FFmpeg源代码简单分析：常见结构体的初始化和销毁（AVFormatContext，AVFrame等）

本文简单分析FFmpeg常见结构体的初始化和销毁函数的源代码。常见的结构体在文章：

《 FFMPEG中最关键的结构体之间的关系 》中已经有过叙述，包括：

AVFormatContext：统领全局的基本结构体。主要用于处理封装格式（FLV/MKV/RMVB等）。

AVIOContext：输入输出对应的结构体，用于输入输出（读写文件，RTMP协议等）。

AVStream，AVCodecContext：视音频流对应的结构体，用于视音频编解码。

AVFrame：存储非压缩的数据（视频对应RGB/YUV像素数据，音频对应PCM采样数据）

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）

他们之间的关系如下图所示（详细信息可以参考上文提到的文章）。

下文简单分析一下上述几个结构体的初始化和销毁函数。这些函数列表如下。

结构体

初始化

销毁

AVFormatContext

avformat_alloc_context()

avformat_free_context()

AVIOContext

avio_alloc_context()

 

AVStream

avformat_new_stream()

 

AVCodecContext

avcodec_alloc_context3()

 

AVFrame

av_frame_alloc();

av_image_fill_arrays()

av_frame_free()

AVPacket

av_init_packet();

av_new_packet()

av_free_packet()

下面进入正文。

AVFormatContext

AVFormatContext的初始化函数是avformat_alloc_context()，销毁函数是avformat_free_context()。

avformat_alloc_context()

avformat_alloc_context()的声明位于libavformat\avformat.h，如下所示。

/** * Allocate an AVFormatContext. * avformat_free_context() can be used to freethe context and everything * allocated by the framework within it. */AVFormatContext*avformat_alloc_context(void);

avformat_alloc_context()的定义位于libavformat\options.c。代码如下所示。

AVFormatContext *avformat_alloc_context(void){  AVFormatContext *ic;  ic = av_malloc(sizeof(AVFormatContext));  if (!ic) return ic;  avformat_get_context_defaults(ic);  ic->internal = av_mallocz(sizeof(*ic->internal));  if (!ic->internal) {    avformat_free_context(ic);    return NULL;  }  return ic;}

从代码中可以看出，avformat_alloc_context()调用av_malloc()为AVFormatContext结构体分配了内存，而且同时也给AVFormatContext中的internal字段分配内存（这个字段是FFmpeg内部使用的，先不分析）。此外调用了一个avformat_get_context_defaults()函数。该函数用于设置AVFormatContext的字段的默认值。它的定义也位于libavformat\options.c，确切的说就位于avformat_alloc_context()上面。我们看一下该函数的定义。

static void avformat_get_context_defaults(AVFormatContext *s){  memset(s, 0, sizeof(AVFormatContext));  s->av_class = &av_format_context_class;  av_opt_set_defaults(s);}

从代码可以看出，avformat_get_context_defaults()首先调用memset()将AVFormatContext的所有字段置0。而后调用了一个函数av_opt_set_defaults()。av_opt_set_defaults()用于给字段设置默认值。

avformat_alloc_context()代码的函数调用关系如下图所示。

avformat_free_context()

avformat_free_context()的声明位于libavformat\avformat.h，如下所示。

/** * Free an AVFormatContext and all its streams. * @param s context to free */void avformat_free_context(AVFormatContext *s);

avformat_free_context()的定义位于libavformat\options.c。代码如下所示。

void avformat_free_context(AVFormatContext *s){  int i;  if (!s)    return;  av_opt_free(s);  if (s->iformat && s->iformat->priv_class && s->priv_data)    av_opt_free(s->priv_data);  if (s->oformat && s->oformat->priv_class && s->priv_data)    av_opt_free(s->priv_data);  for (i = s->nb_streams - 1; i >= 0; i--) {    ff_free_stream(s, s->streams[i]);  }  for (i = s->nb_programs - 1; i >= 0; i--) {    av_dict_free(&s->programs[i]->metadata);    av_freep(&s->programs[i]->stream_index);    av_freep(&s->programs[i]);  }  av_freep(&s->programs);  av_freep(&s->priv_data);  while (s->nb_chapters--) {    av_dict_free(&s->chapters[s->nb_chapters]->metadata);    av_freep(&s->chapters[s->nb_chapters]);  }  av_freep(&s->chapters);  av_dict_free(&s->metadata);  av_freep(&s->streams);  av_freep(&s->internal);  flush_packet_queue(s);  av_free(s);}

