hevc的头信息保存在stsd的hev中，如下图，68 78 63 43对应的hvcC就是hevc的标识，阴影部分就是重要的vps,sps,pps数据。

ffmpeg中读取头hvcC信息，在mov.c中，如下：

{ MKTAG('h','v','c','C'), mov_read_glbl },  //碰到hvcC头，就用mov_read_glbl处理。

/**
 * This function reads atom content and puts data in extradata without tag
 * nor size unlike mov_read_extradata.
 */
static int mov_read_glbl(MOVContext *c, AVIOContext *pb, MOVAtom atom)
{
    AVStream *st;
    int ret;

    if (c->fc->nb_streams < 1)
        return 0;
    st = c->fc->streams[c->fc->nb_streams-1];

    if ((uint64_t)atom.size > (1<<30))
        return AVERROR_INVALIDDATA;

    if (atom.size >= 10) {
        // Broken files created by legacy versions of libavformat will
        // wrap a whole fiel atom inside of a glbl atom.
        unsigned size = avio_rb32(pb);
        unsigned type = avio_rl32(pb);
        avio_seek(pb, -8, SEEK_CUR);
        if (type == MKTAG('f','i','e','l') && size == atom.size)
            return mov_read_default(c, pb, atom);
    }

 // codecpar定义为AVCodecParameters *codecpar;

    if (st->codecpar->extradata_size > 1 && st->codecpar->extradata) {
        av_log(c, AV_LOG_WARNING, "ignoring multiple glbl\n");
        return 0;
    }
    av_freep(&st->codecpar->extradata);
    ret = ff_get_extradata(c->fc, st->codecpar, pb, atom.size);
    if (ret < 0)
        return ret;

    return 0;
}


int ff_get_extradata(AVFormatContext *s, AVCodecParameters *par, AVIOContext *pb, int size)
{
    int ret = ff_alloc_extradata(par, size);    //分配size大小的空间，也就是st->codecpar->extradata_size为size。
    if (ret < 0)
        return ret;
    ret = avio_read(pb, par->extradata, size);   //将数据读到par->extradata，保存起来。
    if (ret != size) {
        av_freep(&par->extradata);
        par->extradata_size = 0;
        av_log(s, AV_LOG_ERROR, "Failed to read extradata of size %d\n", size);
        return ret < 0 ? ret : AVERROR_INVALIDDATA;
    }

    return ret;
}