Should variable definition be in header files?

Question

My very basic knowledge of C and compilation process has gone rusty lately. I was trying to figure out answer to the following question but I could not con

Answer 1

You have two .c files. They get compiled separately. Each one includes your header file. Once. Each one gets a definition. They conflict at link time.

The conventional solution is:

#ifdef DEFINE_SOMETHING
int something = 0;
#endif

Then you #define DEFINE_SOMETHING in only one .c file.