从代码中可以看出，avformat_free_context()调用了各式各样的销毁函数：av_opt_free()，av_freep()，av_dict_free()。这些函数分别用于释放不同种类的变量，在这里不再详细讨论。在这里看一个释放AVStream的函数ff_free_stream()。该函数的定义位于libavformat\options.c（其实就在avformat_free_context()上方）。

void ff_free_stream(AVFormatContext *s, AVStream *st) {  int j;  av_assert0(s->nb_streams>0);  av_assert0(s->streams[ s->nb_streams - 1 ] == st);  for (j = 0; j < st->nb_side_data; j++)    av_freep(&st->side_data[j].data);  av_freep(&st->side_data);  st->nb_side_data = 0;  if (st->parser) {    av_parser_close(st->parser);  }  if (st->attached_pic.data)    av_free_packet(&st->attached_pic);  av_dict_free(&st->metadata);  av_freep(&st->probe_data.buf);  av_freep(&st->index_entries);  av_freep(&st->codec->extradata);  av_freep(&st->codec->subtitle_header);  av_freep(&st->codec);  av_freep(&st->priv_data);  if (st->info)    av_freep(&st->info->duration_error);  av_freep(&st->info);  av_freep(&s->streams[ --s->nb_streams ]);}

从代码中可以看出，与释放AVFormatContext类似，释放AVStream的时候，也是调用了av_freep()，av_dict_free()这些函数释放有关的字段。如果使用了parser的话，会调用av_parser_close()关闭该parser。

AVIOContext

avio_alloc_context()

AVIOContext的初始化函数是avio_alloc_context()，销毁的时候使用av_free()释放掉其中的缓存即可。它的声明位于libavformat\avio.h中，如下所示。

/** * Allocate and initialize an AVIOContext for buffered I/O. It must be later * freed with av_free(). * * @param buffer Memory block for input/output operations via AVIOContext. *        The buffer must be allocated with av_malloc() and friends. * @param buffer_size The buffer size is very important for performance. *        For protocols with fixed blocksize it should be set to this blocksize. *        For others a typical size is a cache page, e.g. 4kb. * @param write_flag Set to 1 if the buffer should be writable, 0 otherwise. * @param opaque An opaque pointer to user-specific data. * @param read_packet  A function for refilling the buffer, may be NULL. * @param write_packet A function for writing the buffer contents, may be NULL. *        The function may not change the input buffers content. * @param seek A function for seeking to specified byte position, may be NULL. * * @return Allocated AVIOContext or NULL on failure. */AVIOContext *avio_alloc_context(                  unsigned char *buffer,                  int buffer_size,                  int write_flag,                  void *opaque,                  int (*read_packet)(void *opaque, uint8_t *buf, int buf_size),                  int (*write_packet)(void *opaque, uint8_t *buf, int buf_size),                  int64_t (*seek)(void *opaque, int64_t offset, int whence));

avio_alloc_context()定义位于libavformat\aviobuf.c中，如下所示。

AVIOContext *avio_alloc_context(  unsigned char *buffer,  int buffer_size,  int write_flag,  void *opaque,  int (*read_packet)(void *opaque, uint8_t *buf, int buf_size),  int (*write_packet)(void *opaque, uint8_t *buf, int buf_size),  int64_t (*seek)(void *opaque, int64_t offset, int whence)){    AVIOContext *s = av_mallocz(sizeof(AVIOContext));    if (!s)        return NULL;    ffio_init_context(s, buffer, buffer_size, write_flag, opaque,  read_packet, write_packet, seek);    return s;}

