Defining something to itself in C preprocessor

Question

I ran into these lines:

#define bool  bool
#define false false
#define true  true

I don\'t think I need to say more than \"wtf?\", but just

Answer 1

It allows the user code to conditionally compile based on whether those macros are or aren't defined:

#if defined(bool)
    /*...*/
#else
    /*...*/
#endif

It basically saves you from having to pollute the global namespace with yet another name (like HAVE_BOOL), provided that the implementation lets its users know that iff it provides a bool, it will also provide a macro with the same name that expands to it (or the implementation may simply use this internally for its own preprocessor conditionals).