从头用脚分析FFmpeg源码 - avformat_write_header

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_WARNINGS
    if (!s->url && !(s->url = av_strdup(s->filename))) {
FF_ENABLE_DEPRECATION_WARNINGS
#else
    if (!s->url && !(s->url = av_strdup(""))) {
#endif
        ret = AVERROR(ENOMEM);
        goto fail;
    }

#if FF_API_LAVF_AVCTX
FF_DISABLE_DEPRECATION_WARNINGS
    if (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 checks
    if (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_WARNINGS
        if (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);
            else
                avpriv_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 here
                par->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;
                }
            } else
                par->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MOVTrack
    if (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;
}