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;