FFmpeg AVPacket 结构体剖析以及实战

前沿

        AVPacket:存储压缩数据(视频对应H.264等码流数据,音频对应AAC/MP3等码流数据),简单来说就是携带一个NAL视频单元,或者多个NAL音频单元


重要结构体成员分析

        AVBufferRef *buf; //当前AVPacket中压缩数据的引用计数,以及保存压缩数据的指针地址(压缩数据申请的空间在这里)
        uint8_t *data;//保存压缩数据的指针地址(data同时指向了buf中的data)
        int   size;//压缩数据的长度
        int   stream_index;//视频还是音频的索引


实战(构建包含一个NAL单元(长度为nLen)的AVPacket)

     AVPacket pkt1, *packet  = &pkt1;
     av_new_packet(packet, nLen);
     memcpy(packet->data, data, nLen);
     packet->size = nLen;
     packet->stream_index = 0;
然后就可以将packet加入链表等待解码出一帧数据,或者调用avcodec_decode_video2进行解码,解码之后,可以调用av_free_packet或者  av_packet_unref释放资源


释疑
1)为什么不直接对packet->data申请内存,然后进行数据的拷贝?按结构体中定义说明 AVBufferRef只是数据的引用计数,可以为NULL,代表没有任何的引用

所以上面的代码修改(不推荐):
         av_init_packet(packet);//初始化结构体,尤其是AVBufferRef *buf,避免
         packet->data = (uint8_t *)malloc(sizeof(uint8_t)* nByte);
         memcpy(packet->data, data, nLen);
         packet->size = nLen;
         packet->stream_index = 0;
注意:av_init_packet(packet);//初始化结构体,尤其是AVBufferRef *buf,避免在解码的时候,访问到非法的指针地址。
不推荐:无法使用av_free_packet或者  av_packet_unref进行资源的释放,必须手动释放掉packet->data申请的内存,因为这两个函数释放资源针对的都是AVPacket结构体中的buf,而不是data

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);
    }
}

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;
}


2)为什么 AVBufferRef里面中的data跟外层的data指针是一样的,但是size长度不一样
根据
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;
}
可以得知两个data指针指向的是同一块内存,但是在申请内存的时候,进行了字节的对齐
多申请AV_INPUT_BUFFER_PADDING_SIZE个字节的数据作为结尾的填充

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_INPUT_BUFFER_PADDING_SIZE是为了进行数据的对齐,方便数据的访问
static int packet_alloc(AVBufferRef **buf, int size)
{
    int ret;
    if (size < 0 || size >= INT_MAX - AV_INPUT_BUFFER_PADDING_SIZE)
        return AVERROR(EINVAL);

    ret = av_buffer_realloc(buf, size + AV_INPUT_BUFFER_PADDING_SIZE);
    if (ret < 0)
        return ret;

    memset((*buf)->data + size, 0, AV_INPUT_BUFFER_PADDING_SIZE);

    return 0;
}
创建一个AVBufferRef实例,然后申请size长度的内存分配给AVBufferRef实例中的data
int av_buffer_realloc(AVBufferRef **pbuf, int size)
{
    AVBufferRef *buf = *pbuf;
    uint8_t *tmp;

    if (!buf) {
        /* allocate a new buffer with av_realloc(), so it will be reallocatable
         * later */
        uint8_t *data = av_realloc(NULL, size);
        if (!data)
            return AVERROR(ENOMEM);

        buf = av_buffer_create(data, size, av_buffer_default_free, NULL, 0);
        if (!buf) {
            av_freep(&data);
            return AVERROR(ENOMEM);
        }

        buf->buffer->flags |= BUFFER_FLAG_REALLOCATABLE;
        *pbuf = buf;

        return 0;
    } else if (buf->size == size)
        return 0;

    if (!(buf->buffer->flags & BUFFER_FLAG_REALLOCATABLE) ||
        !av_buffer_is_writable(buf) || buf->data != buf->buffer->data) {
        /* cannot realloc, allocate a new reallocable buffer and copy data */
        AVBufferRef *new = NULL;

        av_buffer_realloc(&new, size);
        if (!new)
            return AVERROR(ENOMEM);

        memcpy(new->data, buf->data, FFMIN(size, buf->size));

        buffer_replace(pbuf, &new);
        return 0;
    }

    tmp = av_realloc(buf->buffer->data, size);
    if (!tmp)
        return AVERROR(ENOMEM);

