using global variable in dll and exe

Question

i have an global variable in an common header file. Eg

commonHeader.h

int commonInt = 0;

i have 3 dll projects in which i want to use it , so i include above header, but it give me error symbol defined multiple times , #pragma once also did't work.

if i make above variable extern , and define it in my exe i get linker errors in my dll.

all my dll need above header. one of my dll need other 2 dll's header file (probably making multiple include of syombol)

how i can resolve above issue , i want only one variable across dll and exe.

i am using VS 2010 prof on windows 7.

thanks in advance.

Answer 1

You're violating the One Definition Rule (§ 3.2) by having that global variable definition in a header file. Instead you were correct to only declare it in a header file with extern and then have the definition in a single implementation file.

But in order to have this work with dlls you also have to declare it as exported by the exe and imported by the dlls with __declspec(dllexport) and __declspec(dllimport), using appropriate macros to choose the right __declspec depending on whether you're compiling the exe or the dlls.

Answer 2

You should only declare globals in a header. They should be defined in an implementation (source) file.

In your header you should have:

// commonHeader.h

extern int commonInt;    // global *declaration*

and then in one of your implementation files you should have:

// some_file.cpp

int commonInt = 0;       // global *definition* (and initialisation)

Of course global variables should be avoided wherever reasonably possible - excessive use of globals is a "code smell", but sometimes it can not be avoided.