IEEE754浮点值的便携式序列化
我最近一直致力于一个需要存储和加载大量数据的系统，包括单精度浮点值。 我决定标准化整数的网络字节顺序，并决定以big-endian格式存储浮点值，即：
|-- Byte 0 --| |-- Byte 1 -| Byte 2 Byte 3 # ####### # ####### ######## ######## Sign Exponent Mantissa 1b 8b, MSB first 23b, MSB first 理想情况下，我想提供像htonl()和ntohl()这样的函数，因为我已经使用它们来擦除整数，我也希望以尽可能多的平台独立性的方式实现它（同时假设float类型对应于IEEE754 32位浮点值）。 有没有办法，可能使用ieee754.h ，这样做？
我有一个似乎有效的答案，我将在下面发布，但它似乎非常缓慢和低效，我将不胜感激任何有关如何使其更快和/或更可靠的建议。
更简单，并且取决于与您相同的假设（浮点数和整数类型具有相同的字节顺序，并且几乎普遍有效 – 实际上您将永远不会遇到不是真的系统）：
#include float htonf(float val) { uint32_t rep; memcpy(&rep, &val, sizeof rep); rep = htonl(rep); memcpy(&val, &rep, sizeof rep); return val; }
任何相当不错的编译器都会优化掉两个memcpy调用; 它们存在以击败过度严格的别名优化，因此最终会像htonl一样高效，加上单个函数调用的开销。
正如上面的问题所提到的，我对我的问题有一个解决方案，但我并不特别依赖它，我欢迎其他答案，所以我在这里发布而不是在问题中。 特别是，它似乎很慢，我不确定它是否会打破严格的混叠，以及其他潜在的问题。
#include float htonf (float val) { union ieee754_float u; float v; uint8_t *un = (uint8_t *) &v; uf = val; un[0] = (u.ieee.negative << 7) + ((u.ieee.exponent & 0xfe) >> 1); un[1] = ((u.ieee.exponent & 0x01) << 7) + ((u.ieee.mantissa & 0x7f0000) >> 16); un[2] = (u.ieee.mantissa & 0xff00) >> 8; un[3] = (u.ieee.mantissa & 0xff); return v; } float ntohf (float val) { union ieee754_float u; uint8_t *un = (uint8_t *) &val; u.ieee.negative = (un[0] & 0x80) >> 7; u.ieee.exponent = (un[0] & 0x7f) << 1; u.ieee.exponent += (un[1] & 0x80) >> 7; u.ieee.mantissa = (un[1] & 0x7f) << 16; u.ieee.mantissa += un[2] << 8; u.ieee.mantissa += un[3]; return uf; }
这是一个便携式IEEE 754写例程。 无论主机上的浮点表示如何，它都将以IEEE 754格式写入双精度。
/* * write a double to a stream in ieee754 format regardless of host * encoding. * x - number to write * fp - the stream * bigendian - set to write big bytes first, elee write litle bytes * first * Returns: 0 or EOF on error * Notes: different NaN types and negative zero not preserved. * if the number is too big to represent it will become infinity * if it is too small to represent it will become zero. */ static int fwriteieee754(double x, FILE *fp, int bigendian) { int shift; unsigned long sign, exp, hibits, hilong, lowlong; double fnorm, significand; int expbits = 11; int significandbits = 52; /* zero (can't handle signed zero) */ if (x == 0) { hilong = 0; lowlong = 0; goto writedata; } /* infinity */ if (x > DBL_MAX) { hilong = 1024 + ((1 << (expbits - 1)) - 1); hilong <<= (31 - expbits); lowlong = 0; goto writedata; } /* -infinity */ if (x < -DBL_MAX) { hilong = 1024 + ((1 << (expbits - 1)) - 1); hilong <<= (31 - expbits); hilong |= (1 << 31); lowlong = 0; goto writedata; } /* NaN - dodgy because many compilers optimise out this test, but *there is no portable isnan() */ if (x != x) { hilong = 1024 + ((1 << (expbits - 1)) - 1); hilong <<= (31 - expbits); lowlong = 1234; goto writedata; } /* get the sign */ if (x < 0) { sign = 1; fnorm = -x; } else { sign = 0; fnorm = x; } /* 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--; } /* check for denormalized numbers */ if (shift < -1022) { while (shift < -1022) { fnorm /= 2.0; shift++; } shift = -1023; } /* out of range. Set to infinity */ else if (shift > 1023) { hilong = 1024 + ((1 << (expbits - 1)) - 1); hilong <<= (31 - expbits); hilong |= (sign << 31); lowlong = 0; goto writedata; } else fnorm = fnorm - 1.0; /* take the significant bit off mantissa */ /* calculate the integer form of the significand */ /* hold it in a double for now */ significand = fnorm * ((1LL << significandbits) + 0.5f); /* get the biased exponent */ exp = shift + ((1 << (expbits - 1)) - 1); /* shift + bias */ /* put the data into two longs (for convenience) */ hibits = (long)(significand / 4294967296); hilong = (sign << 31) | (exp << (31 - expbits)) | hibits; x = significand - hibits * 4294967296; lowlong = (unsigned long)(significand - hibits * 4294967296); writedata: /* write the bytes out to the stream */ if (bigendian) { fputc((hilong >> 24) & 0xFF, fp); fputc((hilong >> 16) & 0xFF, fp); fputc((hilong >> 8) & 0xFF, fp); fputc(hilong & 0xFF, fp); fputc((lowlong >> 24) & 0xFF, fp); fputc((lowlong >> 16) & 0xFF, fp); fputc((lowlong >> 8) & 0xFF, fp); fputc(lowlong & 0xFF, fp); } else { fputc(lowlong & 0xFF, fp); fputc((lowlong >> 8) & 0xFF, fp); fputc((lowlong >> 16) & 0xFF, fp); fputc((lowlong >> 24) & 0xFF, fp); fputc(hilong & 0xFF, fp); fputc((hilong >> 8) & 0xFF, fp); fputc((hilong >> 16) & 0xFF, fp); fputc((hilong >> 24) & 0xFF, fp); } return ferror(fp); }