从代码中可以看出，avio_alloc_context()首先调用av_mallocz()为AVIOContext分配内存。而后调用了一个函数ffio_init_context()。该函数完成了真正的初始化工作。我们看一下ffio_init_context()函数的定义。

int ffio_init_context(AVIOContext *s,          unsigned char *buffer,          int buffer_size,          int write_flag,          void *opaque,          int (*read_packet)(void *opaque, uint8_t *buf, int buf_size),          int (*write_packet)(void *opaque, uint8_t *buf, int buf_size),          int64_t (*seek)(void *opaque, int64_t offset, int whence)){  s->buffer  = buffer;  s->orig_buffer_size =  s->buffer_size = buffer_size;  s->buf_ptr = buffer;  s->opaque  = opaque;  s->direct  = 0;  url_resetbuf(s, write_flag ? AVIO_FLAG_WRITE : AVIO_FLAG_READ);  s->write_packet= write_packet;  s->read_packet = read_packet;  s->seek= seek;  s->pos = 0;  s->must_flush  = 0;  s->eof_reached = 0;  s->error   = 0;  s->seekable= seek ? AVIO_SEEKABLE_NORMAL : 0;  s->max_packet_size = 0;  s->update_checksum = NULL;  if (!read_packet && !write_flag) {    s->pos = buffer_size;    s->buf_end = s->buffer + buffer_size;  }  s->read_pause = NULL;  s->read_seek  = NULL;  return 0;}

从函数的代码可以看出，ffio_init_context()对AVIOContext中的缓存，函数指针等等进行了赋值。

AVStream，AVCodecContext

AVStream的初始化函数是avformat_new_stream()，销毁函数使用销毁AVFormatContext的avformat_free_context()就可以了。

avformat_new_stream()

avformat_new_stream()的声明位于libavformat\avformat.h中，如下所示。

/** * Add a new stream to a media file. * * When demuxing, it is called by the demuxer in read_header(). If the * flag AVFMTCTX_NOHEADER is set in s.ctx_flags, then it may also * be called in read_packet(). * * When muxing, should be called by the user before avformat_write_header(). * * User is required to call avcodec_close() and avformat_free_context() to * clean up the allocation by avformat_new_stream(). * * @param s media file handle * @param c If non-NULL, the AVCodecContext corresponding to the new stream * will be initialized to use this codec. This is needed for e.g. codec-specific * defaults to be set, so codec should be provided if it is known. * * @return newly created stream or NULL on error. */AVStream *avformat_new_stream(AVFormatContext *s, const AVCodec *c);

avformat_new_stream()的定义位于libavformat\utils.c中，如下所示。

AVStream *avformat_new_stream(AVFormatContext *s, const AVCodec *c){  AVStream *st;  int i;  AVStream **streams;  if (s->nb_streams >= INT_MAX/sizeof(*streams))    return NULL;  streams = av_realloc_array(s->streams, s->nb_streams + 1, sizeof(*streams));  if (!streams)    return NULL;  s->streams = streams;  st = av_mallocz(sizeof(AVStream));  if (!st)    return NULL;  if (!(st->info = av_mallocz(sizeof(*st->info)))) {    av_free(st);    return NULL;  }  st->info->last_dts = AV_NOPTS_VALUE;  st->codec = avcodec_alloc_context3(c);  if (s->iformat) {    /* no default bitrate if decoding */    st->codec->bit_rate = 0;    /* default pts setting is MPEG-like */    avpriv_set_pts_info(st, 33, 1, 90000);  }  st->index  = s->nb_streams;  st->start_time = AV_NOPTS_VALUE;  st->duration   = AV_NOPTS_VALUE;  /* we set the current DTS to 0 so that formats without any timestamps   * but durations get some timestamps, formats with some unknown   * timestamps have their first few packets buffered and the   * timestamps corrected before they are returned to the user */  st->cur_dts   = s->iformat ? RELATIVE_TS_BASE : 0;  st->first_dts = AV_NOPTS_VALUE;  st->probe_packets = MAX_PROBE_PACKETS;  st->pts_wrap_reference = AV_NOPTS_VALUE;  st->pts_wrap_behavior = AV_PTS_WRAP_IGNORE;  st->last_IP_pts = AV_NOPTS_VALUE;  st->last_dts_for_order_check = AV_NOPTS_VALUE;  for (i = 0; i < MAX_REORDER_DELAY + 1; i++)    st->pts_buffer[i] = AV_NOPTS_VALUE;  st->sample_aspect_ratio = (AVRational) { 0, 1 };#if FF_API_R_FRAME_RATE  st->info->last_dts  = AV_NOPTS_VALUE;#endif  st->info->fps_first_dts = AV_NOPTS_VALUE;  st->info->fps_last_dts  = AV_NOPTS_VALUE;  st->inject_global_side_data = s->internal->inject_global_side_data;  s->streams[s->nb_streams++] = st;  return st;}

