CRT Library Features

This topic discusses the various .lib files that comprise the C run-time libraries as well as their associated compiler options and preprocessor directives.

C Run-Time Libraries (CRT)

The following libraries contain the C run-time library functions.

If you link your program from the command line without a compiler option that specifies a C run-time library, the linker will use LIBCMT.LIB. This is different from previous versions of Visual C++ which used LIBC.LIB, the single-threaded library, instead.

Using the statically linked CRT implies that any state information saved by the C runtime library will be local to that instance of the CRT. For example, if you use strtok, _strtok_l, wcstok, _wcstok_l, _mbstok, _mbstok_l when using a statically linked CRT, the position of the strtok parser is unrelated to the strtok state used in code in the same process (but in a different DLL or EXE) that is linked to another instance of the static CRT. In contrast, the dynamically linked CRT shares state for all code within a process that is dynamically linked to the CRT. This concern does not apply if you use the new more secure versions of these functions; for example, strtok_s does not have this problem.

Because a DLL built by linking to a static CRT will have its own CRT state, it is not recommended to link statically to the CRT in a DLL unless the consequences of this are specifically desired and understood. For example, if you call _set_se_translator in an executable that loads the DLL linked to its own static CRT, any hardware exceptions generated by the code in the DLL will not be caught by the translator, but hardware exceptions generated by code in the main executable will be caught.

If you are using the /clr compiler switch, your code will be linked with a static library, msvcmrt.lib. The static library provides a proxy between your managed code and the native CRT. You cannot use the statically linked CRT ( /MT or /MTd options) with /clr. Use the dynamically-linked libraries (/MD or /MDd) instead.

If you are using the /clr:pure compiler switch, your code will be linked with the static library msvcurt.lib. As with /clr, you cannot link with the statically linked library.

For more information on using the CRT with /clr, see Mixed (Native and Managed) Assemblies; for /clr:pure, see Pure and Verifiable Code (C++/CLI).

To build a debug version of your application, the _DEBUG flag must be defined and the application must be linked with a debug version of one of these libraries. For more information about using the debug versions of the library files, see CRT Debugging Techniques.

This version of Visual C++ is not conformant with the C99 standard.

Standard C++ Library

Note   LIBCP.LIB and LIBCPD.LIB (via the old /ML and /MLd options) have been removed. Use LIBCPMT.LIB and LIBCPMTD.LIB instead via the /MT and /MTd options.

When you build a release version of your project, one of the basic C run-time libraries (LIBCMT.LIB, MSVCMRT.LIB, MSVCRT.LIB) is linked by default, depending on the compiler option you choose (multithreaded, DLL, /clr). If you include one of the C++ Standard Library Header Files in your code, a Standard C++ Library will be linked in automatically by Visual C++ at compile time. For example:

#include <ios> 

To change which basic C run-time library is linked in Visual Studio, open the property pages for your project. Open the Configuration Properties, C/C++, Code Generation page and change the Runtime Library setting. To change the Common Language Runtime library, open the property pages for your project. Open the Configuration Properties, C/C++, General page and then change the Common Language RunTime Support setting.

What is the difference between msvcrt.dll and msvcr120.dll?

The msvcrt.dll is now a "known DLL," meaning that it is a system component owned and built by Windows. It is intended for future use only by system-level components.

What problems exist if an application uses both msvcrt.dll and msvcr120.dll?

If you have a .lib or .obj file that needs to link to msvcrt.lib, then you should not have to recompile it to work with the new msvcrt.lib in Visual C++. The .lib or .obj file may rely on the sizes, field offsets, or member function names of various CRT classes or variables, and those should all still exist in a compatible way. When you relink against msvcrt.lib, your final EXE and DLL image will now have a dependency on msvcr120.dll instead of msvcrt.dll.

If you have more than one DLL or EXE, then you may have more than one CRT, whether or not you are using different versions of Visual C++. For example, statically linking the CRT into multiple DLLs can present the same problem. Developers encountering this problem with static CRTs have been instructed to compile with /MD to use the CRT DLL. Now that the CRT DLL has been renamed to msvcr120.dll, applications may have some components linked to msvcrt.dll and others to msvcr120.dll. If your DLLs pass CRT resources across the msvcrt.dll and msvcr120.dll boundary, you will encounter issues with mismatched CRTs and need to recompile your project with Visual C++.

If your program is using more than one version of the CRT, some care is needed when passing certain CRT objects (such as file handles, locales and environment variables) across DLL boundaries. For more information on the issues involved and how to resolve them, see Potential Errors Passing CRT Objects Across DLL Boundaries.

See Also

Other Resources

C Run-Time Library Reference