如下图的解码流程，AVPacket中的位置
FFmpeg源码中通过AVPacket存储压缩后的音视频数据。它通常由解复用器（demuxers）输出，然后作为输入传递给解码器。
或者从编码器作为输出接收，然后传递给多路复用器（muxers）。
对于视频，它通常包含一个压缩帧；对于音频，它可能包含几个压缩帧。编码器允许输出不包含压缩音视频数据、只包含side data(边数据：例如，在编码结束时更新一些流参数)的空的数据包（ empty packets）。
AVPacket 结构体 所在的文件名 libavcodec/packet.h
/*
* AVPacket public API
*
* This file is part of FFmpeg.
*
* FFmpeg is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation; either
* version 2.1 of the License, or (at your option) any later version.
*
* FFmpeg is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with FFmpeg; if not, write to the Free Software
* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
*/
#ifndef AVCODEC_PACKET_H
#define AVCODEC_PACKET_H
#include <stddef.h>
#include <stdint.h>
#include "libavutil/attributes.h"
#include "libavutil/buffer.h"
#include "libavutil/dict.h"
#include "libavutil/rational.h"
#include "libavutil/version.h"
#include "libavcodec/version_major.h"
/**
* @defgroup lavc_packet AVPacket
*
* Types and functions for working with AVPacket.
* @{
*/
enum AVPacketSideDataType {
/**
* An AV_PKT_DATA_PALETTE side data packet contains exactly AVPALETTE_SIZE
* bytes worth of palette. This side data signals that a new palette is
* present.
*/
AV_PKT_DATA_PALETTE,
/**
* The AV_PKT_DATA_NEW_EXTRADATA is used to notify the codec or the format
* that the extradata buffer was changed and the receiving side should
* act upon it appropriately. The new extradata is embedded in the side
* data buffer and should be immediately used for processing the current
* frame or packet.
*/
AV_PKT_DATA_NEW_EXTRADATA,
/**
* An AV_PKT_DATA_PARAM_CHANGE side data packet is laid out as follows:
* @code
* u32le param_flags
* if (param_flags & AV_SIDE_DATA_PARAM_CHANGE_CHANNEL_COUNT)
* s32le channel_count
* if (param_flags & AV_SIDE_DATA_PARAM_CHANGE_CHANNEL_LAYOUT)
* u64le channel_layout
* if (param_flags & AV_SIDE_DATA_PARAM_CHANGE_SAMPLE_RATE)
* s32le sample_rate
* if (param_flags & AV_SIDE_DATA_PARAM_CHANGE_DIMENSIONS)
* s32le width
* s32le height
* @endcode
*/
AV_PKT_DATA_PARAM_CHANGE,
/**
* An AV_PKT_DATA_H263_MB_INFO side data packet contains a number of
* structures with info about macroblocks relevant to splitting the
* packet into smaller packets on macroblock edges (e.g. as for RFC 2190).
* That is, it does not necessarily contain info about all macroblocks,
* as long as the distance between macroblocks in the info is smaller
* than the target payload size.
* Each MB info structure is 12 bytes, and is laid out as follows:
* @code
* u32le bit offset from the start of the packet
* u8 current quantizer at the start of the macroblock
* u8 GOB number
* u16le macroblock address within the GOB
* u8 horizontal MV predictor
* u8 vertical MV predictor
* u8 horizontal MV predictor for block number 3
* u8 vertical MV predictor for block number 3
* @endcode
*/
AV_PKT_DATA_H263_MB_INFO,
/**
* This side data should be associated with an audio stream and contains
* ReplayGain information in form of the AVReplayGain struct.
*/
AV_PKT_DATA_REPLAYGAIN,
/**
* This side data contains a 3x3 transformation matrix describing an affine
* transformation that needs to be applied to the decoded video frames for
* correct presentation.
*
* See libavutil/display.h for a detailed description of the data.
