c - double 的符号、尾数和指数

Question

我从 How to get the sign, mantissa and exponent of a floating point number 找到了以下代码

#include <stdio.h>
typedef union {
  float f;
  struct {
    unsigned int mantisa : 23;
    unsigned int exponent : 8;
    unsigned int sign : 1;
  } parts;
} double_cast;

int main() {
  double_cast d1;
  d1.f = 0.15625;
  printf("sign = %x\n",d1.parts.sign);
  printf("exponent = %x\n",d1.parts.exponent);
  printf("mantisa = %x\n",d1.parts.mantisa);
  return 0;
}

但是如何将其转换为尾数值为 52、指数为 11、符号为 1 的 double 值？

Answer 1

只需扩展逻辑并像这样定义您的 union ，使用标准 int 64 来实现可移植性:

#include <stdint.h>

typedef union {
  double f;
  struct {
    uint64_t mantisa : 52;
    uint64_t exponent : 11;
    uint64_t sign : 1;
  } parts;
} double_cast;

https://en.wikipedia.org/wiki/Double-precision_floating-point_format

顺便说一句:浮点映射也应该使用标准类型，而不是 int:

typedef union {
  float f;
  struct {
    uint32_t mantisa : 23;
    uint32_t exponent : 8;
    uint32_t sign : 1;
  } parts;
} float_cast;