从代码中可以看出，avformat_new_stream()首先调用av_mallocz()为AVStream分配内存。接着给新分配的AVStream的各个字段赋上默认值。然后调用了另一个函数avcodec_alloc_context3()初始化AVStream中的AVCodecContext。

avcodec_alloc_context3()

avcodec_alloc_context3()的声明位于libavcodec\avcodec.h中，如下所示。

/** * Allocate an AVCodecContext and set its fields to default values. The * resulting struct should be freed with avcodec_free_context(). * * @param codec if non-NULL, allocate private data and initialize defaults *              for the given codec. It is illegal to then call avcodec_open2() *              with a different codec. *              If NULL, then the codec-specific defaults won't be initialized, *              which may result in suboptimal default settings (this is *              important mainly for encoders, e.g. libx264). * * @return An AVCodecContext filled with default values or NULL on failure. * @see avcodec_get_context_defaults */AVCodecContext *avcodec_alloc_context3(const AVCodec *codec);

下面我们看一下avcodec_alloc_context3()的定义。下面我们看一下avcodec_alloc_context3()的定义。avcodec_alloc_context3()的定义位于libavcodec\options.c中。

AVCodecContext *avcodec_alloc_context3(const AVCodec *codec){  AVCodecContext *avctx= av_malloc(sizeof(AVCodecContext));  if (!avctx)    return NULL;  if(avcodec_get_context_defaults3(avctx, codec) < 0){    av_free(avctx);    return NULL;  }  return avctx;}

从代码中可以看出，avcodec_alloc_context3()首先调用av_malloc()为AVCodecContext分配存储空间，然后调用了一个函数avcodec_get_context_defaults3()用于设置该AVCodecContext的默认值。avcodec_get_context_defaults3()的定义如下。

int avcodec_get_context_defaults3(AVCodecContext *s, const AVCodec *codec){  int flags=0;  memset(s, 0, sizeof(AVCodecContext));  s->av_class = &av_codec_context_class;  s->codec_type = codec ? codec->type : AVMEDIA_TYPE_UNKNOWN;  if (codec)    s->codec_id = codec->id;  if(s->codec_type == AVMEDIA_TYPE_AUDIO)    flags= AV_OPT_FLAG_AUDIO_PARAM;  else if(s->codec_type == AVMEDIA_TYPE_VIDEO)    flags= AV_OPT_FLAG_VIDEO_PARAM;  else if(s->codec_type == AVMEDIA_TYPE_SUBTITLE)    flags= AV_OPT_FLAG_SUBTITLE_PARAM;  av_opt_set_defaults2(s, flags, flags);  s->time_base   = (AVRational){0,1};  s->get_buffer2 = avcodec_default_get_buffer2;  s->get_format  = avcodec_default_get_format;  s->execute = avcodec_default_execute;  s->execute2= avcodec_default_execute2;  s->sample_aspect_ratio = (AVRational){0,1};  s->pix_fmt = AV_PIX_FMT_NONE;  s->sample_fmt  = AV_SAMPLE_FMT_NONE;  s->timecode_frame_start = -1;  s->reordered_opaque= AV_NOPTS_VALUE;  if(codec && codec->priv_data_size){    if(!s->priv_data){      s->priv_data= av_mallocz(codec->priv_data_size);      if (!s->priv_data) {        return AVERROR(ENOMEM);      }    }    if(codec->priv_class){      *(const AVClass**)s->priv_data = codec->priv_class;      av_opt_set_defaults(s->priv_data);    }  }  if (codec && codec->defaults) {    int ret;    const AVCodecDefault *d = codec->defaults;    while (d->key) {      ret = av_opt_set(s, d->key, d->value, 0);      av_assert0(ret >= 0);      d++;    }  }  return 0;}

