C / C++ / MFC

// [file Main.cpp]
#include "mod.h"

void main()
{
	iGlobalNumber = 2;
}
--------------------------------------
// [file Mod.cpp]
#include "mod.H"

void Test()
{
	iGlobalNumber = 1;
}
--------------------------------------
// [file Mod.H]
#ifndef _MY_HEADER_FILE_H_
#define _MY_HEADER_FILE_H_


int iGlobalNumber;


#endif // _MY_HEADER_FILE_H_
---------------------------------------

mod.obj : error LNK2005: "int iGlobalNumber" (?iGlobalNumber@@3HA) already defined in main.obj

[the whole pseudo code in one single source file]
/* First, the compiler compiles main.cpp. It embeds the content of the included *.h files into main.cpp */

int iGlobalNumber;

void main()
{
    iGlobalNumber = 2;
}

/* Then, the compiler compiles mod.cpp. It embeds the content of the included *.h files into mod.cpp */

int iGlobalNumber; // Again because the #include "mod.H", this is why already defined.

void Test()
{
    iGlobalNumber = 1;
}

[file mod.h]
#ifndef _MY_HEADER_FILE_H_
#define _MY_HEADER_FILE_H_
// ...
void Test();
// ...
#endif // _MY_HEADER_FILE_H_

--------------------------------
[file main.cpp]
#include "mod.h"

int iGlobalNumber; // Declare this name only ONCE.

void main()
{
	iGlobalNumber = 2;
	Test();
}

-----------------------------------
[file mod.cpp]
extern int iGlobalNumber; // Tell the linker that it is from outside.

void Test()
{
	iGlobalNumber = 1;
}
------------------------------------