*/
AV_PKT_DATA_DISPLAYMATRIX,
/**
* This side data should be associated with a video stream and contains
* Stereoscopic 3D information in form of the AVStereo3D struct.
*/
AV_PKT_DATA_STEREO3D,
/**
* This side data should be associated with an audio stream and corresponds
* to enum AVAudioServiceType.
*/
AV_PKT_DATA_AUDIO_SERVICE_TYPE,
/**
* This side data contains quality related information from the encoder.
* @code
* u32le quality factor of the compressed frame. Allowed range is between 1 (good) and FF_LAMBDA_MAX (bad).
* u8 picture type
* u8 error count
* u16 reserved
* u64le[error count] sum of squared differences between encoder in and output
* @endcode
*/
AV_PKT_DATA_QUALITY_STATS,
/**
* This side data contains an integer value representing the stream index
* of a "fallback" track. A fallback track indicates an alternate
* track to use when the current track can not be decoded for some reason.
* e.g. no decoder available for codec.
*/
AV_PKT_DATA_FALLBACK_TRACK,
/**
* This side data corresponds to the AVCPBProperties struct.
*/
AV_PKT_DATA_CPB_PROPERTIES,
/**
* Recommmends skipping the specified number of samples
* @code
* u32le number of samples to skip from start of this packet
* u32le number of samples to skip from end of this packet
* u8 reason for start skip
* u8 reason for end skip (0=padding silence, 1=convergence)
* @endcode
*/
AV_PKT_DATA_SKIP_SAMPLES,
/**
* An AV_PKT_DATA_JP_DUALMONO side data packet indicates that
* the packet may contain "dual mono" audio specific to Japanese DTV
* and if it is true, recommends only the selected channel to be used.
* @code
* u8 selected channels (0=main/left, 1=sub/right, 2=both)
* @endcode
*/
AV_PKT_DATA_JP_DUALMONO,
/**
* A list of zero terminated key/value strings. There is no end marker for
* the list, so it is required to rely on the side data size to stop.
*/
AV_PKT_DATA_STRINGS_METADATA,
/**
* Subtitle event position
* @code
* u32le x1
* u32le y1
* u32le x2
* u32le y2
* @endcode
*/
AV_PKT_DATA_SUBTITLE_POSITION,
/**
* Data found in BlockAdditional element of matroska container. There is
* no end marker for the data, so it is required to rely on the side data
* size to recognize the end. 8 byte id (as found in BlockAddId) followed
* by data.
*/
AV_PKT_DATA_MATROSKA_BLOCKADDITIONAL,
/**
* The optional first identifier line of a WebVTT cue.
*/
AV_PKT_DATA_WEBVTT_IDENTIFIER,
/**
* The optional settings (rendering instructions) that immediately
* follow the timestamp specifier of a WebVTT cue.
*/
AV_PKT_DATA_WEBVTT_SETTINGS,
/**
* A list of zero terminated key/value strings. There is no end marker for
* the list, so it is required to rely on the side data size to stop. This
* side data includes updated metadata which appeared in the stream.
*/
AV_PKT_DATA_METADATA_UPDATE,
/**
* MPEGTS stream ID as uint8_t, this is required to pass the stream ID
* information from the demuxer to the corresponding muxer.
*/
AV_PKT_DATA_MPEGTS_STREAM_ID,
/**
* Mastering display metadata (based on SMPTE-2086:2014). This metadata
* should be associated with a video stream and contains data in the form
* of the AVMasteringDisplayMetadata struct.
*/
AV_PKT_DATA_MASTERING_DISPLAY_METADATA,
/**
* This side data should be associated with a video stream and corresponds
* to the AVSphericalMapping structure.
*/
AV_PKT_DATA_SPHERICAL,
/**
* Content light level (based on CTA-861.3). This metadata should be
* associated with a video stream and contains data in the form of the
* AVContentLightMetadata struct.
*/
AV_PKT_DATA_CONTENT_LIGHT_LEVEL,
/**
* ATSC A53 Part 4 Closed Captions. This metadata should be associated with
* a video stream. A53 CC bitstream is stored as uint8_t in AVPacketSideData.data.
