Link Error : xxx is already defined in *****.LIB :: What exactly is wrong?

Question

Problem:

I'm trying to use a library named DCMTK which used some other external libraries ( zlib, libtiff, libpng, libxml2, libiconv ). I've downloaded these external libraries (*.LIB & *.h files ) from the same website. Now, when I compile the DCMTK library I'm getting link errors (793 errors) like this:

Error   2   error LNK2005: __encode_pointer already defined in MSVCRTD.lib(MSVCR90D.dll)    LIBCMTD.lib dcmmkdir 
Error   3   error LNK2005: __decode_pointer already defined in MSVCRTD.lib(MSVCR90D.dll)    LIBCMTD.lib dcmmkdir 
Error   4   error LNK2005: __CrtSetCheckCount already defined in MSVCRTD.lib(MSVCR90D.dll)  LIBCMTD.lib dcmmkdir 
Error   5   error LNK2005: __invoke_watson already defined in MSVCRTD.lib(MSVCR90D.dll) LIBCMTD.lib dcmmkdir 
Error   6   error LNK2005: __errno already defined in MSVCRTD.lib(MSVCR90D.dll) LIBCMTD.lib dcmmkdir 
Error   7   error LNK2005: __configthreadlocale already defined in MSVCRTD.lib(MSVCR90D.dll)    LIBCMTD.lib dcmmkdir 
Error   8   error LNK2005: _exit already defined in MSVCRTD.lib(MSVCR90D.dll)   LIBCMTD.lib dcmmkdir 

Documentation:

This seems to be a popular error for this library so, they do have a FAQ entry addressing this issue which ( http://forum.dcmtk.org/viewtopic.php?t=35 ) says:

• The problem is that the linker tries to combine different, incompatible versions of the Visual C++ runtime library into a single binary.
• This happens when not all parts of your project and the libraries you link against are generated with the same code generation options in Visual C++.

• Do not use the /NODEFAULTLIB workaround, because strange software crashes may follow. Fix the problem!

• DCMTK is by default compiled with the "Multithreaded" or "Multithreaded Debug" code generation option (the latter for Debug mode).

• Either change the project settings of all of your code to use these code generation options,
• or change the code generation for all DCMTK modules and re-compile.
• MFC users beware: DCMTK should be compiled with "Multithreaded DLL" or "Multithreaded DLL Debug" settings if you want to link the libraries into an MFC application.

Solution to same problem for others:

Huge Amount of Linker Issues with Release Build Only says:

It seems that your release build is trying to link to something that was built debug. You probably have a broken dependency in your build, (or you missed rebuilding something to release by hand if your project is normally built in pieces).

More technically, you seem to be linking projects built with different C Run Time library settings, one with "Multi-Threaded", another one with "Multi-Threaded Debug". Adjust the settings for all the projects to use the very same flavour of the library and the issue should go away

Questions:

Till now I used to think that Name mangling is the only problem that may cause linking failures if its not been standardized. Just now I knew there are other things also which can cause same effect.

1. Whats up with the "Debug Mode" (Multi-Threaded Debug) and "Release Mode" (Multi-Threaded)? What exactly is happening under the hood? Why exactly this thing is causing linking error?

2. I wonder if there is something called "Single-Threaded Debug" and "Single-Threaded" which again causes the same thing.

3. Documentation talks something about "Code Generation Options". What Code Generation Options? WTH are they?

4. Documentation specifically warns us not to use /NODEFAULTLIB workaround. (example /NODEFAULTLIB :msvcrt ). Why? How would I cause troubles? what exactly is it?

5. Please explain the last point in the documentation for MFC users. Because I'm going to use MFC later in this project. Explain Why should we do it? What troubles would it cause if I don't.
6. Anything more you'd like to mention? I mean regarding similar errors. I'm very interested in Linker & its problems. So, if there are any similar things you can mentions them or some keywords atleast.

Answer 1

Whats up with the "Debug Mode" (Multi-Threaded Debug) and "Release Mode" (Multi-Threaded)? What exactly is happening under the hood? Why exactly this thing is causing linking error?

The linker drags in libraries for several different reasons. The simplest is that a library is listed on the linker command line, or in the linker answer file on the linker command line. But any object files, whether compiled in your project or packed into a library, can also contain linker options including requesting particular libraries be linked in. In fact, the Visual C++ compiler automatically embeds such linker options matching the project options you use when compiling.

At link time, all the linker options from all object files and objects in static library files get combined. If more than one CRT library filename is requested, the linker reads in all of them and them you get naming conflicts, where the linker doesn't know which one to use.

I wonder if there is something called "Single-Threaded Debug" and "Single-Threaded" which again causes the same thing.

There used to be, but the last few versions of Visual C++ have only shipped multi-thread compatible libraries.

Documentation talks something about "Code Generation Options". What Code Generation Options? WTH are they?

Look inside your project options.

Documentation specifically warns us not to use /NODEFAULTLIB workaround. (example /NODEFAULTLIB :msvcrt ). Why? How would I cause troubles? what exactly is it?

If you use /NODEFAULTLIB, all the linker settings stored within object files and objects in libraries get ignored. You'll end up with no runtime library and maybe missing other libraries. You can add them back in by hand, but it's still a big mess.

Please explain the last point in the documentation for MFC users. Because I'm going to use MFC later in this project. Explain Why should we do it? What troubles would it cause if I don't. Anything more you'd like to mention? I mean regarding similar errors. I'm very interested in Linker & its problems. So, if there are any similar things you can mentions them or some keywords atleast.

MFC applications and the MFC library have to use the same memory management functions, so that memory allocated by MFC can be freed by the application and vice-versa. FILE handles and other resources are also shared. The MFC DLLs are already compiled to use the CRT in a DLL, and in order to be able to share resources you need to use the same CRT, which means using a DLL too.