Steven Engelhardt

When developing C/C++ applications on Windows, there are multiple versions of the C run-time library to choose from:

• Single-Threaded (statically linked)
• Multi-Threaded (statically linked)
• Multi-Threaded DLL
• Debug Single-Threaded (statically linked)
• Debug Multi-Threaded (statically linked)
• Debug Multi-Threaded DLL

Most of the time you don’t need to concern yourself with this setting. However, different versions of the C run-time library may not play well together.

For example, I recently discovered that if you try to link a piece of code which uses a static version of the C run-time library to a static library which uses a DLL version of the C run-time library, you will get errors of the form:

Linking...
MSVCRTD.lib(MSVCRTD.dll) : error LNK2005: _printf already defined in LIBCD.lib(printf.obj)
LINK : warning LNK4098: defaultlib "MSVCRTD" conflicts with use of other libs; use /NODEFAULTLIB:library
Debug/Exe.exe : fatal error LNK1169: one or more multiply defined symbols found
Error executing link.exe.

Therefore, if you run across errors like this, check that all your projects use the same C run-time library. In Microsoft Visual C++ 6.0, the choice of C run-time library is provided under Project Settings > C/C++ > Code Generation > Use run-time library. Here is a picture of the dialog: