It's just that the #if checks for the MSVC compiler, not for Windows as a platform. And on that compiler you will never have LP64, but LLP64: https://en.wikiped

It's just that the #if checks for the MSVC compiler, not for Windows as a platform. And on that compiler you will never have LP64, but LLP64: https://en.wikipedia.org/wiki/64-bit_computing#64-bit_data_models https://en.wikipedia.org/wiki/64-bit_computing#64-bit_data_m... I just roll my own for this because I don't need all the functionality, and generally find such cross-platform libraries to only really test on Linux, and it's too much of a hassle.