KnowledgeBase Archive

An Archive of Early Microsoft KnowledgeBase Articles

View on GitHub

Q140584: HOWTO: Link with the Correct C Run-Time (CRT) Library

Article: Q140584
Product(s): Microsoft C Compiler
Version(s): 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
Operating System(s): 
Keyword(s): kbCRT kbVC200 kbVC210 kbVC220 kbVC400 kbVC500 kbVC600
Last Modified: 02-NOV-2001

-------------------------------------------------------------------------------
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 <object>.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.