How to implement process-global variable in C++?

Question

Normally using a variable in a .cpp file results in the variable being globally available, like this:

.h file:
extern int myGlobal;
void work();

.cpp file:
int myGlobal = 42;
void work(){ myGlobal++; }

When the .cpp file is put in a static library and more than one shared library (DLL) or executable links against the static library, each one has its own copy of myGlobal. work() would modify its own version of the variable.

My question now: is there a way to get a process-wide unique variable or pointer to that variable? Similar what thread-local storage would be for thread-wide variables. It doesn't have to be platform independent. Bonus points if it works in Win32 :)

Answer 1

Simple: make all the DLLs in the process link to a single DLL that exposes the variable.

Answer 2

Either don't statically link to it (stick myGlobal in a DLL), or only link against the static library once (potentially more of a pain in the neck to achieve).

Answer 3

I suggest you have a look at Boost.Interprocess, in particular this page on sharing memory between multiple processes. There you can find how to use this library to create a shared memory space, open one that's already created (actually "open or create"), resize it to your needs and access it. There's a simple example that should get you going quite fast.

Answer 4

Perhaps this is overkill for a single process, but I've used shared sections to achieve something similar to what you describe.