avformat_new_stream()函数的调用结构如下所示。

AVFrame

AVFrame的初始化函数是av_frame_alloc()，销毁函数是av_frame_free()。在这里有一点需要注意，旧版的FFmpeg都是使用avcodec_alloc_frame()初始化AVFrame的，但是我在写这篇文章的时候，avcodec_alloc_frame()已经被标记为“过时的”了，为了保证与时俱进，决定分析新的API——av_frame_alloc()。

av_frame_alloc()

av_frame_alloc()的声明位于libavutil\frame.h，如下所示。

/** * Allocate an AVFrame and set its fields to default values.  The resulting * struct must be freed using av_frame_free(). * * @return An AVFrame filled with default values or NULL on failure. * * @note this only allocates the AVFrame itself, not the data buffers. Those * must be allocated through other means, e.g. with av_frame_get_buffer() or * manually. */AVFrame *av_frame_alloc(void);

av_frame_alloc()的定义位于libavutil\frame.c。代码如下所示。

AVFrame *av_frame_alloc(void){  AVFrame *frame = av_mallocz(sizeof(*frame));  if (!frame)    return NULL;  frame->extended_data = NULL;  get_frame_defaults(frame);  return frame;}

从代码可以看出，av_frame_alloc()首先调用av_mallocz()为AVFrame结构体分配内存。而后调用了一个函数get_frame_defaults()用于设置一些默认参数。get_frame_defaults()定义如下。

static void get_frame_defaults(AVFrame *frame){  if (frame->extended_data != frame->data)    av_freep(&frame->extended_data);  memset(frame, 0, sizeof(*frame));  frame->pts   =  frame->pkt_dts   =  frame->pkt_pts   = AV_NOPTS_VALUE;  av_frame_set_best_effort_timestamp(frame, AV_NOPTS_VALUE);  av_frame_set_pkt_duration (frame, 0);  av_frame_set_pkt_pos  (frame, -1);  av_frame_set_pkt_size (frame, -1);  frame->key_frame   = 1;  frame->sample_aspect_ratio = (AVRational){ 0, 1 };  frame->format  = -1; /* unknown */  frame->extended_data   = frame->data;  frame->color_primaries = AVCOL_PRI_UNSPECIFIED;  frame->color_trc   = AVCOL_TRC_UNSPECIFIED;  frame->colorspace  = AVCOL_SPC_UNSPECIFIED;  frame->color_range = AVCOL_RANGE_UNSPECIFIED;  frame->chroma_location = AVCHROMA_LOC_UNSPECIFIED;}

从av_frame_alloc()的代码我们可以看出，该函数并没有为AVFrame的像素数据分配空间。因此AVFrame中的像素数据的空间需要自行分配空间，例如使用avpicture_fill()，av_image_fill_arrays()等函数。

av_frame_alloc()函数的调用结构如下所示。

avpicture_fill()

avpicture_fill()的声明位于libavcodec\avcodec.h，如下所示。

/** * Setup the picture fields based on the specified image parameters * and the provided image data buffer. * * The picture fields are filled in by using the image data buffer * pointed to by ptr. * * If ptr is NULL, the function will fill only the picture linesize * array and return the required size for the image buffer. * * To allocate an image buffer and fill the picture data in one call, * use avpicture_alloc(). * * @param picture       the picture to be filled in * @param ptr           buffer where the image data is stored, or NULL * @param pix_fmt       the pixel format of the image * @param width         the width of the image in pixels * @param height        the height of the image in pixels * @return the size in bytes required for src, a negative error code * in case of failure * * @see av_image_fill_arrays() */int avpicture_fill(AVPicture *picture, const uint8_t *ptr,                   enum AVPixelFormat pix_fmt, int width, int height);

