What's the difference between /MD and /MDd (Use Run-Time library)?

5
votes

When I added the following line to my code:

std::string sFrameTag

I got the following linker error:

Error 34 error LNK2005: "public: __thiscall std::basic_string,class std::allocator >::~basic_string,class std::allocator >(void)" (??1?$basic_string@DU? $char_traits@D@std@@V?$allocator@D@2@@std@@QAE@XZ) already defined in VFPAnalyzerApi.lib(VFPEvaluation.obj) msvcprtd.lib

I took a look at the project properties, under C/C++ -> code generation -> runtime Library and changed it from /MDd to /MD and somehow the error disappeared.. However, I don't really understand what's the difference and why it worked. Can someone please explain what exactly have I changed and why it made a difference?

Thanks!!!

5
c++
One is the multi-threaded-DLL debug lib, the other is the multi-threaded-DLL release lib. You went from the former to the latter. The objects exposed, particularly with the debug allocators wired into the standard objects of the debug-c-runtime-DLL, result in different mangled names that are exported from the runtime libraries. Thus a cross-link will not work - WhozCraig

5 Answers

6
votes

MSDN has good docs on this.

In short, /MDd links the necessary parts of a debug build of the C & C++ standard libraries into your DLL. /MD is similar but uses a NDEBUG version of the standard libraries. If you try to link code that combines debug and release standard libraries, you'll get symbol clashes as both libraries will provide the same set of functions.

3
votes

/MD uses the multi-threaded - DLL runtime libraries. /MDd uses the debug version of the same libraries

You are probably using another library that was built with MD and is thus linking to the release version of the runtime libraries. (thus the already defined error)

1
votes

There are multiple versions of the runtime library (debug, multithreaded, etc). When building a project, you need to make sure that everything is using the same version, or you'd get errors like the one in your question.

For more information, see /MD, /MT, /LD (Use Run-Time Library).

1
votes

The difference is that /Mdd specifies a debug version of the runtime, while /MD does not. /MDd sets the _DEBUG symbol, so maybe that's your issue if std::string sFrameTag was declared inside an #ifdef _DEBUG statement. Check this MSDN article for reference.

1
votes

Runtime libraries contains implementations of all standard library classes and functions. You have to provide these implementations, such that your program can work, and Visual Studio gives you two options:

  • Runtime libraries are not compiled into binaries of your program, however it will require the Visual Studio Runtimes to be installed prior to your application (or it won't work)
  • Runtime libraries will be compiled into binaries of your program - that will slightly increase its size, but it won't require any additional runtimes to work.

Additionally, you may choose, whether you want the debug or release version of these libraries. There are four choices:

  • /MDd - Multi threaded Debug DLL
  • /MD - Multi threaded DLL
  • /MTd - Multi threaded Debug
  • /MT - Multi threaded

If you use Visual Studio, you can modify these options in project properties: Configuration Properties | C/C++ | Code Generation.

You have to choose the option, which best suits your needs. However, you should provide apropriate static library, which contains required function transports. You switched from release to debug configuration and that's why compiler complained (default Debug and Release configurations already have appropriate library set to be linked).