FFmpeg源代码简单分析：avformat_write_header()

打算写两篇文章简单分析FFmpeg的写文件用到的3个函数：avformat_write_header()，av_write_frame()以及av_write_trailer()。其中av_write_frame()用于写视频数据，avformat_write_header()用于写视频文件头，而av_write_trailer()用于写视频文件尾。
本文首先分析avformat_write_header()。
PS：
需要注意的是，尽管这3个函数功能是配套的，但是它们的前缀却不一样，写文件头Header的函数前缀是“avformat_”，其他两个函数前缀是“av_”（不太明白其中的原因）。
avformat_write_header()的声明位于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);

简单解释一下它的参数的含义：

s：用于输出的AVFormatContext。
options：额外的选项，目前没有深入研究过，一般为NULL。

函数正常执行后返回值等于0。

该函数最典型的例子可以参考：
最简单的基于FFMPEG的视频编码器（YUV编码为H.264）

函数调用关系图

avformat_write_header()的调用关系如下图所示。

avformat_write_header()

avformat_write_header()的定义位于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;
}

从源代码可以看出，avformat_write_header()完成了以下工作：

（1）调用init_muxer()初始化复用器
（2）调用AVOutputFormat的write_header()

下面看一下这两个函数。

init_muxer()

init_muxer()用于初始化复用器，它的定义如下所示。

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_TB
FF_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()代码很长，但是它所做的工作比较简单，可以概括成两个字：检查。函数的流程可以概括成以下几步：

（1）将传入的AVDictionary形式的选项设置到AVFormatContext
（2）遍历AVFormatContext中的每个AVStream，并作如下检查：

a)AVStream的time_base是否正确设置。如果发现AVStream的time_base没有设置，则会调用avpriv_set_pts_info()进行设置。

b)对于音频，检查采样率设置是否正确；对于视频，检查宽、高、宽高比。

c)其他一些检查，不再详述。

AVOutputFormat->write_header()

avformat_write_header()中最关键的地方就是调用了AVOutputFormat的write_header()。write_header()是AVOutputFormat中的一个函数指针，指向写文件头的函数。不同的AVOutputFormat有不同的write_header()的实现方法。在这里我们举例子看一下FLV封装格式对应的AVOutputFormat，它的定义位于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,
};

从ff_flv_muxer的定义中可以看出，write_header()指向的函数为flv_write_header()。我们继续看一下flv_write_header()函数。flv_write_header()的定义同样位于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;
    //设置参数
    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) {
            	//设置帧率，从AVStream拷贝过来
                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);
            }
            //视频编码的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);
            }
            //音频编码的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;
    //开始写入
    //Signature
    avio_write(pb, "FLV", 3);
    //Version
    avio_w8(pb, 1);
    //“！！”意思是把非0转换成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结束
    //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;
}

从源代码可以看出，flv_write_header()完成了FLV文件头的写入工作。该函数的工作可以大体分为以下两部分：

（1）给FLVContext设置参数
（2）写文件头，以及相关的Tag

写文件头的代码很短，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);

可以参考下图中FLV文件头的定义比对一下上面的代码。

雷霄骅
[email protected]
http://blog.csdn.net/leixiaohua1020