* The number of bytes of CC data is AVPacketSideData.size.
*/
AV_PKT_DATA_A53_CC,
/**
* This side data is encryption initialization data.
* The format is not part of ABI, use av_encryption_init_info_* methods to
* access.
*/
AV_PKT_DATA_ENCRYPTION_INIT_INFO,
/**
* This side data contains encryption info for how to decrypt the packet.
* The format is not part of ABI, use av_encryption_info_* methods to access.
*/
AV_PKT_DATA_ENCRYPTION_INFO,
/**
* Active Format Description data consisting of a single byte as specified
* in ETSI TS 101 154 using AVActiveFormatDescription enum.
*/
AV_PKT_DATA_AFD,
/**
* Producer Reference Time data corresponding to the AVProducerReferenceTime struct,
* usually exported by some encoders (on demand through the prft flag set in the
* AVCodecContext export_side_data field).
*/
AV_PKT_DATA_PRFT,
/**
* ICC profile data consisting of an opaque octet buffer following the
* format described by ISO 15076-1.
*/
AV_PKT_DATA_ICC_PROFILE,
/**
* DOVI configuration
* ref:
* dolby-vision-bitstreams-within-the-iso-base-media-file-format-v2.1.2, section 2.2
* dolby-vision-bitstreams-in-mpeg-2-transport-stream-multiplex-v1.2, section 3.3
* Tags are stored in struct AVDOVIDecoderConfigurationRecord.
*/
AV_PKT_DATA_DOVI_CONF,
/**
* Timecode which conforms to SMPTE ST 12-1:2014. The data is an array of 4 uint32_t
* where the first uint32_t describes how many (1-3) of the other timecodes are used.
* The timecode format is described in the documentation of av_timecode_get_smpte_from_framenum()
* function in libavutil/timecode.h.
*/
AV_PKT_DATA_S12M_TIMECODE,
/**
* HDR10+ dynamic metadata associated with a video frame. The metadata is in
* the form of the AVDynamicHDRPlus struct and contains
* information for color volume transform - application 4 of
* SMPTE 2094-40:2016 standard.
*/
AV_PKT_DATA_DYNAMIC_HDR10_PLUS,
/**
* The number of side data types.
* This is not part of the public API/ABI in the sense that it may
* change when new side data types are added.
* This must stay the last enum value.
* If its value becomes huge, some code using it
* needs to be updated as it assumes it to be smaller than other limits.
*/
AV_PKT_DATA_NB
};
#define AV_PKT_DATA_QUALITY_FACTOR AV_PKT_DATA_QUALITY_STATS //DEPRECATED
typedef struct AVPacketSideData {
uint8_t *data;
size_t size;
enum AVPacketSideDataType type;
} AVPacketSideData;
/**
* 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).
*
* 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().
*
* sizeof(AVPacket) being a part of the public ABI is deprecated. once
* av_init_packet() is removed, new packets will only be able to be allocated
* with av_packet_alloc(), and new fields may be added to the end of the struct
* with a minor bump.
*
* @see av_packet_alloc
* @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
/**
* for some private data of the user
*/
void *opaque;
/**
* AVBufferRef for free use by the API user. FFmpeg will never check the
* contents of the buffer ref. FFmpeg calls av_buffer_unref() on it when
* the packet is unreferenced. av_packet_copy_props() calls create a new
* reference with av_buffer_ref() for the target packet's opaque_ref field.
*
* This is unrelated to the opaque field, although it serves a similar
* purpose.
*/
AVBufferRef *opaque_ref;
/**
* Time base of the packet's timestamps.
* In the future, this field may be set on packets output by encoders or
* demuxers, but its value will be by default ignored on input to decoders
* or muxers.
