Portable way to serialize float as 32-bit integer

Question

I have been struggling with finding a portable way to serialize 32-bit float variables in C and C++ to be sent to and from microcontrollers. I want the format to be well-def

Answer 1

You seem to have a bug in serialize_float: the last 4 lines should read:

buffer[(*index)++] = (res >> 24) & 0xFF;
buffer[(*index)++] = (res >> 16) & 0xFF;
buffer[(*index)++] = (res >> 8) & 0xFF;
buffer[(*index)++] = res & 0xFF;

Your method might not work correctly for infinities and/or NaNs because of the offset by 126 instead of 128. Note that you can validate it by extensive testing: there are only 4 billion values, trying all possibilities should not take very long.

The actual representation in memory of float values may differ on different architectures, but IEEE 854 (or more precisely IEC 60559) is largely prevalent today. You can verify if your particular targets are compliant or not by checking if __STDC_IEC_559__ is defined. Note however that even if you can assume IEEE 854, you must handle potentially different endianness between the systems. You cannot assume the endianness of floats to be the same as that of integers for the same platform.

Note also that the simple cast would be incorrect: uint32_t res = *(uint32_t *)&number; violates the strict aliasing rule. You should either use a union or use memcpy(&res, &number, sizeof(res));