#define many times without #undef，is it legal？

Question

For example, I define AA for three times, is it legal？：

#include
#define AA 10
#define AA 20
#define AA 30
int main() {
    printf(\"         


        

Answer 1

No it is not. The C standard clearly states that it is a constraint violation, 6.10.3 p.2:

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.

So as for all constraint violations, your compiler is only obliged to issue a "diagnostic" that is an explanatory message. It may or may not continue to compile your code.

To state it more directly, your code erroneous and your compiler must tell you.