Shared DLL Module State

Global and static variables in a DLL module are, by default, not shared between processes which load the DLL, as a result, each process has its own instance of a DLL’s global and static variables.

To share DLL global and static variables amongst all processes which load the DLL, you must place the shared variables in a new data segment;

#pragma data_seg(".DATASEGNAME")
int nSomeSharedVariable = 0; // initialise variable
#pragma data_seg()

You then need to tell the linker about this data segment and mark it “shared”.

#pragma comment(linker, "/SECTION:.DATASEGNAME,RWS")

Having done this, variable nSomeSharedVariable is shared between all processes which load the DLL.  This obviously means that you must protect such variables from access by multiple threads.

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