#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

Lets start by correcting your printf to something useful: printf("%d", AA);

Compiling it with gcc will produce two warnings that "AA" is redefined. Warnings are really important and should be avoided in C, but the result will be as expected (30).