DOCUMENT:Q140584 02-NOV-2001 [visualc] TITLE :HOWTO: Link with the Correct C Run-Time (CRT) Library PRODUCT :Microsoft C Compiler PROD/VER::2.0,2.1,2.2,4.0,4.0a,4.1,4.2,4.2b,5.0,5.0sp1,5.0sp2,5.0sp3,6.0,6.0SP3,6.0sp4,6.0sp5 OPER/SYS: KEYWORDS:kbCRT kbVC200 kbVC210 kbVC220 kbVC400 kbVC500 kbVC600 ====================================================================== ------------------------------------------------------------------------------- The information in this article applies to: - Microsoft Visual C++, 32-bit Editions, versions 2.0, 2.1, 2.2, 4.0, 4.0a, 4.1, 4.2, 4.2b, 5.0, 5.0sp1, 5.0sp2, 5.0sp3, 6.0, 6.0SP3, 6.0sp4, 6.0sp5 ------------------------------------------------------------------------------- SUMMARY ======= There are six types of reusable libraries: Static Single Threaded Library (Debug/Release), Static Multithreaded Library (Debug/Release), and Dynamic Link Library (DLL)(Debug/Release). The DLL is multithread-afe 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 above. 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 using 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 .obj Look in the section titled RAW DATA #1. In the right-most column, the default libraries will be listed. 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 studio 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 Additional query words: ====================================================================== Keywords : kbCRT kbVC200 kbVC210 kbVC220 kbVC400 kbVC500 kbVC600 Technology : kbVCsearch kbVC400 kbAudDeveloper kbVC220 kbVC410 kbVC420 kbVC500 kbVC600 kbVC200 kbVC210 kbVC32bitSearch kbVC500SP1 kbVC500SP2 kbVC500SP3 kbVC600SP3 kbVC600SP4 kbVC600SP5 kbVC400a kbVC420b kbVC500Search Version : :2.0,2.1,2.2,4.0,4.0a,4.1,4.2,4.2b,5.0,5.0sp1,5.0sp2,5.0sp3,6.0,6.0SP3,6.0sp4,6.0sp5 Issue type : kbhowto ============================================================================= THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY. Copyright Microsoft Corporation 2001.