Undefine a function-like macro in C?

问题

I am trying to do some hacks over the glibc, and I wanted to know whether it's possible to redefine function-like macros ?

For example, <tgmath.h> has the following macro:

#define expm1(Val) __TGMATH_UNARY_REAL_ONLY (Val, expm1)

How to redefine expm1 as :

#define expm1(Val) __TGMATH_UNARY_REAL_IMAG (Val, expm1, cexpm1)

I suppose that I have to cancel the previous definition but I do not know exactly how to do that.

回答1:

Exactly. Just undefine it first.

#ifdef expm1
#undef expm1
#endif

#define expm1(Val) __TGMATH_UNARY_REAL_IMAG (Val, expm1, cexpm1)

来源：https://stackoverflow.com/questions/15958638/undefine-a-function-like-macro-in-c