PRB: LNK2005 Errors When Link C Run-Time Libraries Are Linked Before MFC Libraries
Q148652
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The information in this article applies to:
Microsoft Visual C++, versions 4.0, 4.1
Microsoft Visual C++, 32-bit Enterprise Edition, versions 5.0, 6.0
Microsoft Visual C++, 32-bit Professional Edition, versions 5.0, 6.0
Microsoft Visual C++, 32-bit Learning Edition, version 6.0
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SYMPTOMS
When the C Run-Time (CRT) library and Microsoft Foundation Class (MFC) libraries are linked in the wrong order, LNK2005 errors similar to the following may occur:
nafxcwd.lib(afxmem.obj) : error LNK2005:
"void * __cdecl operator new(unsigned int)"(??2@YAPAXI@Z) already
defined in LIBCMTD.lib(new.obj)
nafxcwd.lib(afxmem.obj) : error LNK2005:
"void __cdecl operator delete(void *)"(??3@YAXPAX@Z) already defined
in LIBCMTD.lib(dbgnew.obj)
nafxcwd.lib(afxmem.obj) : error LNK2005:
"void * __cdecl operator new(unsigned int,int,char const *,int)"
(??2@YAPAXIHPBDH@Z) already defined in LIBCMTD.lib(dbgnew.obj)
mfcs40d.lib(dllmodul.obj): error LNK2005: _DllMain@12 already defined in
MSVCRTD.LIB (dllmain.obj)
mfcs42d.lib(dllmodul.obj): error LNK2005: _DllMain@12 already defined in
msvcrtd.lib(dllmain.obj)
CAUSE
The CRT libraries use weak external linkage for the new, delete, and DllMain functions. The MFC libraries also contain new, delete, and DllMain functions, which requires MFC to be linked before the CRT libraries.
RESOLUTION
There are two ways to resolve this problem. The first solution involves forcing the linker to link the libraries in the correct order. The second solution allows you to find the module that is causing the problem and correct it.
Solution One: Force Linker to Link Libraries in Correct Order
Open the Project Settings dialog box by clicking Settings on the Build menu.
In the Settings For view, click to select (highlight) the project configuration that is getting the link errors.
Click the Link tab.
Click to select INPUT in the Category combo box.
In the Libraries to Ignore edit box, insert the library names (for example, Nafxcwd.lib Libcmtd.lib).
NOTE: The linker command-line equivalent in /NOD:<library name>.
In the Object/library Modules edit box, insert the library names. You must ensure that these are listed in order and as the first two libraries in the line (for example, Nafxcwd.lib Libcmtd.lib).
Solution Two: Locate and Correct the Problem Module
Perform the following steps to view the current library link order:
Open the Project Settings dialog box by clicking Settings on the Build menu.
In the Settings For view, click to select (highlight) the project configuration that is getting the link errors.
Click the Link tab.
Type the following in the Project Options dialog box:
/verbose:lib
Rebuild your project. The libraries will be listed in the output window during the linking process.
STATUS
This behavior is by design.
MORE INFORMATION
When you use the MFC libraries, you must make sure they are linked before the CRT library. This can be done by ensuring that every file in your project includes Msdev\Mfc\Include\Afx.h first, either directly (#include <Afx.h>) or indirectly (#include <Stdafx.h>). The Afx.h include file forces the correct order of the libraries, by using the #pragma comment (lib,"<libname>") directive.
If the source file has a .c extension, or the file has a .cpp extension but does not use MFC, you can create and include a small header file (Forcelib.h) at the top of the module. This new header ensures the correct library search order.
Visual C++ does not contain this header file, but you can easily create this file by performing the following steps:
Open Msdev\Mfc\Include\Afx.h.
Select the lines between #ifndef _AFX_NOFORCE_LIBS and #endif //!_AFX_NOFORCE_LIBS.
Copy the selection to the Windows Clipboard.
Create a new text file.
Paste the contents of the Clipboard into this new file.
Save the file as Msdev\Mfc\Include\Forcelib.h.