*/
AVRational time_base;
} AVPacket;
#if FF_API_INIT_PACKET
attribute_deprecated
typedef struct AVPacketList {
AVPacket pkt;
struct AVPacketList *next;
} AVPacketList;
#endif
#define AV_PKT_FLAG_KEY 0x0001 ///< The packet contains a keyframe
#define AV_PKT_FLAG_CORRUPT 0x0002 ///< The packet content is corrupted
/**
* Flag is used to discard packets which are required to maintain valid
* decoder state but are not required for output and should be dropped
* after decoding.
**/
#define AV_PKT_FLAG_DISCARD 0x0004
/**
* The packet comes from a trusted source.
*
* Otherwise-unsafe constructs such as arbitrary pointers to data
* outside the packet may be followed.
*/
#define AV_PKT_FLAG_TRUSTED 0x0008
/**
* Flag is used to indicate packets that contain frames that can
* be discarded by the decoder. I.e. Non-reference frames.
*/
#define AV_PKT_FLAG_DISPOSABLE 0x0010
enum AVSideDataParamChangeFlags {
#if FF_API_OLD_CHANNEL_LAYOUT
/**
* @deprecated those are not used by any decoder
*/
AV_SIDE_DATA_PARAM_CHANGE_CHANNEL_COUNT = 0x0001,
AV_SIDE_DATA_PARAM_CHANGE_CHANNEL_LAYOUT = 0x0002,
#endif
AV_SIDE_DATA_PARAM_CHANGE_SAMPLE_RATE = 0x0004,
AV_SIDE_DATA_PARAM_CHANGE_DIMENSIONS = 0x0008,
};
/**
* Allocate an AVPacket and set its fields to default values. The resulting
* struct must be freed using av_packet_free().
*
* @return An AVPacket filled with default values or NULL on failure.
*
* @note this only allocates the AVPacket itself, not the data buffers. Those
* must be allocated through other means such as av_new_packet.
*
* @see av_new_packet
*/
AVPacket *av_packet_alloc(void);
/**
* Create a new packet that references the same data as src.
*
* This is a shortcut for av_packet_alloc()+av_packet_ref().
*
* @return newly created AVPacket on success, NULL on error.
*
* @see av_packet_alloc
* @see av_packet_ref
*/
AVPacket *av_packet_clone(const AVPacket *src);
/**
* Free the packet, if the packet is reference counted, it will be
* unreferenced first.
*
* @param pkt packet to be freed. The pointer will be set to NULL.
* @note passing NULL is a no-op.
*/
void av_packet_free(AVPacket **pkt);
#if FF_API_INIT_PACKET
/**
* Initialize optional fields of a packet with default values.
*
* Note, this does not touch the data and size members, which have to be
* initialized separately.
*
* @param pkt packet
*
* @see av_packet_alloc
* @see av_packet_unref
*
* @deprecated This function is deprecated. Once it's removed,
sizeof(AVPacket) will not be a part of the ABI anymore.
*/
attribute_deprecated
void av_init_packet(AVPacket *pkt);
#endif
/**
* Allocate the payload of a packet and initialize its fields with
* default values.
*
* @param pkt packet
* @param size wanted payload size
* @return 0 if OK, AVERROR_xxx otherwise
*/
int av_new_packet(AVPacket *pkt, int size);
/**
* Reduce packet size, correctly zeroing padding
*
* @param pkt packet
* @param size new size
*/
void av_shrink_packet(AVPacket *pkt, int size);
/**
* Increase packet size, correctly zeroing padding
*
* @param pkt packet
* @param grow_by number of bytes by which to increase the size of the packet
*/
int av_grow_packet(AVPacket *pkt, int grow_by);
/**
* Initialize a reference-counted packet from av_malloc()ed data.
*
* @param pkt packet to be initialized. This function will set the data, size,
* and buf fields, all others are left untouched.
* @param data Data allocated by av_malloc() to be used as packet data. If this
* function returns successfully, the data is owned by the underlying AVBuffer.
* The caller may not access the data through other means.
