ffmpeg——avformat_write_header写文件头

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一：介绍

ffmpeg 的 Mux 主要分为三步操作：

avformat_write_header ：写文件头

av_write_frame/av_interleaved_write_frame：写packet

av_write_trailer ：写文件尾

本文主要分析 avformat_write_header 。

可额外参考： http://blog.csdn.net/leixiaohua1020/article/details/44116215

二：分析

avformat_write_header：

int avformat_write_header(AVFormatContext *s, AVDictionary **options){    int ret = 0;//初始化Mux，一些参数检查和设置    if ((ret = init_muxer(s, options)) < 0)        return ret;// Write_header    if (!s->oformat->check_bitstream) {        ret = write_header_internal(s);        if (ret < 0)            goto fail;    }//初始化pts    if ((ret = init_pts(s)) < 0)        goto fail;    if (s->avoid_negative_ts < 0) {        av_assert2(s->avoid_negative_ts == AVFMT_AVOID_NEG_TS_AUTO);        if (s->oformat->flags & (AVFMT_TS_NEGATIVE | AVFMT_NOTIMESTAMPS)) {            s->avoid_negative_ts = 0;        } else            s->avoid_negative_ts = AVFMT_AVOID_NEG_TS_MAKE_NON_NEGATIVE;    }    return 0;fail:    if (s->oformat->deinit)        s->oformat->deinit(s);    return ret;}

来具体看看 avformat_write_header 都具体做了什么：

init_muxer （初始化Mux，参数检查与设置）：

static int init_muxer(AVFormatContext *s, AVDictionary **options){    int ret = 0, i;    AVStream *st;    AVDictionary *tmp = NULL;    AVCodecParameters *par = NULL;    AVOutputFormat *of = s->oformat;    const AVCodecDescriptor *desc;    AVDictionaryEntry *e;//通过AVClass接口设置AVFormatContext 的Options参数(一般options为NULL,所以可以忽略)    if (options)        av_dict_copy(&tmp, *options, 0);    if ((ret = av_opt_set_dict(s, &tmp)) < 0)        goto fail;//通过AVClass接口设置AVFormatContext->priv_data的Options参数。//(还记得么? priv_data保存的是AVOutputFormat 的预存AVClass接口。例如MOV封装预存的是个MOVMuxContext )    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;//新版ffmpeg3.1.4 废除了codec(忽略此部分)#if FF_API_LAVF_AVCTXFF_DISABLE_DEPRECATION_WARNINGS    if (s->nb_streams && s->streams[0]->codec->flags & AV_CODEC_FLAG_BITEXACT) {        if (!(s->flags & AVFMT_FLAG_BITEXACT)) {#if FF_API_LAVF_BITEXACT            av_log(s, AV_LOG_WARNING,                   "Setting the AVFormatContext to bitexact mode, because "                   "the AVCodecContext is in that mode. This behavior will "                   "change in the future. To keep the current behavior, set "                   "AVFormatContext.flags |= AVFMT_FLAG_BITEXACT.\n");            s->flags |= AVFMT_FLAG_BITEXACT;#else            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");#endif        }    }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;    }//循环处理每个流通道Stream，完成对codecpar ,time_base 的参数检查    for (i = 0; i < s->nb_streams; i++) {        st  = s->streams[i];        par = st->codecpar;//ffmpeg3.1.4 codec 已废除#if FF_API_LAVF_CODEC_TB && FF_API_LAVF_AVCTXFF_DISABLE_DEPRECATION_WARNINGS        if (!st->time_base.num && st->codec->time_base.num) {            av_log(s, AV_LOG_WARNING, "Using AVStream.codec.time_base as a "                   "timebase hint to the muxer is deprecated. Set "                   "AVStream.time_base instead.\n");            avpriv_set_pts_info(st, 64, st->codec->time_base.num, st->codec->time_base.den);        }FF_ENABLE_DEPRECATION_WARNINGS#endif//ffmpeg3.1.4 codec 已废除#if FF_API_LAVF_AVCTXFF_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        /* update internal context from codecpar, old bsf api needs this         * FIXME: remove when autobsf uses new bsf API */         //内部使用,无需关注        ret = avcodec_parameters_to_context(st->internal->avctx, st->codecpar);        if (ret < 0)            goto fail;//检查AVStream->time_base 是否被设置，如果没有:// 音频设置为: 1/sample_rate    ;    视频设置为:1/90000        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);        }//对音视频参数检查        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;        }//检查输出封装(AVOutputFormat)是否支持设定的流编码格式。(在AVOutputFormat支持的编码列表中寻找匹配)        desc = avcodec_descriptor_get(par->codec_id);        if (desc && desc->props & AV_CODEC_PROP_REORDER)            st->internal->reorder = 1;        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)) {                    char tagbuf[32], tagbuf2[32];                    av_get_codec_tag_string(tagbuf, sizeof(tagbuf), par->codec_tag);                    av_get_codec_tag_string(tagbuf2, sizeof(tagbuf2), av_codec_get_tag(s->oformat->codec_tag, par->codec_id));                    av_log(s, AV_LOG_ERROR,                           "Tag %s/0x%08x incompatible with output codec id '%d' (%s)\n",                           tagbuf, par->codec_tag, par->codec_id, tagbuf2);                    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++;    }//此处实际上在前面已经做过    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;        }    }//metadata 的设置(根据需要设置)    /* 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;    }    if (s->oformat->init && (ret = s->oformat->init(s)) < 0) {        if (s->oformat->deinit)            s->oformat->deinit(s);        goto fail;    }    return 0;fail:    av_dict_free(&tmp);    return ret;}