avpicture_fill()的定义位于libavcodec\avpicture.c，如下所示。

PS：目测这个函数未来也有可能成为“过时的”函数，因为通过观察这一年FFmpeg代码的变化，发现FFmpeg组织似乎想把AVFrame相关的函数（原先定义在AVCodec的头文件中）从AVCodec的代码中分离出来，形成一套单独的API。所以很多和AVFrame相关的名称为avcodec_XXX()的函数都被标记上了“过时的”标记。当然，上述推测也是我自己猜测的。

int avpicture_fill(AVPicture *picture, const uint8_t *ptr,                   enum AVPixelFormat pix_fmt, int width, int height){    return av_image_fill_arrays(picture->data, picture->linesize,                                ptr, pix_fmt, width, height, 1);}

从代码中可以看出，avpicture_fill()仅仅是简单调用了一下av_image_fill_arrays()。也就是说这两个函数实际上是等同的。

av_image_fill_arrays()

av_image_fill_arrays()的声明位于libavutil\imgutils.h中，如下所示。

/** * Setup the data pointers and linesizes based on the specified image * parameters and the provided array. * * The fields of the given image are filled in by using the src * address which points to the image data buffer. Depending on the * specified pixel format, one or multiple image data pointers and * line sizes will be set.  If a planar format is specified, several * pointers will be set pointing to the different picture planes and * the line sizes of the different planes will be stored in the * lines_sizes array. Call with !src to get the required * size for the src buffer. * * To allocate the buffer and fill in the dst_data and dst_linesize in * one call, use av_image_alloc(). * * @param dst_data      data pointers to be filled in * @param dst_linesizes linesizes for the image in dst_data to be filled in * @param src           buffer which will contain or contains the actual image data, can be NULL * @param pix_fmt       the pixel format of the image * @param width         the width of the image in pixels * @param height        the height of the image in pixels * @param align         the value used in src for linesize alignment * @return the size in bytes required for src, a negative error code * in case of failure */int av_image_fill_arrays(uint8_t *dst_data[4], int dst_linesize[4],                         const uint8_t *src,                         enum AVPixelFormat pix_fmt, int width, int height, int align);

av_image_fill_arrays()的定义位于libavutil\imgutils.c中。

int av_image_fill_arrays(uint8_t *dst_data[4], int dst_linesize[4],             const uint8_t *src,             enum AVPixelFormat pix_fmt, int width, int height, int align){  int ret, i;  if ((ret = av_image_check_size(width, height, 0, NULL)) < 0)    return ret;  if ((ret = av_image_fill_linesizes(dst_linesize, pix_fmt, width)) < 0)    return ret;  for (i = 0; i < 4; i++)    dst_linesize[i] = FFALIGN(dst_linesize[i], align);  if ((ret = av_image_fill_pointers(dst_data, pix_fmt, width, NULL, dst_linesize)) < 0)    return ret;  return av_image_fill_pointers(dst_data, pix_fmt, height, (uint8_t *)src, dst_linesize);}

av_image_fill_arrays()函数中包含3个函数：av_image_check_size()，av_image_fill_linesizes()，av_image_fill_pointers()。av_image_check_size()用于检查输入的宽高参数是否合理，即不能太大或者为负数。av_image_fill_linesizes()用于填充dst_linesize。av_image_fill_pointers()则用于填充dst_data。它们的定义相对比较简单，不再详细分析。

av_image_check_size()代码如下所示。

int av_image_check_size(unsigned int w, unsigned int h, int log_offset, void *log_ctx){  ImgUtils imgutils = { &imgutils_class, log_offset, log_ctx };  if ((int)w>0 && (int)h>0 && (w+128)*(uint64_t)(h+128) < INT_MAX/8)    return 0;  av_log(&imgutils, AV_LOG_ERROR, "Picture size %ux%u is invalid\n", w, h);  return AVERROR(EINVAL);}

