CODEWITHSUNDEEP
c++windowsdll
Victor Ronin
Victor Ronin

Reputation: 23268

Limitation on usages of global variables in DLL (for Windows)

First of all, I know global variables are evil :) However, there is legit case why I need to use one.

I know that there are very strict limitation on what can be executed in DllMain (no LoadLibraries, no COM initialization and so on).

And I know that global variables are initialized just prior DllMain DLL_PROCESS_ATTACH.

Do I have the same limitation while global variable initialization?

I found Microsoft article: http://msdn.microsoft.com/en-us/library/988ye33t However, it doesn't have any details related to the limitations.

I saw also another Stackoverflow questions: What happens to global variables declared in a DLL? Here is snippet from there "There are things that are forbidden to do in the DllMain. Those things are probably forbidden, too, in the constructors."

However, it looks like commentor isn't sure whether such limitations are really exist.

I would appreciate any information on this subject.

Upvotes: 0

Views: 2282

Answers (1)

Matteo Italia
Matteo Italia

Reputation: 126787

As explained in this answer of mine, the real dll entrypoint (i.e. the real DllMain) is taken by the CRT, that, on DLL_PROCESS_ATTACH, initializes its stuff, calls the constructors of the globals and then call the "fake" DllMain (i.e. what you as a programmer see as DllMain).

The MSDN documentation calls DllMain the real dll entrypoint, so all the restrictions apply also to the constructors of global objects, since they are called by it.

This is yet another reason to avoid globals: the code in their constructors is called from DllMain, but this is not evident, so in case of problems coming from doing "forbidden" things in DllMain it may take a lot of time before acknowledging that the offending code is in such constructors.

Upvotes: 6

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