FFmpeg原始碼簡單分析：avformat_write_header()，

FFmpeg原始碼簡單分析：avformat_write_header()，
打算寫兩篇文章簡單分析FFmpeg的寫檔案用到的3個函數：avformat_write_header()，av_write_frame()以及av_write_trailer()。其中av_write_frame()用於寫視頻資料，avformat_write_header()用於寫視頻檔案頭，而av_write_trailer()用於寫視頻檔案尾。
本文首先分析avformat_write_header()。
PS：
需要注意的是，儘管這3個函數功能是配套的，但是它們的首碼卻不一樣，寫檔案頭Header的函數首碼是“avformat_”，其他兩個函數首碼是“av_”（不太明白其中的原因）。
avformat_write_header()的聲明位於libavformat\avformat.h，如下所示。

/** * 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 0 on success, negative AVERROR on failure. * * @see av_opt_find, av_dict_set, avio_open, av_oformat_next. */int avformat_write_header(AVFormatContext *s, AVDictionary **options);

簡單解釋一下它的參數的含義：

s：用於輸出的AVFormatContext。
options：額外的選項，目前沒有深入研究過，一般為NULL。

函數正常執行後傳回值等於0。

該函數最典型的例子可以參考：
最簡單的基於FFMPEG的視頻編碼器（YUV編碼為H.264）

函數呼叫歷程圖

avformat_write_header()的調用關係如所示。

avformat_write_header()avformat_write_header()的定義位於libavformat\mux.c，如下所示。

int avformat_write_header(AVFormatContext *s, AVDictionary **options){    int ret = 0;    if (ret = init_muxer(s, options))        return ret;    if (s->oformat->write_header) {        ret = s->oformat->write_header(s);        if (ret >= 0 && s->pb && s->pb->error < 0)            ret = s->pb->error;        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);    }    if ((ret = init_pts(s)) < 0)        return ret;    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;}

從原始碼可以看出，avformat_write_header()完成了以下工作：

（1）調用init_muxer()初始化複用器
（2）調用AVOutputFormat的write_header()

下面看一下這兩個函數。

init_muxer()init_muxer()用於初始化複用器，它的定義如下所示。

static int init_muxer(AVFormatContext *s, AVDictionary **options){    int ret = 0, i;    AVStream *st;    AVDictionary *tmp = NULL;    AVCodecContext *codec = NULL;    AVOutputFormat *of = s->oformat;    AVDictionaryEntry *e;    if (options)        av_dict_copy(&tmp, *options, 0);    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_LAVF_BITEXACT    if (s->nb_streams && s->streams[0]->codec->flags & CODEC_FLAG_BITEXACT)        s->flags |= AVFMT_FLAG_BITEXACT;#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;    }    for (i = 0; i < s->nb_streams; i++) {        st    = s->streams[i];        codec = st->codec;#if FF_API_LAVF_CODEC_TBFF_DISABLE_DEPRECATION_WARNINGS        if (!st->time_base.num && 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, codec->time_base.num, codec->time_base.den);        }FF_ENABLE_DEPRECATION_WARNINGS#endif        if (!st->time_base.num) {            /* fall back on the default timebase values */            if (codec->codec_type == AVMEDIA_TYPE_AUDIO && codec->sample_rate)                avpriv_set_pts_info(st, 64, 1, codec->sample_rate);            else                avpriv_set_pts_info(st, 33, 1, 90000);        }        switch (codec->codec_type) {        case AVMEDIA_TYPE_AUDIO:            if (codec->sample_rate <= 0) {                av_log(s, AV_LOG_ERROR, "sample rate not set\n");                ret = AVERROR(EINVAL);                goto fail;            }            if (!codec->block_align)                codec->block_align = codec->channels *                                     av_get_bits_per_sample(codec->codec_id) >> 3;            break;        case AVMEDIA_TYPE_VIDEO:            if ((codec->width <= 0 || codec->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, codec->sample_aspect_ratio)                && FFABS(av_q2d(st->sample_aspect_ratio) - av_q2d(codec->sample_aspect_ratio)) > 0.004*av_q2d(st->sample_aspect_ratio)            ) {                if (st->sample_aspect_ratio.num != 0 &&                    st->sample_aspect_ratio.den != 0 &&                    codec->sample_aspect_ratio.num != 0 &&                    codec->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,                           codec->sample_aspect_ratio.num,                           codec->sample_aspect_ratio.den);                    ret = AVERROR(EINVAL);                    goto fail;                }            }            break;        }        if (of->codec_tag) {            if (   codec->codec_tag                && codec->codec_id == AV_CODEC_ID_RAWVIDEO                && (   av_codec_get_tag(of->codec_tag, codec->codec_id) == 0                    || av_codec_get_tag(of->codec_tag, codec->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                codec->codec_tag = 0;            }            if (codec->codec_tag) {                if (!validate_codec_tag(s, st)) {                    char tagbuf[32], tagbuf2[32];                    av_get_codec_tag_string(tagbuf, sizeof(tagbuf), codec->codec_tag);                    av_get_codec_tag_string(tagbuf2, sizeof(tagbuf2), av_codec_get_tag(s->oformat->codec_tag, codec->codec_id));                    av_log(s, AV_LOG_ERROR,                           "Tag %s/0x%08x incompatible with output codec id '%d' (%s)\n",                           tagbuf, codec->codec_tag, codec->codec_id, tagbuf2);                    ret = AVERROR_INVALIDDATA;                    goto fail;                }            } else                codec->codec_tag = av_codec_get_tag(of->codec_tag, codec->codec_id);        }        if (of->flags & AVFMT_GLOBALHEADER &&            !(codec->flags & CODEC_FLAG_GLOBAL_HEADER))            av_log(s, AV_LOG_WARNING,                   "Codec for stream %d does not use global headers "                   "but container format requires global headers\n", i);        if (codec->codec_type != AVMEDIA_TYPE_ATTACHMENT)            s->internal->nb_interleaved_streams++;    }    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;    }    return 0;fail:    av_dict_free(&tmp);    return ret;}