* @param size size of data in bytes, without the padding. I.e. the full buffer
* size is assumed to be size + AV_INPUT_BUFFER_PADDING_SIZE.
*
* @return 0 on success, a negative AVERROR on error
*/
int av_packet_from_data(AVPacket *pkt, uint8_t *data, int size);
/**
* Allocate new information of a packet.
*
* @param pkt packet
* @param type side information type
* @param size side information size
* @return pointer to fresh allocated data or NULL otherwise
*/
uint8_t* av_packet_new_side_data(AVPacket *pkt, enum AVPacketSideDataType type,
size_t size);
/**
* Wrap an existing array as a packet side data.
*
* @param pkt packet
* @param type side information type
* @param data the side data array. It must be allocated with the av_malloc()
* family of functions. The ownership of the data is transferred to
* pkt.
* @param size side information size
* @return a non-negative number on success, a negative AVERROR code on
* failure. On failure, the packet is unchanged and the data remains
* owned by the caller.
*/
int av_packet_add_side_data(AVPacket *pkt, enum AVPacketSideDataType type,
uint8_t *data, size_t size);
/**
* Shrink the already allocated side data buffer
*
* @param pkt packet
* @param type side information type
* @param size new side information size
* @return 0 on success, < 0 on failure
*/
int av_packet_shrink_side_data(AVPacket *pkt, enum AVPacketSideDataType type,
size_t size);
/**
* Get side information from packet.
*
* @param pkt packet
* @param type desired side information type
* @param size If supplied, *size will be set to the size of the side data
* or to zero if the desired side data is not present.
* @return pointer to data if present or NULL otherwise
*/
uint8_t* av_packet_get_side_data(const AVPacket *pkt, enum AVPacketSideDataType type,
size_t *size);
const char *av_packet_side_data_name(enum AVPacketSideDataType type);
/**
* Pack a dictionary for use in side_data.
*
* @param dict The dictionary to pack.
* @param size pointer to store the size of the returned data
* @return pointer to data if successful, NULL otherwise
*/
uint8_t *av_packet_pack_dictionary(AVDictionary *dict, size_t *size);
/**
* Unpack a dictionary from side_data.
*
* @param data data from side_data
* @param size size of the data
* @param dict the metadata storage dictionary
* @return 0 on success, < 0 on failure
*/
int av_packet_unpack_dictionary(const uint8_t *data, size_t size,
AVDictionary **dict);
/**
* Convenience function to free all the side data stored.
* All the other fields stay untouched.
*
* @param pkt packet
*/
void av_packet_free_side_data(AVPacket *pkt);
/**
* Setup a new reference to the data described by a given packet
*
* If src is reference-counted, setup dst as a new reference to the
* buffer in src. Otherwise allocate a new buffer in dst and copy the
* data from src into it.
*
* All the other fields are copied from src.
*
* @see av_packet_unref
*
* @param dst Destination packet. Will be completely overwritten.
* @param src Source packet
*
* @return 0 on success, a negative AVERROR on error. On error, dst
* will be blank (as if returned by av_packet_alloc()).
*/
int av_packet_ref(AVPacket *dst, const AVPacket *src);
/**
* Wipe the packet.
*
* Unreference the buffer referenced by the packet and reset the
* remaining packet fields to their default values.
*
* @param pkt The packet to be unreferenced.
*/
void av_packet_unref(AVPacket *pkt);
/**
* Move every field in src to dst and reset src.
*
* @see av_packet_unref
*
* @param src Source packet, will be reset
* @param dst Destination packet
*/
void av_packet_move_ref(AVPacket *dst, AVPacket *src);
/**
* Copy only "properties" fields from src to dst.
*
* Properties for the purpose of this function are all the fields
* beside those related to the packet data (buf, data, size)
*
* @param dst Destination packet
* @param src Source packet
*
* @return 0 on success AVERROR on failure.
*/
int av_packet_copy_props(AVPacket *dst, const AVPacket *src);
/**
* Ensure the data described by a given packet is reference counted.
