FFmpeg数据结构AVPacket

struct AVPacket packet;

 _______              ______________
|       |            |              |
| input |  demuxer   | encoded data |   decoder
| file  | ---------> | packets      | -----+
|_______|            |______________|      |
                                           v
                                       _________
                                      |         |
                                      | decoded |
                                      | frames  |
                                      |_________|
 ________             ______________       |
|        |           |              |      |
| output | <-------- | encoded data | <----+
| file   |   muxer   | packets      |   encoder
|________|           |______________|

/**
 * 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().
 *
 * @see av_packet_ref
 * @see av_packet_unref
 */
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.
     */
    AVPacketSideData *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.
     */
    int64_t duration;

    int64_t pos;                            ///< byte position in stream, -1 if unknown

#if FF_API_CONVERGENCE_DURATION
    /**
     * @deprecated Same as the duration field, but as int64_t. This was required
     * for Matroska subtitles, whose duration values could overflow when the
     * duration field was still an int.
     */
    attribute_deprecated
    int64_t convergence_duration;
#endif
} AVPacket;

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;
        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;
}

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;
}