目录
- 结构体说明
AV_PKT_FLAG内存管理AVBufferRef- AVPacket常用函数
av_packet_alloc:分配一个packet内存空间，buffer未分配空间av_packet_free:释放packet的内存空间av_new_packet:利用AVBuffRef和size分配packet。buff已经分配空间av_free_packet:释放指定packet的空间，不建议使用packet_alloc：利用buf和size为packet分配空间av_init_packet：初始化packet成员，不会为buffer分配空间av_packet_ref：AVPacket的引用计数+1av_packet_unref:AVPacket的引用计数-1,为零时这释放内存av_packet_move:AVPacket数据转移，引用计数不变av_packet_clone:利用已有的packet克隆一份packet，新的packet引用计数为1,源packet引用计数不变av_packet_make_refcounted:创建可使用引用技术的pktav_packet_make_writable:创建一个可写引用的pkt，避免内存的拷贝
用于存储压缩编码后的相关数据信息的结构体
结构体说明
/**
* 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.
* 此结构存储的是压缩后的数据，通常由解复用器导出，然后传递给解码器，或者作为编码器的输出，然后传递给多路复用器
* 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).
* 对于视频压缩数据，AVPacket通常包含一帧压缩数据；对于音频，AVPacket可能包含多帧压缩数据。
* 当没有压缩数据，仅仅包含端数据时，编码器允许输出空的AVPacket（比如编码结束后的冲刷编码器）。
* 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.
* 真实存储数据的引用，采用引用计数机制，如果数据为NULL时，这引用计数为0
*/
AVBufferRef *buf;
/**
* Presentation timestamp in AVStream->time_base units; the time at which
* the decompressed packet will be presented to the user.
* 向用户展示数据的事件戳（单位不是秒，而是AVStream->time_base）
* Can be AV_NOPTS_VALUE if it is not stored in the file.
* 如果未存储在文件中，pts = AV_NOPTS_VALUE
* 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.
* pts必须不小于dts，因为编码之前用户不可能显示此数据的信息。
* 此类时间戳(单位time_base)必须转换为真正的事件戳(单位秒)才能显示
*/
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.
* 编码时间戳（单位AVStream->time_base）
* 如果未存储dts，这dts = AV_NOPTS_VALUE
*/
int64_t dts;
uint8_t *data; // 压缩编码的数据
int size; // data的长度
int stream_index; // AVStream的标识，表示属于音频还是视频流
/**
* A combination of AV_PKT_FLAG values
*/
int flags; // AV_PKT_FLAG的集合，比如标识关键帧
/**
* 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; // 不建议使用
#endif
} AVPacket;
AV_PKT_FLAG
#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
内存管理AVBufferRef
/**
* A reference to a data buffer.
* 对数据buffer的引用
* 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; // 数据的真实存储位置
/**
* 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; // 数据
/**
* Size of data in bytes.
*/
int size; // 数据长度
} AVBufferRef;
/**
* A reference counted buffer type. It is opaque and is meant to be used through
* references (AVBufferRef).
*/
typedef struct AVBuffer AVBuffer;
struct AVBuffer {
uint8_t *data; /**< data described by this buffer */
int size; /**< size of data in bytes */
/**
* number of existing AVBufferRef instances referring to this buffer
*/
atomic_uint refcount; // 引用计数
/**
* a callback for freeing the data
*/
void (*free)(void *opaque, uint8_t *data);
/**
* an opaque pointer, to be used by the freeing callback
*/
void *opaque;
/**
* A combination of BUFFER_FLAG_*
*/
int flags;
};
/**
* The buffer is always treated as read-only.
*/
#define BUFFER_FLAG_READONLY (1 << 0)
/**
* The buffer was av_realloc()ed, so it is reallocatable.
*/
#define BUFFER_FLAG_REALLOCATABLE (1 << 1)
AVPacket常用函数
av_packet_alloc:分配一个packet内存空间，buffer未分配空间
/**
* 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)
{
AVPacket *pkt = av_mallocz(sizeof(AVPacket));
if (!pkt)
return pkt;
av_packet_unref(pkt);
return pkt;
}
av_packet_free:释放packet的内存空间
/**
* 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 (!pkt || !*pkt)
return;
av_packet_unref(*pkt);
av_freep(pkt);
}
av_new_packet:利用AVBuffRef和size分配packet。buff已经分配空间
/**
* 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)
{
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_free_packet:释放指定packet的空间，不建议使用
/**
* Free a packet.
*
* @deprecated Use av_packet_unref
*
* @param pkt packet to free
*/
attribute_deprecated
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);
}
}
packet_alloc：利用buf和size为packet分配空间
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;
}
av_init_packet：初始化packet成员，不会为buffer分配空间
/**
* 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
*/
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_ref：AVPacket的引用计数+1
/**
* 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
* @param src Source packet
*
* @return 0 on success, a negative AVERROR on error.
*/
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_unref:AVPacket的引用计数-1,为零时这释放内存
/**
* 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)
{
av_packet_free_side_data(pkt);
av_buffer_unref(&pkt->buf);
av_init_packet(pkt);
pkt->data = NULL;
pkt->size = 0;
}
av_packet_move:AVPacket数据转移，引用计数不变
/**
* 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)
{
*dst = *src;
av_init_packet(src);
src->data = NULL;
src->size = 0;
}
av_packet_clone:利用已有的packet克隆一份packet，新的packet引用计数为1,源packet引用计数不变
/**
* 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)
{
AVPacket *ret = av_packet_alloc();
if (!ret)
return ret;
if (av_packet_ref(ret, src))
av_packet_free(&ret);
return ret;
}
av_packet_make_refcounted:创建可使用引用技术的pkt
/**
* 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)
{
int ret;
if (pkt->buf)
return 0;
ret = packet_alloc(&pkt->buf, pkt->size);
if (ret < 0)
return ret;
av_assert1(!pkt->size || pkt->data);
if (pkt->size)
memcpy(pkt->buf->data, pkt->data, pkt->size);
pkt->data = pkt->buf->data;
return 0;
}
av_packet_make_writable:创建一个可写引用的pkt，避免内存的拷贝
/**
* 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)
{
AVBufferRef *buf = NULL;
int ret;
if (pkt->buf && av_buffer_is_writable(pkt->buf))
return 0;
ret = packet_alloc(&buf, pkt->size);
if (ret < 0)
return ret;
av_assert1(!pkt->size || pkt->data);
if (pkt->size)
memcpy(buf->data, pkt->data, pkt->size);
av_buffer_unref(&pkt->buf);
pkt->buf = buf;
pkt->data = buf->data;
return 0;
}