Mixing Library Types

If your DLL is linked with MSVCRT.LIB, any modules that call your DLL should also be linked with MSVCRT.LIB. Otherwise, unpredictable results can occur, because the module and the DLL have separate copies of all CRT functions and global variables. To avoid problems, link both the executable and the DLL with MSVCRT.LIB. This allows both the executable and the DLL to use the common set of functions and data contained in MSVCRT.DLL. C run-time data such as stream handles can then be shared by both the executable and the DLL.

If you must mix library types, be sure to use the following guidelines:

• CRT file handles may only be used by the module that created them.
• FILE pointers may only be used by the module that created them.
• Memory allocated with the CRT may only be freed or reallocated by the module that allocated.

If a DLL is linked with LIBC.LIB, and the DLL may be called by a multi-threaded application, multiple threads running in this DLL at the same time will not be supported. This may cause unpredictable results. Therefore, if there is a possibility that the DLL will be called by multi-threaded programs, be sure to link it with one of the libraries that support multi-threaded programs (LIBCMT.LIB or MSVCRT.LIB).