av_image_fill_linesizes()代码如下所示。

int av_image_fill_linesizes(int linesizes[4], enum AVPixelFormat pix_fmt, int width){  int i, ret;  const AVPixFmtDescriptor *desc = av_pix_fmt_desc_get(pix_fmt);  int max_step [4];   /* max pixel step for each plane */  int max_step_comp[4];   /* the component for each plane which has the max pixel step */  memset(linesizes, 0, 4*sizeof(linesizes[0]));  if (!desc || desc->flags & AV_PIX_FMT_FLAG_HWACCEL)    return AVERROR(EINVAL);  av_image_fill_max_pixsteps(max_step, max_step_comp, desc);  for (i = 0; i < 4; i++) {    if ((ret = image_get_linesize(width, i, max_step[i], max_step_comp[i], desc)) < 0)      return ret;    linesizes[i] = ret;  }  return 0;}

av_image_fill_pointers()代码如下所示。

int av_image_fill_pointers(uint8_t *data[4], enum AVPixelFormat pix_fmt, int height,               uint8_t *ptr, const int linesizes[4]){  int i, total_size, size[4] = { 0 }, has_plane[4] = { 0 };  const AVPixFmtDescriptor *desc = av_pix_fmt_desc_get(pix_fmt);  memset(data , 0, sizeof(data[0])*4);  if (!desc || desc->flags & AV_PIX_FMT_FLAG_HWACCEL)    return AVERROR(EINVAL);  data[0] = ptr;  if (linesizes[0] > (INT_MAX - 1024) / height)    return AVERROR(EINVAL);  size[0] = linesizes[0] * height;  if (desc->flags & AV_PIX_FMT_FLAG_PAL ||    desc->flags & AV_PIX_FMT_FLAG_PSEUDOPAL) {    size[0] = (size[0] + 3) & ~3;    data[1] = ptr + size[0]; /* palette is stored here as 256 32 bits words */    return size[0] + 256 * 4;  }  for (i = 0; i < 4; i++)    has_plane[desc->comp[i].plane] = 1;  total_size = size[0];  for (i = 1; i < 4 && has_plane[i]; i++) {    int h, s = (i == 1 || i == 2) ? desc->log2_chroma_h : 0;    data[i] = data[i-1] + size[i-1];    h = (height + (1 << s) - 1) >> s;    if (linesizes[i] > INT_MAX / h)      return AVERROR(EINVAL);    size[i] = h * linesizes[i];    if (total_size > INT_MAX - size[i])      return AVERROR(EINVAL);    total_size += size[i];  }  return total_size;}

avpicture_fill()函数调用关系如下图所示。

av_frame_free()

av_frame_free ()的声明位于libavutil\frame.h，如下所示。

/** * Free the frame and any dynamically allocated objects in it, * e.g. extended_data. If the frame is reference counted, it will be * unreferenced first. * * @param frame frame to be freed. The pointer will be set to NULL. */void av_frame_free(AVFrame **frame);

av_frame_free ()的定义位于libavutil\frame.c。代码如下所示。

void av_frame_free(AVFrame **frame){  if (!frame || !*frame)    return;  av_frame_unref(*frame);  av_freep(frame);}

在释放AVFrame结构体之前，首先调用了一个函数av_frame_unref()。av_frame_unref()也是一个FFmpeg的API，它的作用是释放AVFrame中参考的缓存（还没完全弄懂），并且重置AVFrame中的字段。调用这个函数的目的应该是为了确保AVFrame可以被正常释放。代码如下。

void av_frame_unref(AVFrame *frame){  int i;  for (i = 0; i < frame->nb_side_data; i++) {    free_side_data(&frame->side_data[i]);  }  av_freep(&frame->side_data);  for (i = 0; i < FF_ARRAY_ELEMS(frame->buf); i++)    av_buffer_unref(&frame->buf[i]);  for (i = 0; i < frame->nb_extended_buf; i++)    av_buffer_unref(&frame->extended_buf[i]);  av_freep(&frame->extended_buf);  av_dict_free(&frame->metadata);  av_buffer_unref(&frame->qp_table_buf);  get_frame_defaults(frame);}

