无法从 MSVCRT strtod/sscanf/atof 函数获取 NaN

是否有办法从Windows CRT string到float函数获得NaN s?

为什么:我正在用C编写IEEE float到string转换器，没有信息丢失(strtod, sscanf或atof返回原始float)，前提是舍入模式不改变。

我在MinGW或Visual c++下，所以这些调用去msvc++运行时。问题是我无法让它解析任何特殊值(如"Inf"或"NaN")。Inf是OK的(在解析了不适合float的值后返回，例如"1e999")。

  /* Return the shortest string representation of a float with a successful scanf round-trip.
   * Guaranteed to fit in 13 chars (including the final '').
   */
  char* ftoa(char* res, float f) {
     float r = 0;
     int i, j, len, e = floor(log10(f)) + 1;
     char fmt[8];
     union { float f; int32_t i; } u = { f } ;
     if (f > FLT_MAX) { sprintf(res, "1e999"); return res; }
     if (f < -FLT_MAX) { sprintf(res, "-1e999"); return res; }
     if ((u.i & 0x7F800000) == 0x7F800000) {  // NaN
        sprintf(res, u.i == 0x7FC00000 ? "%sNaN" : "%sNaN%d", u.i<0 ? "-" : "", u.i & 0x7FFFFF);
        return res;
     }  
     // compute the shortest string without exponent ("123000", "0.15")
     if (!f || e>-4 && e<10) {
        for (i=0; i<=10; i++) {
           sprintf(fmt, "%%.%df", i);
           sprintf(res, fmt, f);
           sscanf(res, "%f", &r); if (r==f) break;
        }
     }
     if (r==f) len = strlen(res);
     else len = 1e9;
     if (!f) return res;  // handle 0 and -0
     // compute the shortest string with exponent ("123e3", "15e-2")
     for (i=0; i<9; i++) {
        sprintf(res, "%.0fe%d", f * pow(10,-e), e); sscanf(res, "%f", &r); if (r==f) break;
        j = strlen(res); if (j >= lenF) break;
        while (res[j] != 'e') j--;
        res[j-1]--; sscanf(res, "%f", &r); if (r==f) break;  // try +-1
        res[j-1]+=2; sscanf(res, "%f", &r); if (r==f) break;
        e--;
     }
     if (len <= strlen(res)) sprintf(res, fmt, f);
     return res;
  }