init_muxer()代碼很長，但是它所做的工作比較簡單，可以概括成兩個字：檢查。函數的流程可以概括成以下幾步：

（1）將傳入的AVDictionary形式的選項設定到AVFormatContext
（2）遍曆AVFormatContext中的每個AVStream，並作如下檢查：

a)AVStream的time_base是否正確設定。如果發現AVStream的time_base沒有設定，則會調用avpriv_set_pts_info()進行設定。

b)對於音頻，檢查採樣率設定是否正確；對於視頻，檢查寬、高、寬高比。

c)其他一些檢查，不再詳述。

AVOutputFormat->write_header()avformat_write_header()中最關鍵的地方就是調用了AVOutputFormat的write_header()。write_header()是AVOutputFormat中的一個函數指標，指向寫檔案頭的函數。不同的AVOutputFormat有不同的write_header()的實現方法。在這裡我們舉例子看一下FLV封裝格式對應的AVOutputFormat，它的定義位於libavformat\flvenc.c，如下所示。

AVOutputFormat ff_flv_muxer = {    .name           = "flv",    .long_name      = NULL_IF_CONFIG_SMALL("FLV (Flash Video)"),    .mime_type      = "video/x-flv",    .extensions     = "flv",    .priv_data_size = sizeof(FLVContext),    .audio_codec    = CONFIG_LIBMP3LAME ? AV_CODEC_ID_MP3 : AV_CODEC_ID_ADPCM_SWF,    .video_codec    = AV_CODEC_ID_FLV1,    .write_header   = flv_write_header,    .write_packet   = flv_write_packet,    .write_trailer  = flv_write_trailer,    .codec_tag      = (const AVCodecTag* const []) {                          flv_video_codec_ids, flv_audio_codec_ids, 0                      },    .flags          = AVFMT_GLOBALHEADER | AVFMT_VARIABLE_FPS |                      AVFMT_TS_NONSTRICT,};

從ff_flv_muxer的定義中可以看出，write_header()指向的函數為flv_write_header()。我們繼續看一下flv_write_header()函數。flv_write_header()的定義同樣位於libavformat\flvenc.c，如下所示。

