error LNK2005: _DllMain@12 already defined in MSVCRT.lib

Question

I am getting this linker error.

mfcs80.lib(dllmodul.obj) : error LNK2005: _DllMain@12 already defined in MSVCRT.lib(dllmain.obj)

Answer 1

I had the same error message, but none of the answers here solved it for me. So if you Encounter that Problem when creating a DLL Project that uses MFC, it can be resolved by entering the following line:

extern "C" { int _afxForceUSRDLL; } 

to the cpp file where DllMain is defined. Then your own DllMain implementation is used, rather than the one from dllmain.obj.

When we try to use MFC library, we surely will include afx.h directly or indirectly, then MFC(afx.h) tell the linker to find the symbol of __afxForceUSRDLL and put that object which contains __afxForceUSRDLL into the program, so linker searches and puts dllmodule.obj into our program, because __afxForceUSRDLL is defined in dllmodule.cpp.

That’s the common scenario. When we want to use our own DllMain in a mfc dll project, linker complains that there are two DllMain, one in our code, one in Dllmodule.obj.

So we need to tell the linker to add our dllmain.obj for __afxForceUSRDLL. So we need to define __afxForceUSRDLL in our own cpp file where our own DllMain is defined, then the linker will ignore mfc’s dllmodule.obj and see only one DllMain and never complains.

Source: http://social.msdn.microsoft.com/Forums/en-US/0d78aa6b-1e87-4c01-a4a7-691335b7351a/how-to-build-mfc-application-dll-in-visual-c-2010

Answer 2

In my case I had a problem with the preprocessor directives. For some reason _USRDLL was defined, when it should not have been.

To check this, go to the menu Project , select Project Properties , then select the snippet Configuration Properties --> Preprocessor .

The preprocessor directives will be found there.

Answer 3

For all those who are experiencing this error in ATL projects (mostly when trying to add MFC support), here's the solution I found after days of frustration!

First of all, this link was more helpful to me than all the others. It pointed me into the right direction. The problem occurs, if the "generated files" (containing the proxy and stub code, just as the type guids) for some reason have been removed and readded to the project. This causes Visual Studio to add them in the wrong order!

Usually you first come up with the "ATL requires C++ compilation" error, but you may have fixed this by turning out the Yc/Yu (precompiled headers) setting for that file.

What you should do next is unloading your project and edit it. Search for the item groups that define the build and include order (ClCompile and ClInclude). Check their order and settings.

The compiles should appear in this order:

1. dllmain.cpp (with CompileAsManaged set to false and PrecompiledHeader left empty).
2. Library source (MyLib.cpp, containing DllCanUnloadNow and so on)
3. Proxy/Stub code (MyLib_i.c; with same settings as dllmain.cpp)
4. stdafx.cpp (with PrecompiledHeader set to Create)
5. All the other library source files (your actual libraries content)
6. xdlldata.c (with the same settings as dllmain.cpp)

The includes should then be ordered like this:

1. dllmain.h
2. MyLib_i.h
3. Resource.h
4. stdafx.h
5. targetver.h
6. ... (actual library headers)
7. xdlldata.h

Fixing the build order fixed my project and I was able to create a new clean build.

Answer 4

I have personally got rid of this error this way: right-clicked project in the Solution Explorer, selected Properties from pop-up menu, clicked Linker tab and added mfcs71ud.lib into Additional Dependencies. If you're using Visual Studio 2005, it should be "80" instead of "71" and so on.

Answer 5

If you're defining your own DllMain, in your project settings you need to set 'Use of MFC' in the 'Configuration Properties/General' to 'Use Standard Windows Libraries'.

You should do a clean rebuild after changing it.