*
* @note This function does not ensure that the reference will be writable.
* Use av_packet_make_writable instead for that purpose.
*
* @see av_packet_ref
* @see av_packet_make_writable
*
* @param pkt packet whose data should be made reference counted.
*
* @return 0 on success, a negative AVERROR on error. On failure, the
* packet is unchanged.
*/
int av_packet_make_refcounted(AVPacket *pkt);
/**
* Create a writable reference for the data described by a given packet,
* avoiding data copy if possible.
*
* @param pkt Packet whose data should be made writable.
*
* @return 0 on success, a negative AVERROR on failure. On failure, the
* packet is unchanged.
*/
int av_packet_make_writable(AVPacket *pkt);
/**
* Convert valid timing fields (timestamps / durations) in a packet from one
* timebase to another. Timestamps with unknown values (AV_NOPTS_VALUE) will be
* ignored.
*
* @param pkt packet on which the conversion will be performed
* @param tb_src source timebase, in which the timing fields in pkt are
* expressed
* @param tb_dst destination timebase, to which the timing fields will be
* converted
*/
void av_packet_rescale_ts(AVPacket *pkt, AVRational tb_src, AVRational tb_dst);
/**
* @}
*/
#endif // AVCODEC_PACKET_H
AVPacket结构体
AVPacket结构体声明在FFmpeg源码（本文演示用的FFmpeg源码版本为7.0.1）的头文件libavcodec/packet.h中：
/**
* @}
*/
/**
* @defgroup lavc_packet AVPacket
*
* Types and functions for working with AVPacket.
* @{
*/
/**
* 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).
*
* 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().
*
* sizeof(AVPacket) being a part of the public ABI is deprecated. once
* av_init_packet() is removed, new packets will only be able to be allocated
* with av_packet_alloc(), and new fields may be added to the end of the struct
* with a minor bump.
*
* @see av_packet_alloc
* @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
/**
* for some private data of the user
*/
void *opaque;
/**
* AVBufferRef for free use by the API user. FFmpeg will never check the
* contents of the buffer ref. FFmpeg calls av_buffer_unref() on it when
* the packet is unreferenced. av_packet_copy_props() calls create a new
* reference with av_buffer_ref() for the target packet's opaque_ref field.
*
* This is unrelated to the opaque field, although it serves a similar
* purpose.
*/
AVBufferRef *opaque_ref;
/**
* Time base of the packet's timestamps.
* In the future, this field may be set on packets output by encoders or
* demuxers, but its value will be by default ignored on input to decoders
* or muxers.
*/
AVRational time_base;
} AVPacket;
成员变量buf：
AVBufferRef类型指针，指向"存储packet（一个数据包的压缩后的音视频）数据的引用计数缓冲区"。如果为NULL，表示该packet数据未被引用计数。AVPacket通过AVBufferRef来管理引用计数缓冲区。通过AVBufferRef结构体里面的AVBuffer里面的成员变量refcount记录资源使用的次数，控制资源的释放。
关于AVBufferRef类型可以参考：FFmpeg引用计数数据缓冲区相关的结构体：AVBuffer、AVBufferRef。-CSDN博客
成员变量pts：
显示时间戳（presentation time stamp）。即帧显示的时间刻度，用来告诉播放器在哪个时间点显示此帧（可以简单理解为这帧视频或音频数据的播放时间）。其单位不是秒，而是以AVStream->time_base为单位。pts必须大于或等于dts，因为一帧视频或音频数据必须在解码后才能播放，所以一帧视频或音频数据的显示/播放时间必须大于或等于解码时间。
成员变量dts：
解码时间戳（Decompression timestamp）。即帧解码的时间刻度，用来告诉播放器在哪个时间点解码此帧。其单位不是秒，而是以AVStream->time_base为单位。
成员变量data：
指针，指向"存放压缩后的一帧（对于视频通常包含一个压缩帧，对于音频可能包含几个压缩帧）音视频数据的缓冲区"。
成员变量size：
成员变量data指向的缓冲区的大小，单位为字节。
成员变量stream_index：
索引，用来标识该AVPacket所属的视频/音频流的序号，表示这是第几路流。注意：它是从0而不是从1开始的。
成员变量flags：
AV_PKT_FLAG的组合。值有如下选择：
#define AV_PKT_FLAG_KEY 0x0001 ///< The packet contains a keyframe
#define AV_PKT_FLAG_CORRUPT 0x0002 ///< The packet content is corrupted
/**
* Flag is used to discard packets which are required to maintain valid
* decoder state but are not required for output and should be dropped
* after decoding.
