Question

Say I have a DLL that has the following static/global:

ClassA Object;

Along with the implementation of ClassA, it also contains a ‘regular’ ClassB, which will not work properly if ClassA has not been constructed yet (which is why I’ve made ClassA is a static/global).

In Windows, I believe that the DLL loader will load this DLL on the call to ClassB’s constructor, correct? At this point, ClassA will be constructed and then ClassB’s construction will follow. If a second thread comes along and constructs ClassB, ClassA will not be constructed as it has already been constructed.

Now, my question is — what if ClassB is constructed simultaneously by two threads. So Thread 1 will start to construct ClassA. Will Thread 2 wait until ClassA is completely constructed before executing ClassB’s constructor?

In other words, does LoadLibrary() use a CriticalSection to ensure thread-safe initialization of a DLL’s static/globals? My hunch is ‘yes’, but I can’t seem to find any documentation saying one way or the other.

Answer 1

DllMain is called by the Windows loader while holding an internal critical section known as the “loader lock,” so your static constructors will be called during the DLL_PROCESS_ATTACH event, which only occurs once, when your DLL is first loaded.