    buf->buffer->data = buf->data = tmp;
    buf->buffer->size = buf->size = size;
    return 0;
}
//释放AVBufferRef申请内存
void av_buffer_unref(AVBufferRef **buf)
{
    if (!buf || !*buf)
        return;

    buffer_replace(buf, NULL);
}

static void buffer_replace(AVBufferRef **dst, AVBufferRef **src)
{
    AVBuffer *b;

    b = (*dst)->buffer;

    if (src) {
        **dst = **src;
        av_freep(src);
    } else
        av_freep(dst);

    if (atomic_fetch_add_explicit(&b->refcount, -1, memory_order_acq_rel) == 1) {
        b->free(b->opaque, b->data);
        av_freep(&b);
    }
}

结构体定义
/**
 * A reference to a data buffer.
 *
 * The size of this struct is not a part of the public ABI and it is not meant
 * to be allocated directly.
 */
typedef struct AVBufferRef {
    AVBuffer *buffer;

    /**
     * The data buffer. It is considered writable if and only if
     * this is the only reference to the buffer, in which case
     * av_buffer_is_writable() returns 1.
     */
    uint8_t *data;
    /**
     * Size of data in bytes.
     */
    int      size;
} AVBufferRef;

/**
 * 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.
 *
 * 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 or destruct (deprecated)
 * fields. If either is set, the packet data is dynamically allocated and is
 * valid indefinitely until av_free_packet() is called (which in turn calls
 * av_buffer_unref()/the destruct callback to free the data). If neither is set,
 * the packet data is typically backed by some static buffer somewhere and is
 * only valid for a limited time (e.g. until the next read call when demuxing).
 *
 * The side data is always allocated with av_malloc() and is freed in
 * av_free_packet().
 */
typedef struct AVPacket {
    /**
     * A reference to the reference-counted buffer where the packet data is
     * stored.
     * May be NULL, then the packet data is not reference-counted.
     */
    AVBufferRef *buf;
    /**
     * Presentation timestamp in AVStream->time_base units; the time at which
     * the decompressed packet will be presented to the user.
     * Can be AV_NOPTS_VALUE if it is not stored in the file.
     * pts MUST be larger or equal to dts as presentation cannot happen before
     * decompression, unless one wants to view hex dumps. Some formats misuse
     * the terms dts and pts/cts to mean something different. Such timestamps
     * must be converted to true pts/dts before they are stored in AVPacket.
     */
    int64_t pts;
    /**
     * Decompression timestamp in AVStream->time_base units; the time at which
     * the packet is decompressed.
     * Can be AV_NOPTS_VALUE if it is not stored in the file.
     */
    int64_t dts;
    uint8_t *data;
    int   size;
    int   stream_index;
    /**
     * A combination of AV_PKT_FLAG values
     */
    int   flags;
    /**
     * Additional packet data that can be provided by the container.
     * Packet can contain several types of side information.
     */
    struct {
        uint8_t *data;
        int      size;
        enum AVPacketSideDataType type;
    } *side_data;
    int side_data_elems;

    /**
     * Duration of this packet in AVStream->time_base units, 0 if unknown.
     * Equals next_pts - this_pts in presentation order.
     */
    int   duration;
#if FF_API_DESTRUCT_PACKET
    attribute_deprecated
    void  (*destruct)(struct AVPacket *);
    attribute_deprecated
    void  *priv;
#endif
    int64_t pos;                            ///< byte position in stream, -1 if unknown

    /**
     * Time difference in AVStream->time_base units from the pts of this
     * packet to the point at which the output from the decoder has converged
     * independent from the availability of previous frames. That is, the
     * frames are virtually identical no matter if decoding started from
     * the very first frame or from this keyframe.
     * Is AV_NOPTS_VALUE if unknown.
     * This field is not the display duration of the current packet.
     * This field has no meaning if the packet does not have AV_PKT_FLAG_KEY
     * set.
     *
     * The purpose of this field is to allow seeking in streams that have no
     * keyframes in the conventional sense. It corresponds to the
     * recovery point SEI in H.264 and match_time_delta in NUT. It is also
     * essential for some types of subtitle streams to ensure that all
     * subtitles are correctly displayed after seeking.
     */
    int64_t convergence_duration;
} AVPacket;


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