FFmepg：AVPacket源码完整解析

先看一下源代码中对于AVPacket的描述：

• This structure stores compressed data. It is typically exported by demuxers and then passed as input to decoders, or received as output from encoders and then passed to muxers.
• For video, it should typically contain one compressed frame. For audio it may contain several compressed frames. Encoders are allowed to output empty packets, with no compressed data, containing only side data (e.g. to update some stream parameters at the end of encoding).
• AVPacket is one of the few structs in FFmpeg, whose size is a part of public ABI. Thus it may be allocated on stack and no new fields can be added to it without libavcodec and libavformat major bump.
• The semantics of data ownership depends on the buf field. If it is set, the packet data is dynamically allocated and is valid indefinitely until a call to av_packet_unref() reduces the reference count to 0.
• If the buf field is not set av_packet_ref() would make a copy instead of increasing the reference count.
• The side data is always allocated with av_malloc(), copied by av_packet_ref() and freed by av_packet_unref().

从这段描述中可以得知AVPacket数据结构中很重要的一个字段就是AVBufferRef *buf;
*buf is a reference to the reference-counted buffer where the packet data is stored.
May be NUL L, then the packet data is not reference-counted.

AVPacket结构体中有一些字段被统称为"properties fields"：除了(buf、data、size)以外的所有字段。
所以可以理解为Packet的核心数据在buf、data、size中，其他都是相对次要的属性数据

• av_init_packet

void av_init_packet(AVPacket *pkt)
{
    pkt->pts                  = AV_NOPTS_VALUE;
    pkt->dts                  = AV_NOPTS_VALUE;
    pkt->pos                  = -1;
    pkt->duration             = 0;
#if FF_API_CONVERGENCE_DURATION
FF_DISABLE_DEPRECATION_WARNINGS
    pkt->convergence_duration = 0;
FF_ENABLE_DEPRECATION_WARNINGS
#endif
    pkt->flags                = 0;
    pkt->stream_index         = 0;
    pkt->buf                  = NULL;
    pkt->side_data            = NULL;
    pkt->side_data_elems      = 0;
}

• av_packet_alloc

AVPacket *av_packet_alloc(void)
{
    AVPacket *pkt = av_mallocz(sizeof(AVPacket));
    if (!pkt)
        return pkt;

    av_packet_unref(pkt);

    return pkt;
}

• av_new_packet

int av_new_packet(AVPacket *pkt, int size)
{
    AVBufferRef *buf = NULL;
    int ret = packet_alloc(&buf, size);
    if (ret < 0)
        return ret;

    av_init_packet(pkt);
    pkt->buf      = buf;
    pkt->data     = buf->data;
    pkt->size     = size;

    return 0;
}

• av_packet_from_data

int av_packet_from_data(AVPacket *pkt, uint8_t *data, int size)
{
    if (size >= INT_MAX - AV_INPUT_BUFFER_PADDING_SIZE)
        return AVERROR(EINVAL);

    pkt->buf = av_buffer_create(data, size + AV_INPUT_BUFFER_PADDING_SIZE,
                                av_buffer_default_free, NULL, 0);
    if (!pkt->buf)
        return AVERROR(ENOMEM);

    pkt->data = data;
    pkt->size = size;

    return 0;
}

• av_dup_packet

int av_dup_packet(AVPacket *pkt)
{
    AVPacket tmp_pkt;

    if (!pkt->buf && pkt->data) {
        tmp_pkt = *pkt;
        return copy_packet_data(pkt, &tmp_pkt, 1);
    }
    return 0;
}

