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

- Microsoft Visual C++, 32-bit Edition, versions 2.0, 2.1, 4.0, 5.0

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

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:

  ARTICLE-ID: Q126646

     TITLE     : 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.