static int ogg_build_flac_headers(const uint8_t *extradata, int extradata_size,
                                  OGGStreamContext *oggstream, int bitexact)
{
    const char *vendor = bitexact ? "ffmpeg" : LIBAVFORMAT_IDENT;
    uint8_t *p;
    if (extradata_size != 34)
        return -1;
    oggstream->header_len[0] = 51;
    oggstream->header[0] = av_mallocz(51); // per ogg flac specs
    p = oggstream->header[0];
    bytestream_put_byte(&p, 0x7F);
    bytestream_put_buffer(&p, "FLAC", 4);
    bytestream_put_byte(&p, 1); // major version
    bytestream_put_byte(&p, 0); // minor version
    bytestream_put_be16(&p, 1); // headers packets without this one
    bytestream_put_buffer(&p, "fLaC", 4);
    bytestream_put_byte(&p, 0x00); // streaminfo
    bytestream_put_be24(&p, 34);
    bytestream_put_buffer(&p, extradata, 34);
    oggstream->header_len[1] = 1+3+4+strlen(vendor)+4;
    oggstream->header[1] = av_mallocz(oggstream->header_len[1]);
    p = oggstream->header[1];
    bytestream_put_byte(&p, 0x84); // last metadata block and vorbis comment
    bytestream_put_be24(&p, oggstream->header_len[1] - 4);
    bytestream_put_le32(&p, strlen(vendor));
    bytestream_put_buffer(&p, vendor, strlen(vendor));
    bytestream_put_le32(&p, 0); // user comment list length
    return 0;
}