Explanation of #undef directives in c programming language by examples, questions and answers
Directive #undef is used to undefine any macro constants except global identifiers. It is useful when we want to redefined any macro constants. For example:
#include
#define ABC 25
#ifdef ABC
#undef ABC
#define ABC 50
#else
#define ABC 100
#endif
int main(){
printf("%d",ABC);