avformat_write_header 作用

先看FFmpeg怎么说。

/*** Allocate the stream private data and write the stream header to* an output media file.** @param s Media file handle, must be allocated with avformat_alloc_context().*          Its oformat field must be set to the desired output format;*          Its pb field must be set to an already opened AVIOContext.* @param options  An AVDictionary filled with AVFormatContext and muxer-private options.*                 On return this parameter will be destroyed and replaced with a dict containing*                 options that were not found. May be NULL.** @return AVSTREAM_INIT_IN_WRITE_HEADER on success if the codec had not already been fully initialized in avformat_init,*         AVSTREAM_INIT_IN_INIT_OUTPUT  on success if the codec had already been fully initialized in avformat_init,*         negative AVERROR on failure.** @see av_opt_find, av_dict_set, avio_open, av_oformat_next, avformat_init_output.*/
av_warn_unused_result
int avformat_write_header(AVFormatContext *s, AVDictionary **options);

注释说申请流的私有数据然后把流信息写到媒体文件里面。
看源码之后感觉申请数据没发现什么，这个函数最重要的就干了两个事情，第一个是调用avformat_init_output，初始化mutex，实际上大部分都是检查格式，然后写入options。第二个就是调用对应格式的write_header，完成写入packet之前的一些格式上的初始化操作。

avformat_write_header源码

下面详细介绍一下代码。
里面的重点就是avformat_init_output函数和write_header函数。

int avformat_write_header(AVFormatContext *s, AVDictionary **options)
{int ret = 0;int already_initialized = s->internal->initialized;int streams_already_initialized = s->internal->streams_initialized;if (!already_initialized)// 这里初始化，一般来说，正常路径调用avformat_write_header，都会调用到这里if ((ret = avformat_init_output(s, options)) < 0)return ret;// 还未打开输出文件流if (!(s->oformat->flags & AVFMT_NOFILE) && s->pb)avio_write_marker(s->pb, AV_NOPTS_VALUE, AVIO_DATA_MARKER_HEADER);if (s->oformat->write_header) {// 调用write_header函数，这个也是这里面最重要的函数。// 比如在写入mp4的时候，会根据stream的extradata初始化一些参数。ret = s->oformat->write_header(s);if (ret >= 0 && s->pb && s->pb->error < 0)ret = s->pb->error;if (ret < 0)goto fail;flush_if_needed(s);}if (!(s->oformat->flags & AVFMT_NOFILE) && s->pb)avio_write_marker(s->pb, AV_NOPTS_VALUE, AVIO_DATA_MARKER_UNKNOWN);if (!s->internal->streams_initialized) {if ((ret = init_pts(s)) < 0)goto fail;}return streams_already_initialized;fail:deinit_muxer(s);return ret;
}

avformat_init_output源码

其实就是init_muxer的一个封装，avformat_init_output在回输出多个文件的格式中会调用多次，比如m3u8格式。

int avformat_init_output(AVFormatContext *s, AVDictionary **options)
{int ret = 0;if ((ret = init_muxer(s, options)) < 0)return ret;s->internal->initialized = 1;s->internal->streams_initialized = ret;if (s->oformat->init && ret) {if ((ret = init_pts(s)) < 0)return ret;return AVSTREAM_INIT_IN_INIT_OUTPUT;}return AVSTREAM_INIT_IN_WRITE_HEADER;
}

init_muxer源码

这个函数就三个作用，第一个作用是把options设置到AVFormatContext中，第二个作用就是遍历所有的AVStream，然后检测AVStream中的参数是否正确。 第三个就是看当前的AVOutputFormat是否实现了init方法，然后调用init

