C Run-Time Libraries

The following table lists the release versions of the C run-time library files, along with their associated compiler options and environment variables. Prior to Visual C++ 4.2, the C run-time libraries contained the iostream library functions. In Visual C++ 4.2 and later, the old iostream library functions have been removed from LIBC.LIB, LIBCMT.LIB, and MSVCRT.LIB. (This change was made because the Standard C++ library has been added to Visual C++, and it contains a new set of iostream libraries. Thus, two sets of iostream functions are now included in Visual C++.) The old iostream functions now exist in their own libraries: LIBCI.LIB, LIBCIMT.LIB, and MSVCIRT.LIB. The new iostream functions, as well as many other new functions, exist in the Standard C++ libraries: LIBCP.LIB, LIBCPMT.LIB, and MSVCPRT.LIB.

The Standard C++ library and the old iostream library are incompatible, and only one of them can be linked with your project. See Port to the Standard C++ Library and the for details.

When you build a release version of your project, one of the basic C run-time libraries (LIBC.LIB, LIBCMT.LIB, and MSVCRT.LIB) is linked by default, depending on the compiler option you choose (single-threaded, multithreaded, or DLL). Depending on the headers you use in your code, a library from the Standard C++ libraries or one from the old iostream libraries may also be linked:

  • If you include a in your code, a Standard C++ library will be linked in automatically by Visual C++ at compile time. For example:
   #include <ios>
  • If you include an old iostream library header, an old iostream library will be linked in automatically by Visual C++ at compile time. For example:
   #include <ios.h>

Note that headers from the Standard C++ library and the old iostream library cannot be mixed.

Headers determine whether a Standard C++ library, an old iostream library, or neither will be linked. Compiler options determine which of the libraries to be linked is the default (single-threaded, multithreaded, or DLL). When a specific library compiler option is defined, that library is considered to be the default and its environment variables are automatically defined.

C Run-Time Library (without iostream) Characteristics Option Defined
LIBC.LIB Single threaded, static link /ML
LIBCMT.LIB Multithreaded, static link /MT _MT
MSVCRT.LIB Multithreaded, dynamic link (import library for MSVCRT.DLL) /MD _MT, _DLL
Standard C++ Library Characteristics Option Defined
LIBCP.LIB Single threaded, static link /ML
LIBCPMT.LIB Multithreaded, static link /MT _MT
MSVCPRT.LIB Multithreaded, dynamic link (import library for MSVCRT.DLL) /MD _MT, _DLL
Old Iostream Library Characteristics Option Defined
LIBCI.LIB Single threaded, static link /ML
LIBCIMT.LIB Multithreaded, static link /MT _MT
MSVCIRT.LIB Multithreaded, dynamic link (import library for MSVCIRT.DLL) /MD _MT, _DLL

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 C Run-Time Debug Libraries.