**/
#define AV_PKT_FLAG_DISCARD 0x0004
/**
* The packet comes from a trusted source.
*
* Otherwise-unsafe constructs such as arbitrary pointers to data
* outside the packet may be followed.
*/
#define AV_PKT_FLAG_TRUSTED 0x0008
/**
* Flag is used to indicate packets that contain frames that can
* be discarded by the decoder. I.e. Non-reference frames.
*/
#define AV_PKT_FLAG_DISPOSABLE 0x0010
AV_PKT_FLAG_KEY：数据包包含一个关键帧，即当前AVPacket是关键帧。
AV_PKT_FLAG_CORRUPT：数据包内容已损坏。
AV_PKT_FLAG_DISCARD：用于丢弃需要保持有效解码器状态但不需要输出的数据包，并且在解码后应该丢弃。带有该标志的AVPacket所携带的数据为解码器相关的信息，不会被解码出一幅图像。
AV_PKT_FLAG_TRUSTED：该数据包来自一个可信的源。
AV_PKT_FLAG_DISPOSABLE：用于指示包含可以被解码器丢弃的帧的数据包。也就是非参考帧。
成员变量side_data：
存放"容器可以提供的额外数据包数据"的数组的首地址。数据包可以包含多种类型的边信息，比如，在编码结束的时候更新一些流的参数。指针side_data指向的空间存储了用于解码、展现或处理编码流的辅助信息，它通常由解复用器和编码器导出，可以以分组为单位提供给解码器和复用器，也可以作为全局侧数据(应用于整个编码流)。
成员变量side_data_elems：
side_data的数量。
成员变量duration：
该数据包的持续时间，以AVStream->time_base为单位。值为下一个数据包的pts减去当前数据包的pts。如果值为0表示未知。
成员变量pos：
该数据包在流中的位置，单位为字节。值为-1表示未知。
成员变量opaque：
指针，指向"存放用户使用的私有数据的缓冲区"。
成员变量time_base：
AVRational类型，数据包时间戳的时间基准，即pts和dts的时间基。
例如对于视频来说，这个值可能是 1,25 也就是说，1秒钟我们显示
AVPacket 相关函数说明
av_packet_alloc函数
av_packet_alloc函数定义在源文件libavcodec/avpacket.c中：
需要通过av_packet_free() 释放
* @note this only allocates the AVPacket itself, not the data buffers. Those
* must be allocated through other means such as av_new_packet.
需要注意的是： 通过 av_packet_alloc的只能 allocates avpacket自己，里面并没有内容
/**
* Allocate an AVPacket and set its fields to default values. The resulting
* struct must be freed using av_packet_free().
*
* @return An AVPacket filled with default values or NULL on failure.
*
* @note this only allocates the AVPacket itself, not the data buffers. Those
* must be allocated through other means such as av_new_packet.
*
* @see av_new_packet
*/
AVPacket *av_packet_alloc(void);
测试，我们看到在 av_packet_alloc之后，除了avpacket自己有值外，里面的数据都是0或者随机值
void av_packet_free(AVPacket **pkt);
/**
* Free the packet, if the packet is reference counted, it will be
* unreferenced first.
*
* @param pkt packet to be freed. The pointer will be set to NULL.
* @note passing NULL is a no-op.
*/
void av_packet_free(AVPacket **pkt);