FFmpeg源码：avformat_new_stream函数分析

一、avformat_new_stream函数的声明

avformat_new_stream函数定义在FFmpeg源码（本文演示用的FFmpeg源码版本为7.0.1）的头文件libavformat/avformat.h中：

/*** 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 avformat_free_context() to clean up the allocation* by avformat_new_stream().** @param s media file handle* @param c unused, does nothing** @return newly created stream or NULL on error.*/
AVStream *avformat_new_stream(AVFormatContext *s, const struct AVCodec *c);

该函数作用是：给一个媒体文件增加新流。即给形参s的streams数组增加新的元素，新元素会被初始化。

形参s：输出型参数，指向一个AVFormatContext对象。AVFormatContext结构体中有一个成员变量streams，为指针的指针。streams指向一个AVStream指针数组，该数组的每个元素都指向一个AVStream结构，每一个AVStream结构都存贮媒体文件/媒体流中一路流的信息，流的类型可以为视频流、音频流、字幕流等。

AVFormatContext结构体中的nb_streams成员存放该媒体文件/媒体流中流的个数，也是AVStream指针数组（形参s的streams数组）的元素个数。

执行avformat_new_stream函数后，形参s的streams数组会增加新的元素，也就是会给媒体文件增加新流：

typedef struct AVFormatContext {
//.../*** Number of elements in AVFormatContext.streams.** Set by avformat_new_stream(), must not be modified by any other code.*/unsigned int nb_streams;/*** A list of all streams in the file. New streams are created with* avformat_new_stream().** - demuxing: streams are created by libavformat in avformat_open_input().*             If AVFMTCTX_NOHEADER is set in ctx_flags, then new streams may also*             appear in av_read_frame().* - muxing: streams are created by the user before avformat_write_header().** Freed by libavformat in avformat_free_context().*/AVStream **streams;
//...
} AVFormatContext;

形参c：该参数没有被用到，可忽略。

返回值：成功返回新流对应的AVStream结构，失败返回NULL。

二、avformat_new_stream函数的定义

avformat_new_stream函数定义在源文件libavformat/options.c中：

AVStream *avformat_new_stream(AVFormatContext *s, const AVCodec *c)
{FFFormatContext *const si = ffformatcontext(s);FFStream *sti;AVStream *st;AVStream **streams;if (s->nb_streams >= s->max_streams) {av_log(s, AV_LOG_ERROR, "Number of streams exceeds max_streams parameter"" (%d), see the documentation if you wish to increase it\n",s->max_streams);return NULL;}streams = av_realloc_array(s->streams, s->nb_streams + 1, sizeof(*streams));if (!streams)return NULL;s->streams = streams;sti = av_mallocz(sizeof(*sti));if (!sti)return NULL;st = &sti->pub;st->av_class = &stream_class;st->codecpar = avcodec_parameters_alloc();if (!st->codecpar)goto fail;sti->fmtctx = s;if (s->iformat) {sti->avctx = avcodec_alloc_context3(NULL);if (!sti->avctx)goto fail;sti->info = av_mallocz(sizeof(*sti->info));if (!sti->info)goto fail;#if FF_API_R_FRAME_RATEsti->info->last_dts      = AV_NOPTS_VALUE;
#endifsti->info->fps_first_dts = AV_NOPTS_VALUE;sti->info->fps_last_dts  = AV_NOPTS_VALUE;/* default pts setting is MPEG-like */avpriv_set_pts_info(st, 33, 1, 90000);/* 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 */sti->cur_dts = RELATIVE_TS_BASE;} else {sti->cur_dts = AV_NOPTS_VALUE;}st->index      = s->nb_streams;st->start_time = AV_NOPTS_VALUE;st->duration   = AV_NOPTS_VALUE;sti->first_dts     = AV_NOPTS_VALUE;sti->probe_packets = s->max_probe_packets;sti->pts_wrap_reference = AV_NOPTS_VALUE;sti->pts_wrap_behavior  = AV_PTS_WRAP_IGNORE;sti->last_IP_pts = AV_NOPTS_VALUE;sti->last_dts_for_order_check = AV_NOPTS_VALUE;for (int i = 0; i < MAX_REORDER_DELAY + 1; i++)sti->pts_buffer[i] = AV_NOPTS_VALUE;st->sample_aspect_ratio = (AVRational) { 0, 1 };sti->transferred_mux_tb = (AVRational) { 0, 1 };;#if FF_API_AVSTREAM_SIDE_DATAsti->inject_global_side_data = si->inject_global_side_data;
#endifsti->need_context_update = 1;s->streams[s->nb_streams++] = st;return st;
fail:ff_free_stream(&st);return NULL;
}

