There are six types of reusable libraries:

•	Static Single Threaded Library (Debug/Release)
•	Static Multithreaded Library (Debug/Release)
•	Dynamic Link Library (DLL)(Debug/Release)

Note Each library has a debug version and a release version.

The DLL is multithread-safe and a single-thread version of the CRT library is not provided for DLLs. If the reusable library or any user of the library is using multiple threads, then the library needs to be a multithread-safe library type.

Note Debug libraries and compiler switches /MLd, /MTd, and /MDd are only available in Visual C++ versions 4.0 and later.

The following table shows which compiler switch should be used for building each of the six types of reusable libraries (all DLL types are multithread-safe). Any project that uses the reusable library should use the same compiler switch. When using the /ML(default), /MLd, /MT, /MTd, /MD, or /MDd compiler switches, the compiler places the default library name (listed under the Library column) in the object file.

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
				

You can view an object module to determine which switch was used when it was built by using this command:

   dumpbin /all <object>.obj
				

Look in the section titled RAW DATA #1. In the right-most column, the default libraries will be listed.

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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.

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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
				

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