# Portability of binary serialization of double/float type in C++

Brian “Beej Jorgensen” Hall gives in his Guide to Network Programming some code to pack float (resp. double) to uint32_t (resp. uint64_t) to be able to safely transmit it over the network between two machine that may not both agree to their representation. It has some limitation, mainly it does not support NaN and infinity.

Here is his packing function:

#define pack754_32(f) (pack754((f), 32, 8))
#define pack754_64(f) (pack754((f), 64, 11))

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