三、avformat_new_stream函数的内部实现分析

avformat_new_stream函数内部，首先会判断该媒体文件/媒体流中已经存在的流的个数是否超过“限制的最大数量”，“限制的最大数量”（s->max_streams）默认是1000，该值可以设置。如果超过，打印日志：“Number of streams exceeds max_streams parameter”，avformat_new_stream函数返回NULL：

    if (s->nb_streams >= s->max_streams) {av_log(s, AV_LOG_ERROR, "Number of streams exceeds max_streams parameter"" (%d), see the documentation if you wish to increase it\n",s->max_streams);return NULL;}

通过下面语句，给新流对应的AVStream指针分配内存，给形参s的streams数组增加新的元素（即增加新流对应的AVStream指针）。注意：这一步只会给AVStream的指针分配内存，不会给AVStream本身分配内存。关于av_realloc_array函数的用法可以参考：《FFmpeg源码：av_malloc_array、av_realloc_array函数分析》：

    streams = av_realloc_array(s->streams, s->nb_streams + 1, sizeof(*streams));if (!streams)return NULL;s->streams = streams;

给新流对应的FFStream和AVStream结构本身分配内存：

    sti = av_mallocz(sizeof(*sti));if (!sti)return NULL;st = &sti->pub;
//...s->streams[s->nb_streams++] = st;

给新流对应的AVStream结构的其它成员分配内存和初始化。语句：avpriv_set_pts_info(st, 33, 1, 90000)是给AVStream的time_base设置一个默认值，关于avpriv_set_pts_info函数的用法可以参考：《FFmpeg源码：avpriv_set_pts_info函数分析》：

    st->av_class = &stream_class;st->codecpar = avcodec_parameters_alloc();if (!st->codecpar)goto fail;sti->fmtctx = s;if (s->iformat) {sti->avctx = avcodec_alloc_context3(NULL);if (!sti->avctx)goto fail;sti->info = av_mallocz(sizeof(*sti->info));if (!sti->info)goto fail;#if FF_API_R_FRAME_RATEsti->info->last_dts      = AV_NOPTS_VALUE;
#endifsti->info->fps_first_dts = AV_NOPTS_VALUE;sti->info->fps_last_dts  = AV_NOPTS_VALUE;/* default pts setting is MPEG-like */avpriv_set_pts_info(st, 33, 1, 90000);/* 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 */sti->cur_dts = RELATIVE_TS_BASE;} else {sti->cur_dts = AV_NOPTS_VALUE;}st->index      = s->nb_streams;st->start_time = AV_NOPTS_VALUE;st->duration   = AV_NOPTS_VALUE;sti->first_dts     = AV_NOPTS_VALUE;sti->probe_packets = s->max_probe_packets;sti->pts_wrap_reference = AV_NOPTS_VALUE;sti->pts_wrap_behavior  = AV_PTS_WRAP_IGNORE;sti->last_IP_pts = AV_NOPTS_VALUE;sti->last_dts_for_order_check = AV_NOPTS_VALUE;for (int i = 0; i < MAX_REORDER_DELAY + 1; i++)sti->pts_buffer[i] = AV_NOPTS_VALUE;st->sample_aspect_ratio = (AVRational) { 0, 1 };sti->transferred_mux_tb = (AVRational) { 0, 1 };;#if FF_API_AVSTREAM_SIDE_DATAsti->inject_global_side_data = si->inject_global_side_data;
#endifsti->need_context_update = 1;