Question

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Inline functions vs Preprocessor macros

I want to know the difference between the inline function and macro function.

1) is inline function is the same of macro function ?

2) I know that both are not called but they are replaced by its code in the compilation phase. is not?

3) If there is difference, Could you specify it?

Answer 1

Inline replaces a call to a function with the body of the function, however, inline is just a request to the compiler that could be ignored (you could still pass some flags to the compiler to force inline or use always_inline attribute with gcc).

A macro on the other hand, is expanded by the preprocessor before compilation, so it’s just like text substitution, also macros are not type checked, inline functions are. There’s a comparison in the wiki.

For the sake of completeness, you could still have some kind of type safety with macros, using gcc’s __typeof__ for example, the following generate almost identical code and both cause warnings if used with the wrong types:

#define max(a,b) \
  ({ __typeof__ (a) _a = (a); \
      __typeof__ (b) _b = (b); \
    _a > _b ? _a : _b; })

__attribute__((always_inline)) int max(int a, int b) {
   return (a > b ? a : b);
}

Note: sometimes typeless macros are just what’s needed, for example, have a look at how uthash uses macros to make any C structure hashable without resorting to casts.