write_header_internal （写文件头）:

static int write_header_internal(AVFormatContext *s){    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) {        int ret = s->oformat->write_header(s);        if (ret >= 0 && s->pb && s->pb->error < 0)            ret = s->pb->error;        s->internal->write_header_ret = ret;        if (ret < 0)            return ret;        if (s->flush_packets && s->pb && s->pb->error >= 0 && s->flags & AVFMT_FLAG_FLUSH_PACKETS)            avio_flush(s->pb);    }    s->internal->header_written = 1;    if (!(s->oformat->flags & AVFMT_NOFILE) && s->pb)        avio_write_marker(s->pb, AV_NOPTS_VALUE, AVIO_DATA_MARKER_UNKNOWN);    return 0;}

可见 writer_header_internal 直接调用匹配的 AVOutputFormat 的 write_header方法。因此，具体的 write_header 方法由各种封装自己定义并实现。（例如MOV封装：avformat_write_header——>write_header_internal——>AVOutputFormat(MOV)——>mov_write_header）。

priv_pts （设置AVStream->priv_pts）：

<span style="font-size:14px;">static int init_pts(AVFormatContext *s){    int i;    AVStream *st;    /* init PTS generation */    for (i = 0; i < s->nb_streams; i++) {        int64_t den = AV_NOPTS_VALUE;        st = s->streams[i];        switch (st->codecpar->codec_type) {        case AVMEDIA_TYPE_AUDIO:            den = (int64_t)st->time_base.num * st->codecpar->sample_rate;            break;        case AVMEDIA_TYPE_VIDEO:            den = (int64_t)st->time_base.num * st->time_base.den;            break;        default:            break;            }        if (!st->priv_pts)            st->priv_pts = av_mallocz(sizeof(*st->priv_pts));        if (!st->priv_pts)            return AVERROR(ENOMEM);        if (den != AV_NOPTS_VALUE) {            if (den <= 0)                return AVERROR_INVALIDDATA;            frac_init(st->priv_pts, 0, 0, den);        }    }    return 0;}</span>

此处设置的是priv_pts(将后续使用)。

注：关于time的设置很重要，有AVStream->time_base , AVPacket->pts 等，这里也需要留意一下。

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