
                    }
                    pktl = pktl->next;
                }
                if (eof && next_pkt->pts == AV_NOPTS_VALUE && last_dts != AV_NOPTS_VALUE) {
                    // Fixing the last reference frame had none pts issue (For MXF etc).
                    // We only do this when
                    // 1. eof.
                    // 2. we are not able to resolve a pts value for current packet.
                    // 3. the packets for this stream at the end of the files had valid dts.
                    next_pkt->pts = last_dts + next_pkt->duration;
                }
                pktl = s->internal->packet_buffer;
            }

            /* read packet from packet buffer, if there is data */
            st = s->streams[next_pkt->stream_index];
            if (!(next_pkt->pts == AV_NOPTS_VALUE && st->discard < AVDISCARD_ALL &&
                  next_pkt->dts != AV_NOPTS_VALUE && !eof)) {
                ret = ff_packet_list_get(&s->internal->packet_buffer,
                                               &s->internal->packet_buffer_end, pkt);
                goto return_packet;
            }
        }

        ret = read_frame_internal(s, pkt);
        if (ret < 0) {
            if (pktl && ret != AVERROR(EAGAIN)) {
                eof = 1;
                continue;
            } else
                return ret;
        }

        ret = ff_packet_list_put(&s->internal->packet_buffer,
                                 &s->internal->packet_buffer_end,
                                 pkt, FF_PACKETLIST_FLAG_REF_PACKET);
        av_packet_unref(pkt);
        if (ret < 0)
            return ret;
    }

return_packet:

    st = s->streams[pkt->stream_index];
    if ((s->iformat->flags & AVFMT_GENERIC_INDEX) && pkt->flags & AV_PKT_FLAG_KEY) {
        ff_reduce_index(s, st->index);
        av_add_index_entry(st, pkt->pos, pkt->dts, 0, 0, AVINDEX_KEYFRAME);
    }

    if (is_relative(pkt->dts))
        pkt->dts -= RELATIVE_TS_BASE;
    if (is_relative(pkt->pts))
        pkt->pts -= RELATIVE_TS_BASE;

    return ret;
}

有了前面的知识铺垫，不难分析出read_frame_internal是调用ff_packet_list_put来获取AVPacket的，但是这里要注意ff_packet_list_put并不是简单的将AVPacket给read_frame_internal。

int ff_packet_list_get(AVPacketList **pkt_buffer,
                       AVPacketList **pkt_buffer_end,
                       AVPacket      *pkt)
{
    AVPacketList *pktl;
    av_assert0(*pkt_buffer);
    pktl        = *pkt_buffer;
    *pkt        = pktl->pkt;
    *pkt_buffer = pktl->next;
    if (!pktl->next)
        *pkt_buffer_end = NULL;
    av_freep(&pktl);
    return 0;
}

根据源码可以得出，ff_packet_list_get先通过 *pkt = pktl->pkt; 将原来的数据直接拷贝给pkt，然后就释放掉了pkt1，也就是AVPacketList链表上的一个节点。那么这时候链表上就会消失一个AVPacket，这样做的好出就是，会将AVPacketList链表中的AVPacket慢慢释放掉，从而减少内存占用！

对于AVFrame相关API详解，我留到介绍编解码API流程时详细介绍，敬请期待~