I haven't looked into what your macro definition would do to it, but just running grep -r '\b__f\b' /usr/include/ on my system shows a bunch of uses of th

I haven't looked into what your macro definition would do to it, but just running grep -r '\b__f\b' /usr/include/ on my system shows a bunch of uses of this symbol. Looks like it's generally used as a parameter, though I also see it as a field name in a union, e.g. in /usr/include/math.h: __header_always_inline int __inline_signbitf(float __x) { union { float __f; unsigned int __u; } __u; __u.__f = __x; return (int)(__u.__u >> 31); } I wonder how the compiler likes it if you try to #include that header after you've defined a __f() macro?