主页 / user-16683941

William's questions

Martin Hope
William
Asked: 2024-09-25 02:55:30 +0800 CST
  • 9

我正在阅读有关网络编程的指南，我非常喜欢：https://beej.us/guide/bgnet/html/split/slightly-advanced-techniques.html#serialization

不过，我对一些事情感到困惑。在关于序列化的这一节中，他谈到了出于字节排序的原因而序列化整数，这对我来说很有意义，但他还包括这两个函数 pack754 和 unpack754，用于以 IEEE-754 格式序列化浮点数。

uint64_t pack754(long double f, unsigned bits, unsigned expbits)
{
    long double fnorm;
    int shift;
    long long sign, exp, significand;
    unsigned significandbits = bits - expbits - 1; // -1 for sign bit

    if (f == 0.0) return 0; // get this special case out of the way

    // check sign and begin normalization
    if (f < 0) { sign = 1; fnorm = -f; }
    else { sign = 0; fnorm = f; }

    // get the normalized form of f and track the exponent
    shift = 0;
    while(fnorm >= 2.0) { fnorm /= 2.0; shift++; }
    while(fnorm < 1.0) { fnorm *= 2.0; shift--; }
    fnorm = fnorm - 1.0;

    // calculate the binary form (non-float) of the significand data
    significand = fnorm * ((1LL<<significandbits) + 0.5f);

    // get the biased exponent
    exp = shift + ((1<<(expbits-1)) - 1); // shift + bias

    // return the final answer
    return (sign<<(bits-1)) | (exp<<(bits-expbits-1)) | significand;
}

long double unpack754(uint64_t i, unsigned bits, unsigned expbits)
{
    long double result;
    long long shift;
    unsigned bias;
    unsigned significandbits = bits - expbits - 1; // -1 for sign bit

    if (i == 0) return 0.0;

    // pull the significand
    result = (i&((1LL<<significandbits)-1)); // mask
    result /= (1LL<<significandbits); // convert back to float
    result += 1.0f; // add the one back on

    // deal with the exponent
    bias = (1<<(expbits-1)) - 1;
    shift = ((i>>significandbits)&((1LL<<expbits)-1)) - bias;
    while(shift > 0) { result *= 2.0; shift--; }
    while(shift < 0) { result /= 2.0; shift++; }

    // sign it
    result *= (i>>(bits-1))&1? -1.0: 1.0;

    return result;
}

我感到困惑的是，这些函数的工作原理是先查看符号的第一位，然后查看指数的下一个 X 位，再查看尾数的下一个 Y 位。那么，这是否意味着浮点数在主机上必须已经采用 IEEE-754 格式才能正常工作？

这只是为了解释格式，还是你在现实生活中实际会做的事情？

c