Why we should not define global variables in header files only extern?

Question

#include<stdio.h>

int &fun()
{
   static int x;
   return x;
}   

int main()
{
   fun() = 10;
   printf(" %d ", fun());

   return 0;
}

It is fine with c++ compiler while giving error with c compiler.

Answer 1

You should never not define global variables in header files. You can declare them as extern in header file and define them in a .c source file.

Suppose if you define a variable called int my_int in header file my_header.h and you system has more then one .c file called file1.c and file2.c both are including my_header.h. Then variable my_int would be declared twice and can create unexpected result. Better approach would be to define my_int in one of the .c file and create the exterm int my_int in the header file.

I hope it should help you.