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Q128641: INFO: /Mx Compiler Options and the LIBC, LIBCMT, MSVCRT Libs

Article: Q128641
Product(s): Microsoft C Compiler
Version(s): WINNT:2.0,2.1,4.0,5.0;
Operating System(s): 
Keyword(s): kbCompiler kbVC200 kbVC210 kbVC400 kbVC500
Last Modified: 30-JUL-2001

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The information in this article applies to:

- Microsoft Visual C++, versions 2.0, 2.1, 4.0 
- Microsoft Visual C++, 32-bit Enterprise Edition, version 5.0 
- Microsoft Visual C++, 32-bit Professional Edition, version 5.0 
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SUMMARY
=======

This article describes the /ML, /MT and /MD compile options and the default
libraries that these options specify. Different compile options imply linking
with different default libraries. In the Visual C++ version 2.0 development
environment, these options are found in the C/C++ tab after choosing Project
from the Settings menu. In the Visual C++ version 4.0 development environment,
these options are found in the C/C++ tab after choosing Settings... from the
Build menu. For both, select Code Generation. The "Use Run-Time Library"
category corresponds to the switches described below.

NOTE: The /NODEFAULTLIB linker option can be used to override default libraries
specified by the compiler. If you are having problems linking to the default
libraries, check the usage of this linker option.

MORE INFORMATION
================

/ML - Compile for Single-Threaded Statically Linked Runtime Support
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This is the default option, so if you don't specify /MT or /MD, /ML is implicit.
This option generates a default library search record for LIBC.LIB for each and
every object compiled with this flag.

/MT - Compile for Multi-Threaded Statically Linked Run-Time Support
-------------------------------------------------------------------

Multi-threaded support does not imply that the run-time itself is multi-threaded
or that you must use threads in your application. It just allows you to use
threads in you application if you so desire.

This option generates a default library search record for LIBCMT.LIB for each and
every object compiled with this flag. This option also defines (#define) the
symbol _MT during compilation so that multi-threading support is enabled when
you include (#include) C Run-time Library header files.

/MD - Compile for Multi-Threaded Dynamically Linked Run-Time Support
--------------------------------------------------------------------

This option generates a default library search record for MSVCRT.LIB for each and
every object compiled with this flag.

As with /MT, the symbol _MT is defined during compilation so that multi-threading
support is enabled when you include C Run-time Library header files.

Also, /MD defines the symbol _DLL during compilation so that C Run-Time Library
variables and functions will be imported from MSVCRTxx.DLL rather than be
statically linked.

/MD and /D"_DLL" are REQUIRED for C Run-time variable references when linking
with MSVCRT.LIB.


All modules and static libraries within an application should use the same
compile options (/ML, /MT, or /MD) and link to the library cooresponding to the
selected option. Compiling or linking with two or more of the LIBC, LIBCMT,
MSVCRT libraries will cause LNK2005 "symbol" already defined in "object" errors.
If you are getting these errors, make sure all .OBJ files (including static
libraries) are built with the same /Mx option.

Some possible consequences of not following the above recommendation are:

- Linking code compiled with an explicit or implicit /ML to the LIBCMT.LIB or
  MSVCRT.LIB can cause unresolved externals on static C Run-Time data objects
  such as _errno.

- Linking code compiled with /MT or /MD with the library LIBC causes unresolved
  externals on _beginthread(), _beginthreadex(), _endthread(), and/or
  _endthreadex(). LIBC is not multi-threaded, so it does not contain these
  run-time functions. This later case is very common in Visual C++ versions 2.x
  and 4.0, because MFC is now multi-threaded. For more information, please see
  the following article in the Microsoft Knowledge Base:

  Q126646 PRB: Error Msg: LNK2001 on __beginthreadex and __endthreadex

- When compiling with /MD, a reference to "var1" in your source becomes a
  reference "__imp__var1" in the .OBJ file because all C Run-Time is now held
  in a DLL. If you try to link with the static libraries LIBC or LIBCMT, you
  will get an unresolved external on __imp__var1. If you try to link with
  MSVCRT.LIB when compiling without /MD, you may not always get an unresolved
  external, but you are likely to have other problems.

Additional query words: link.exe link cl.exe cl

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Keywords          : kbCompiler kbVC200 kbVC210 kbVC400 kbVC500 
Technology        : kbVCsearch kbVC400 kbAudDeveloper kbVC500 kbVC200 kbVC210 kbVC32bitSearch kbVC500Search
Version           : WINNT:2.0,2.1,4.0,5.0;
Issue type        : kbinfo

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