• av_copy_packet

int av_copy_packet(AVPacket *dst, const AVPacket *src)
{
    *dst = *src;
    return copy_packet_data(dst, src, 0);
}

copy_packet_data

/* Makes duplicates of data, side_data, but does not copy any other fields */
static int copy_packet_data(AVPacket *pkt, const AVPacket *src, int dup)
{
    pkt->data      = NULL;
    pkt->side_data = NULL;
    pkt->side_data_elems = 0;
    if (pkt->buf) {
        AVBufferRef *ref = av_buffer_ref(src->buf);
        if (!ref)
            return AVERROR(ENOMEM);
        pkt->buf  = ref;
        pkt->data = ref->data;
    } else {
        DUP_DATA(pkt->data, src->data, pkt->size, 1, ALLOC_BUF);
    }
    if (src->side_data_elems && dup) {
        pkt->side_data = src->side_data;
        pkt->side_data_elems = src->side_data_elems;
    }
    if (src->side_data_elems && !dup) {
        return av_copy_packet_side_data(pkt, src);
    }
    return 0;

failed_alloc:
    av_packet_unref(pkt);
    return AVERROR(ENOMEM);
}

• av_packet_free 注意av_packet_free的参数类型是AVPacket **

void av_packet_free(AVPacket **pkt)
{
    if (!pkt || !*pkt)
        return;

    av_packet_unref(*pkt);
    av_freep(pkt);
}

• av_free_packet

#if FF_API_AVPACKET_OLD_API
FF_DISABLE_DEPRECATION_WARNINGS
void av_free_packet(AVPacket *pkt)
{
    if (pkt) {
        if (pkt->buf)
            av_buffer_unref(&pkt->buf);
        pkt->data            = NULL;
        pkt->size            = 0;

        av_packet_free_side_data(pkt);
    }
}
FF_ENABLE_DEPRECATION_WARNINGS
#endif

• av_packet_copy_props

int av_packet_copy_props(AVPacket *dst, const AVPacket *src)
{
    int i;

    dst->pts                  = src->pts;
    dst->dts                  = src->dts;
    dst->pos                  = src->pos;
    dst->duration             = src->duration;
#if FF_API_CONVERGENCE_DURATION
FF_DISABLE_DEPRECATION_WARNINGS
    dst->convergence_duration = src->convergence_duration;
FF_ENABLE_DEPRECATION_WARNINGS
#endif
    dst->flags                = src->flags;
    dst->stream_index         = src->stream_index;

    dst->side_data            = NULL;
    dst->side_data_elems      = 0;
    for (i = 0; i < src->side_data_elems; i++) {
        enum AVPacketSideDataType type = src->side_data[i].type;
        int size          = src->side_data[i].size;
        uint8_t *src_data = src->side_data[i].data;
        uint8_t *dst_data = av_packet_new_side_data(dst, type, size);

        if (!dst_data) {
            av_packet_free_side_data(dst);
            return AVERROR(ENOMEM);
        }
        memcpy(dst_data, src_data, size);
    }

    return 0;
}

• av_packet_unref

void av_packet_unref(AVPacket *pkt)
{
    av_packet_free_side_data(pkt);
    av_buffer_unref(&pkt->buf);
    av_init_packet(pkt);
    pkt->data = NULL;
    pkt->size = 0;
}

• av_packet_ref

int av_packet_ref(AVPacket *dst, const AVPacket *src)
{
    int ret;

    ret = av_packet_copy_props(dst, src);
    if (ret < 0)
        return ret;

    if (!src->buf) {
        ret = packet_alloc(&dst->buf, src->size);
        if (ret < 0)
            goto fail;
        av_assert1(!src->size || src->data);
        if (src->size)
            memcpy(dst->buf->data, src->data, src->size);

        dst->data = dst->buf->data;
    } else {
        dst->buf = av_buffer_ref(src->buf);
        if (!dst->buf) {
            ret = AVERROR(ENOMEM);
            goto fail;
        }
        dst->data = src->data;
    }

    dst->size = src->size;

    return 0;
fail:
    av_packet_free_side_data(dst);
    return ret;
}

• av_packet_clone

AVPacket *av_packet_clone(const AVPacket *src)
{
    AVPacket *ret = av_packet_alloc();

    if (!ret)
        return ret;

    if (av_packet_ref(ret, src))
        av_packet_free(&ret);

    return ret;
}

• av_packet_move_ref

void av_packet_move_ref(AVPacket *dst, AVPacket *src)
{
    *dst = *src;
    av_init_packet(src);
    src->data = NULL;
    src->size = 0;
}

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