Are Multiple singleton instances possible in a shared DLL?

Question

I am going to develop a DLL for an MFC Application, and suppose I have a singleton class in this DLL with some synchronization mechanism. And this DLL is used by other processes, namely EXEs. The question is: is this singleton created only once for all sharing processes or every process has its own singleton?

And How can I solve this multiple singleton problem?

Answer 1

I suppose you are talking about Windows. In that case every process has its own singleton. You could place it in shared memory and use named synchronization primitives to share singleton between processes.

Answer 2

If based on the singleton pattern, it'll end up being one singleton per process. Note that if you run multiple threads within that process there will still only be one singleton.

Answer 3

It depends. By default, all data in a DLL is non-shared and all code is shared. But by using #pragma section ("SharedSingleton", read, write, shared) you create a data section named "SharedSingleton", which is shared across all users of the DLL.

Note that this does introduce security risks! Another troublesome issue you might encounter is the initialization of the singleton; C++ doesn't really understand the concept of shared sections.