c++ - 如何将N位写入QByteArray

Question

我的任务是解码RTP数据包并提取该数据包的有效负载(音频数据)。为了绕过所有RTP header 字段，我具有以下功能:

quint32 readNBitsByRange(quint32 position, quint32 count, const QByteArray &array)
{
    quint32 accuml = 0;
    while (count != 0) {
            const quint32 l = (8 - position % 8);
            const quint32 u = (l < count ? l : count);
            const quint32 f = (8 - u);
            accuml  <<= u;
            accuml   |= ((*(array.data() + position / 8) << (8 - l)) & (((1 << u) - 1) << f)) >> f;
            position += u;
            count    -= u;
    }
    return accuml;
}

作为参数，该函数采用将从其进行读取的位置，位数和应从其进行读取的缓冲区。借助此功能，我可以读取所有RTP header 字段。使用此功能的示例:

int main()
{
    // ... We get The RTP packet in binary form and write it to QByteArray ...
    QByteArray array;
    for (quint32 i = 0; i < rtpBinaryDataLength; ++i) {
        array.push_back(rtpBinaryData[i]);
    }
    
    // 0                   1                   2                   3
    // 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    // |V=2|P|X|  CC   |M|     PT      |       sequence number         |
    // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    // |                           timestamp                           |
    // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    // |           synchronization source (SSRC) identifier            |
    // +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
    // |            contributing source (CSRC) identifiers             |
    // |                             ....                              |
    // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    
    quint16 rtpVersion   = readNBitsByRange(0x000, 0x002, array);
    quint16 rtpPadding   = readNBitsByRange(0x002, 0x001, array);
    quint16 rtpExtension = readNBitsByRange(0x003, 0x001, array);
    
    // .. And so on ...
}

一切正常!但这就是问题所在!现在，我有一个任务将RTP数据包值写入QByteArray。我不知道该怎么做!我们只知道，例如，必须将RTP数据包的4个字段(V，P，X，CC)的值写入缓冲区的第一个字节。
我想有一个示例功能，例如阅读功能，以便您可以轻松地使用它。函数示例:

void writeNBits(quint32 position, quint32 count, quint32 val, QByteArray &array)
{
   // ...
}

我尝试将所有值写入 header ，然后将它们写入QByteArray:

struct RtpHeader
{
    unsigned m_v :2; 
    unsigned m_p :1; 
    unsigned m_x :1; 
    unsigned m_cc:4; 
    unsigned m_m :1; 
    unsigned m_pt:7; 
    uint16_t m_sn;   
    uint32_t m_tm;   
    uint32_t m_ssrc;   
};

int main(int argc, char *argv[])
{
    RtpHeader hdr;
    
    hdr.m_v    = 2;
    hdr.m_p    = 0;
    hdr.m_x    = 0;
    hdr.m_cc   = 0;
    hdr.m_m    = 1;
    hdr.m_pt   = 8;
    hdr.m_sn   = 59133;
    hdr.m_tm   = 240;
    hdr.m_ssrc = 0xDEE0EE8F;

    QByteArray array(reinterpret_cast<const char*>(&hdr), sizeof(hdr));
    for (quint8 i = 0; i < 240; i++) {
        array.push_back(0xD5); // Silence 
    }

    QFile file("./rawRtpPacket.bin");
    file.open(QIODevice::WriteOnly);
    file.write(array);
    file.close();
    
    return EXIT_SUCCESS;
}

但这不是我应该得到的!
例如，我应该收到以下12字节 header 的结果:

80 88 E6 FD  00 00 00 F0  DE E0 EE 8F

但是我得到了不同的结果:

02 11 FD E6  F0 00 00 00  8F EE E0 DE 

如果仔细观察，从4个字节到12个字节，我必须镜像这些值，但是以前两个字节为代价，我不明白为什么会得到一个完全不同的字节。
我写:V-2，P-0，X-0，CC-0，应该得到 10000000，但我得到 00000010

Answer 1

经过一些实验，我发现了一种更正确的方法来将RTP数据包写入QByteArray并实现了我以前的错误。因此，我在RTP中编写了正确的QByteArray数据包条目的样本版本:

class RtpHeader
{
public:

    quint16 m_vp:0x02;
    quint16 m_pf:0x01;
    quint16 m_xf:0x01;
    quint16 m_cc:0x04;
    quint16 m_mb:0x01;
    quint16 m_pt:0x07;
    quint16 m_sn;
    quint32 m_tm;
    quint32 m_ss;

};

class RtpHeaderEncoder
{
public:

    RtpHeaderEncoder(void)                                noexcept = delete;
    RtpHeaderEncoder &operator=(const RtpHeaderEncoder &) noexcept = delete;
    RtpHeaderEncoder &operator=(RtpHeaderEncoder &&)      noexcept = delete;
    RtpHeaderEncoder(const RtpHeaderEncoder &)            noexcept = delete;
    RtpHeaderEncoder(RtpHeaderEncoder &&)                 noexcept = delete;
   ~RtpHeaderEncoder(void)                                noexcept = delete;

    static QByteArray encode(const RtpHeader &hdr)  noexcept;

};

QByteArray RtpHeaderEncoder::encode(const RtpHeader &hdr) noexcept
{
    QByteArray array;

    if ((hdr.m_vp == 0x02) && (hdr.m_pf == 0x00) && (hdr.m_cc <= 0x0F) && (hdr.m_pt <= 0x12)) {

        QDataStream stream(&array, QIODevice::WriteOnly);
        stream << (((hdr.m_vp & 0x00003) << 0x01E)|
                   ((hdr.m_pf & 0x00001) << 0x01D)|
                   ((hdr.m_xf & 0x00001) << 0x01C)|
                   ((hdr.m_cc & 0x0000F) << 0x018)|
                   ((hdr.m_mb & 0x00001) << 0x017)|
                   ((hdr.m_pt & 0x0007F) << 0x010)|
                   ((hdr.m_sn & 0x0FFFF) << 0x000));

        stream << hdr.m_tm << hdr.m_ss;
    }
    return array;
}

int main(int argc, char *argv[])
{
    RtpHeader hdr;
    hdr.m_vp = 2;
    hdr.m_pf = 0;
    hdr.m_xf = 0;
    hdr.m_cc = 0;
    hdr.m_mb = 0;
    hdr.m_pt = 8;
    hdr.m_sn = 1;
    hdr.m_tm = 201452158;
    hdr.m_ss = 111537764;

    QFile file("./rawRtpHeader.bin");
    file.open (QIODevice::WriteOnly);
    file.write(RtpHeaderEncoder::encode(hdr));
    file.close();
    
    return EXIT_SUCCESS;
}

此方法既更安全，又可以确保 fixed RTP数据包头的所有字段均正确填写。如果您需要填充 QByteArray和 payload，那么在用包头填充数组之后，我们就编写有效负载本身。
附言如果对代码有任何疑问或意见，我将很乐意回答并接受批评。