Shared global variable in C++ static library

Question

I have a MS C++ project (let's call it project A) that I am currently compiling as a static library (.lib). It defines a global variable foo. I have two other projects which compile separately (call them B and C, respectively) and each links the shared static library A in. Both B and C are dll's that end up loaded in the same process. I would like to share a single instance of foo from A between B and C in the same process: a singleton. I'm not sure how to accomplish the singleton pattern here with project A since it is statically compiled into B and C separately. If I declare foo as extern in both B and C, I end up with different instances in B and C. Using a standard, simple singleton class pattern with a static getInstance method results in two static foo instantiations.

Is there any way to accomplish this while project A is statically compiled into B and C? Or do I have to make A a DLL?

Answer 1

Yes, you have to make A a shared DLL, or else define it as extern in B and C and link all three statically.

Answer 2

No - they are not shared.

From Richter's 'Windows via C/C++' (p583):

When one process maps a DLL image file into its address space space, the system creates instances of the global and static data variable as well.

So, if you need to share a resource between multiple executables you will need to create a shared kernel object of some sort. I would suggest creating a named file mapping, which you can then use to read and write to from the separate processes (with appropriate Mutex exclusion, of course.)