If I am not mistaken, is_iec559 concerns numerical representation, while __STDC_IEC_559__ is broader, and includes the behavior of numerical operations like 1.0

If I am not mistaken, is_iec559 concerns numerical representation, while __STDC_IEC_559__ is broader, and includes the behavior of numerical operations like 1.0/-0.0 and various functions. Huh. https://en.cppreference.com/w/c/23.html https://en.cppreference.com/w/c/23.html says the "Old feature-test macro" __STDC_IEC_559__ was deprecated in C23, in favor of __STDC_IEC_60559_BFP__ .