Compile redeclaration error of global variable in C++, but not in C

Question

Suppose that I have those three files:

a.h

//a.h header
#include <stdio.h>
int int_variable;

void a_f()
{
   printf("int_variable: %d\n", int_variable)
   int_variable++;
}

b.h

//b.h header
#include <stdio.h>
int int_variable;

void b_f()
{
   printf("int_variable: %d\n", int_variable)
   int_variable++;
}

main.c

//main.c
#include "a.h"
#include "b.h"

int main()
{
   a_f();
   b_f();
   return 0;
}

Why compiling in C++ generates redefinition error, but in C doesn't? I am C++ developer, then in C++ makes sense to me, but why in C this is not an error?

When I executed the C generated code, the output was:

int variable: 0

int variable: 1

Answer 1

In C, the two variables are actually combined into a single variable because neither is explicitly initialized.

If you change both your h files to:

// a.h
int int_variable = 0;

and:

// b.h
int int_variable = 0;

you will get a redefinition error.