AVPacket

AVPacket的初始化函数有两个：av_init_packet()，av_new_packet()。销毁函数是av_free_packet()。在初始化函数中av_init_packet()比较简单，初始化一些字段；而av_new_packet()相对“高级”一些，除了包含av_init_packet()的功能之外，还包含了AVPacket内部内存的分配。下面分别看看这些函数。

av_init_packet()

av_init_packet()的声明位于libavcodec\avcodec.h，如下所示。

/** * 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);

av_init_packet()的定义位于libavcodec\avpacket.c。如下所示。

void av_init_packet(AVPacket *pkt){  pkt->pts  = AV_NOPTS_VALUE;  pkt->dts  = AV_NOPTS_VALUE;  pkt->pos  = -1;  pkt->duration = 0;  pkt->convergence_duration = 0;  pkt->flags= 0;  pkt->stream_index = 0;#if FF_API_DESTRUCT_PACKETFF_DISABLE_DEPRECATION_WARNINGS  pkt->destruct = NULL;FF_ENABLE_DEPRECATION_WARNINGS#endif  pkt->buf  = NULL;  pkt->side_data= NULL;  pkt->side_data_elems  = 0;}

av_new_packet()

av_new_packet()的声明位于libavcodec\avcodec.h。如下所示。

/** * 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);

av_new_packet()的定义位于libavcodec\avpacket.c。如下所示。

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;#if FF_API_DESTRUCT_PACKETFF_DISABLE_DEPRECATION_WARNINGS  pkt->destruct = dummy_destruct_packet;FF_ENABLE_DEPRECATION_WARNINGS#endif  return 0;}

从代码可以看出，av_new_packet()调用了av_init_packet(pkt)。此外还调用了一个函数packet_alloc()。packet_alloc()函数的定义如下。

static int packet_alloc(AVBufferRef **buf, int size){  int ret;  if ((unsigned)size >= (unsigned)size + FF_INPUT_BUFFER_PADDING_SIZE)    return AVERROR(EINVAL);  ret = av_buffer_realloc(buf, size + FF_INPUT_BUFFER_PADDING_SIZE);  if (ret < 0)    return ret;  memset((*buf)->data + size, 0, FF_INPUT_BUFFER_PADDING_SIZE);  return 0;}

packet_alloc()中调用av_buffer_realloc()为AVPacket分配内存。然后调用memset()将分配的内存置0。

PS：发现AVPacket的结构随着FFmpeg的发展越发复杂了。原先AVPacket中的数据仅仅存在一个uint8_t类型的数组里，而现在已经使用一个专门的结构体AVBufferRef存储数据。

av_new_packet()代码的函数调用关系如下图所示。

av_free_packet()

av_free_packet()的声明位于libavcodec\avcodec.h，如下所示。

/** * Free a packet. * * @param pkt packet to free */void av_free_packet(AVPacket *pkt);

av_free_packet()的定义位于libavcodec\avpacket.c。如下所示。

void av_free_packet(AVPacket *pkt){    if (pkt) {FF_DISABLE_DEPRECATION_WARNINGS  if (pkt->buf)      av_buffer_unref(&pkt->buf);#if FF_API_DESTRUCT_PACKET  else if (pkt->destruct)      pkt->destruct(pkt);  pkt->destruct = NULL;#endifFF_ENABLE_DEPRECATION_WARNINGS  pkt->data    = NULL;  pkt->size    = 0;  av_packet_free_side_data(pkt);    }}

从代码可以看出，av_free_packet()调用av_buffer_unref()释放AVPacket中的数据，而后还调用了av_packet_free_side_data()释放了side_data（存储封装格式可以提供的额外的数据）。