What happens when I define macros with the same name

Question

What happens when I have multiple #defines with the same name in one sourcefile:

for example:

#define Dummy 1
#define Dummy 2

I do NOT intend to use it, but saw something similar in production code. Is this covered by the standard?

Answer 1

It's constraint violation and as such, a conforming compiler is required to issue a diagnostic.

C11, 6.10.3 Macro replacement states:

An identifier currently defined as an object-like macro shall not be redefined by another #define preprocessing directive unless the second definition is an object-like macro definition and the two replacement lists are identical. [..]

As noted, it's not a constraint violation if the replacement is identical. So

#define X 1
#define X 2

requires a diagnostic; whereas

#define X 1
#define X 1

is OK. Similar constraints apply for function-like macros (C11, 6.10.3, 2).

Answer 2

This:

#define Dummy 1
#define Dummy 2

is the same as:

#define Dummy 2

But you'll get probably (I'm not sure what the standard says) a warning such as 'Dummy': macro redefinition for the second #define

In other words: the last #define wins.

If you want to do things properly, you should use #undef:

#define Dummy 1
#undef Dummy
#define Dummy 2    // no warning this time

BTW: there are scenarios where it is perfectly OK to change the definition of a macro.