How to link with the correct C Run-Time (CRT) library

Reusable Library            Switch    Library    Macro(s) Defined
----------------------------------------------------------------
Single Threaded             /ML       LIBC       (none)
Static MultiThread          /MT       LIBCMT     _MT
Dynamic Link (DLL)          /MD       MSVCRT     _MT and _DLL
Debug Single Threaded       /MLd      LIBCD      _DEBUG
Debug Static MultiThread    /MTd      LIBCMTD    _DEBUG and _MT
Debug Dynamic Link (DLL)    /MDd      MSVCRTD    _DEBUG, _MT, and _DLL
				

   dumpbin /all <object>.obj
				

MORE INFORMATION

A reusable library and all of its users should use the same CRT library types and therefore the same compiler switch. The macros defined (or not defined) for each of the compiler switches can be used in the header file(s) of your reusable library to enforce the proper compiler switch. The sample code in this article demonstrates how to use these macros.

If you would like users of the library to be able to choose static or DLL CRT, you should provide both static and DLL reusable library types.

If you do choose to mix CRT libraries, remember that you have two separate copies of the CRT, with separate and distinct states, so you must be careful about what you try to do across a CRT-boundary. There are many ways to get into trouble with two CRTs. Here are just a few:

• There are two separate heaps. You cannot allocate (explicitly with new, malloc, or so on -- or implicitly with strdup, strstreambuf::str, or so on), and then pass the pointer across a CRT-boundary to be freed.
• You cannot pass a FILE* or file handle across a CRT-boundary and expect the "stdio low-level IO" to work.
• You cannot set the locale in one and expect the other's locale to be set.

Beginning with Visual C++ 4.0, the linker will issue a warning (LNK4098) if a resulting module attempts to combine more than one copy of the CRT library. For more information, search the Help file for LNK4098.

Sample code

The following code can be used in the reusable library's header file to ensure the consistent use of the correct compiler switch:

// MyReusableStaticSingleThreadReleaseLibrary.h
#if defined(_MT) || defined(_DEBUG)
    #error The /ML compiler switch is required.
#endif

// MyReusableStaticMultithreadReleaseLibrary.h
#if !defined(_MT) || defined(_DLL) || defined(_DEBUG)
    #error The /MT compiler switch is required.
#endif

// MyReusableDynamicLinkReleaseLibrary.h
#if !defined(_MT) || !defined(_DLL) || defined(_DEBUG)
    #error The /MD compiler switch is required.
#endif

// MyReusableStaticSingleThreadDebugLibrary.h
#if defined(_MT) || !defined(_DEBUG)
    #error The /MLd compiler switch is required.
#endif

// MyReusableStaticMultithreadDebugLibrary.h
#if !defined(_MT) || defined(_DLL) || !defined(_DEBUG)
    #error The /MTd compiler switch is required.
#endif

// MyReusableDynamicLinkDebugLibrary.h
#if !defined(_MT) || !defined(_DLL) || !defined(_DEBUG)
    #error The /MDd compiler switch is required.
#endif