static int init_muxer(AVFormatContext *s, AVDictionary **options)
{int ret = 0, i;AVStream *st;AVDictionary *tmp = NULL;AVCodecParameters *par = NULL;const AVOutputFormat *of = s->oformat;const AVCodecDescriptor *desc;AVDictionaryEntry *e;if (options)av_dict_copy(&tmp, *options, 0);// 设置options到AVFormatContext中if ((ret = av_opt_set_dict(s, &tmp)) < 0)goto fail;if (s->priv_data && s->oformat->priv_class && *(const AVClass**)s->priv_data==s->oformat->priv_class &&(ret = av_opt_set_dict2(s->priv_data, &tmp, AV_OPT_SEARCH_CHILDREN)) < 0)goto fail;#if FF_API_FORMAT_FILENAME
FF_DISABLE_DEPRECATION_WARNINGSif (!s->url && !(s->url = av_strdup(s->filename))) {
FF_ENABLE_DEPRECATION_WARNINGS
#elseif (!s->url && !(s->url = av_strdup(""))) {
#endifret = AVERROR(ENOMEM);goto fail;}#if FF_API_LAVF_AVCTX
FF_DISABLE_DEPRECATION_WARNINGSif (s->nb_streams && s->streams[0]->codec->flags & AV_CODEC_FLAG_BITEXACT) {if (!(s->flags & AVFMT_FLAG_BITEXACT)) {av_log(s, AV_LOG_WARNING,"The AVFormatContext is not in set to bitexact mode, only ""the AVCodecContext. If this is not intended, set ""AVFormatContext.flags |= AVFMT_FLAG_BITEXACT.\n");}}
FF_ENABLE_DEPRECATION_WARNINGS
#endif// some sanity checksif (s->nb_streams == 0 && !(of->flags & AVFMT_NOSTREAMS)) {av_log(s, AV_LOG_ERROR, "No streams to mux were specified\n");ret = AVERROR(EINVAL);goto fail;}// 遍历所有的AVStream，然后检测格式for (i = 0; i < s->nb_streams; i++) {st  = s->streams[i];par = st->codecpar;#if FF_API_LAVF_AVCTX
FF_DISABLE_DEPRECATION_WARNINGSif (st->codecpar->codec_type == AVMEDIA_TYPE_UNKNOWN &&st->codec->codec_type    != AVMEDIA_TYPE_UNKNOWN) {av_log(s, AV_LOG_WARNING, "Using AVStream.codec to pass codec ""parameters to muxers is deprecated, use AVStream.codecpar ""instead.\n");ret = avcodec_parameters_from_context(st->codecpar, st->codec);if (ret < 0)goto fail;}
FF_ENABLE_DEPRECATION_WARNINGS
#endif//如果没有设置时间戳，就用avpriv_set_pts_info设置时间戳if (!st->time_base.num) {/* fall back on the default timebase values */if (par->codec_type == AVMEDIA_TYPE_AUDIO && par->sample_rate)avpriv_set_pts_info(st, 64, 1, par->sample_rate);elseavpriv_set_pts_info(st, 33, 1, 90000);}switch (par->codec_type) {case AVMEDIA_TYPE_AUDIO:if (par->sample_rate <= 0) {av_log(s, AV_LOG_ERROR, "sample rate not set\n");ret = AVERROR(EINVAL);goto fail;}if (!par->block_align)par->block_align = par->channels *av_get_bits_per_sample(par->codec_id) >> 3;break;case AVMEDIA_TYPE_VIDEO:if ((par->width <= 0 || par->height <= 0) &&!(of->flags & AVFMT_NODIMENSIONS)) {av_log(s, AV_LOG_ERROR, "dimensions not set\n");ret = AVERROR(EINVAL);goto fail;}if (av_cmp_q(st->sample_aspect_ratio, par->sample_aspect_ratio)&& fabs(av_q2d(st->sample_aspect_ratio) - av_q2d(par->sample_aspect_ratio)) > 0.004*av_q2d(st->sample_aspect_ratio)) {if (st->sample_aspect_ratio.num != 0 &&st->sample_aspect_ratio.den != 0 &&par->sample_aspect_ratio.num != 0 &&par->sample_aspect_ratio.den != 0) {av_log(s, AV_LOG_ERROR, "Aspect ratio mismatch between muxer ""(%d/%d) and encoder layer (%d/%d)\n",st->sample_aspect_ratio.num, st->sample_aspect_ratio.den,par->sample_aspect_ratio.num,par->sample_aspect_ratio.den);ret = AVERROR(EINVAL);goto fail;}}break;}desc = avcodec_descriptor_get(par->codec_id);if (desc && desc->props & AV_CODEC_PROP_REORDER)st->internal->reorder = 1;st->internal->is_intra_only = ff_is_intra_only(par->codec_id);if (of->codec_tag) {if (   par->codec_tag&& par->codec_id == AV_CODEC_ID_RAWVIDEO&& (   av_codec_get_tag(of->codec_tag, par->codec_id) == 0|| av_codec_get_tag(of->codec_tag, par->codec_id) == MKTAG('r', 'a', 'w', ' '))&& !validate_codec_tag(s, st)) {// the current rawvideo encoding system ends up setting// the wrong codec_tag for avi/mov, we override it herepar->codec_tag = 0;}if (par->codec_tag) {if (!validate_codec_tag(s, st)) {const uint32_t otag = av_codec_get_tag(s->oformat->codec_tag, par->codec_id);av_log(s, AV_LOG_ERROR,"Tag %s incompatible with output codec id '%d' (%s)\n",av_fourcc2str(par->codec_tag), par->codec_id, av_fourcc2str(otag));ret = AVERROR_INVALIDDATA;goto fail;}} elsepar->codec_tag = av_codec_get_tag(of->codec_tag, par->codec_id);}if (par->codec_type != AVMEDIA_TYPE_ATTACHMENT)s->internal->nb_interleaved_streams++;}// 检测是否为AVOuputFormat分配上下文空间if (!s->priv_data && of->priv_data_size > 0) {s->priv_data = av_mallocz(of->priv_data_size);if (!s->priv_data) {ret = AVERROR(ENOMEM);goto fail;}if (of->priv_class) {*(const AVClass **)s->priv_data = of->priv_class;av_opt_set_defaults(s->priv_data);if ((ret = av_opt_set_dict2(s->priv_data, &tmp, AV_OPT_SEARCH_CHILDREN)) < 0)goto fail;}}/* set muxer identification string */if (!(s->flags & AVFMT_FLAG_BITEXACT)) {av_dict_set(&s->metadata, "encoder", LIBAVFORMAT_IDENT, 0);} else {av_dict_set(&s->metadata, "encoder", NULL, 0);}for (e = NULL; e = av_dict_get(s->metadata, "encoder-", e, AV_DICT_IGNORE_SUFFIX); ) {av_dict_set(&s->metadata, e->key, NULL, 0);}if (options) {av_dict_free(options);*options = tmp;}// 这里调用init方法// 在mp4中，根据context中设置的flag初始化一些数据，然后根据AVStream初始化对应的MOVTrackif (s->oformat->init) {if ((ret = s->oformat->init(s)) < 0) {if (s->oformat->deinit)s->oformat->deinit(s);return ret;}return ret == 0;}return 0;fail:av_dict_free(&tmp);return ret;
}