static int flv_write_header(AVFormatContext *s){    int i;    AVIOContext *pb = s->pb;    FLVContext *flv = s->priv_data;    int64_t data_size;    //設定參數    for (i = 0; i < s->nb_streams; i++) {        AVCodecContext *enc = s->streams[i]->codec;        FLVStreamContext *sc;        switch (enc->codec_type) {        case AVMEDIA_TYPE_VIDEO:            if (s->streams[i]->avg_frame_rate.den &&                s->streams[i]->avg_frame_rate.num) {            //設定幀率，從AVStream拷貝過來                flv->framerate = av_q2d(s->streams[i]->avg_frame_rate);            }            if (flv->video_enc) {                av_log(s, AV_LOG_ERROR,                       "at most one video stream is supported in flv\n");                return AVERROR(EINVAL);            }            //視頻編碼的AVCodecContext            flv->video_enc = enc;            if (enc->codec_tag == 0) {                av_log(s, AV_LOG_ERROR, "Video codec '%s' for stream %d is not compatible with FLV\n",                       avcodec_get_name(enc->codec_id), i);                return AVERROR(EINVAL);            }            if (enc->codec_id == AV_CODEC_ID_MPEG4 ||                enc->codec_id == AV_CODEC_ID_H263) {                int error = s->strict_std_compliance > FF_COMPLIANCE_UNOFFICIAL;                av_log(s, error ? AV_LOG_ERROR : AV_LOG_WARNING,                       "Codec %s is not supported in the official FLV specification,\n", avcodec_get_name(enc->codec_id));                if (error) {                    av_log(s, AV_LOG_ERROR,                           "use vstrict=-1 / -strict -1 to use it anyway.\n");                    return AVERROR(EINVAL);                }            } else if (enc->codec_id == AV_CODEC_ID_VP6) {                av_log(s, AV_LOG_WARNING,                       "Muxing VP6 in flv will produce flipped video on playback.\n");            }            break;        case AVMEDIA_TYPE_AUDIO:            if (flv->audio_enc) {                av_log(s, AV_LOG_ERROR,                       "at most one audio stream is supported in flv\n");                return AVERROR(EINVAL);            }            //音頻編碼的AVCodecContext            flv->audio_enc = enc;            if (get_audio_flags(s, enc) < 0)                return AVERROR_INVALIDDATA;            if (enc->codec_id == AV_CODEC_ID_PCM_S16BE)                av_log(s, AV_LOG_WARNING,                       "16-bit big-endian audio in flv is valid but most likely unplayable (hardware dependent); use s16le\n");            break;        case AVMEDIA_TYPE_DATA:            if (enc->codec_id != AV_CODEC_ID_TEXT && enc->codec_id != AV_CODEC_ID_NONE) {                av_log(s, AV_LOG_ERROR, "Data codec '%s' for stream %d is not compatible with FLV\n",                       avcodec_get_name(enc->codec_id), i);                return AVERROR_INVALIDDATA;            }            flv->data_enc = enc;            break;        default:            av_log(s, AV_LOG_ERROR, "Codec type '%s' for stream %d is not compatible with FLV\n",                   av_get_media_type_string(enc->codec_type), i);            return AVERROR(EINVAL);        }        avpriv_set_pts_info(s->streams[i], 32, 1, 1000); /* 32 bit pts in ms */        sc = av_mallocz(sizeof(FLVStreamContext));        if (!sc)            return AVERROR(ENOMEM);        s->streams[i]->priv_data = sc;        sc->last_ts = -1;    }    flv->delay = AV_NOPTS_VALUE;    //開始寫入    //Signature    avio_write(pb, "FLV", 3);    //Version    avio_w8(pb, 1);    //“！！”意思是把非0轉換成1    //Flags    avio_w8(pb, FLV_HEADER_FLAG_HASAUDIO * !!flv->audio_enc +                FLV_HEADER_FLAG_HASVIDEO * !!flv->video_enc);    //Header size    avio_wb32(pb, 9);    //Header結束    //Previous Tag Size    avio_wb32(pb, 0);    for (i = 0; i < s->nb_streams; i++)        if (s->streams[i]->codec->codec_tag == 5) {            avio_w8(pb, 8);     // message type            avio_wb24(pb, 0);   // include flags            avio_wb24(pb, 0);   // time stamp            avio_wb32(pb, 0);   // reserved            avio_wb32(pb, 11);  // size            flv->reserved = 5;        }    write_metadata(s, 0);    for (i = 0; i < s->nb_streams; i++) {        AVCodecContext *enc = s->streams[i]->codec;        if (enc->codec_id == AV_CODEC_ID_AAC || enc->codec_id == AV_CODEC_ID_H264 || enc->codec_id == AV_CODEC_ID_MPEG4) {            int64_t pos;            avio_w8(pb, enc->codec_type == AVMEDIA_TYPE_VIDEO ?                    FLV_TAG_TYPE_VIDEO : FLV_TAG_TYPE_AUDIO);            avio_wb24(pb, 0); // size patched later            avio_wb24(pb, 0); // ts            avio_w8(pb, 0);   // ts ext            avio_wb24(pb, 0); // streamid            pos = avio_tell(pb);            if (enc->codec_id == AV_CODEC_ID_AAC) {                avio_w8(pb, get_audio_flags(s, enc));                avio_w8(pb, 0); // AAC sequence header                avio_write(pb, enc->extradata, enc->extradata_size);            } else {                avio_w8(pb, enc->codec_tag | FLV_FRAME_KEY); // flags                avio_w8(pb, 0); // AVC sequence header                avio_wb24(pb, 0); // composition time                ff_isom_write_avcc(pb, enc->extradata, enc->extradata_size);            }            data_size = avio_tell(pb) - pos;            avio_seek(pb, -data_size - 10, SEEK_CUR);            avio_wb24(pb, data_size);            avio_skip(pb, data_size + 10 - 3);            avio_wb32(pb, data_size + 11); // previous tag size        }    }    return 0;}

從原始碼可以看出，flv_write_header()完成了FLV檔案頭的寫入工作。該函數的工作可以大體分為以下兩部分：

（1）給FLVContext設定參數
（2）寫檔案頭，以及相關的Tag

寫檔案頭的代碼很短，如下所示。

avio_write(pb, "FLV", 3);avio_w8(pb, 1);avio_w8(pb, FLV_HEADER_FLAG_HASAUDIO * !!flv->audio_enc +            FLV_HEADER_FLAG_HASVIDEO * !!flv->video_enc);avio_wb32(pb, 9);

可以參考中FLV檔案頭的定義比對一下上面的代碼。

 

雷霄驊
leixiaohua1020@126.com
http://blog.csdn.net/leixiaohua1020