ID: Q94248
This document contains the following sections:
Section 1: Three Forms of C Run-Time (CRT) Library Are Available
Section 2: Using the CRT Libraries When Building a DLL
Section 3: Using NTWIN32.MAK to Simplify the Build Process
Section 4: Problems Encountered When Using Multiple CRT Libraries
Section 5: Mixing Library Types
There are three forms of the C Run-time library provided with the Win32 SDK:
When building a DLL which uses any of the C Run-time libraries, in order to ensure that the CRT is properly initialized, either
1. the initialization function must be named DllMain() and the entry point
must be specified with the linker option -entry:_DllMainCRTStartup@12
- or -
2. the DLL's entry point must explicitly call CRT_INIT() on process attach
and process detach
This permits the C Run-time libraries to properly allocate and initialize C
Run-time data when a process or thread is attaching to the DLL, to properly
clean up C Run-time data when a process is detaching from the DLL, and for
global C++ objects in the DLL to be properly constructed and destructed.
The Win32 SDK samples all use the first method. Use them as an example. Also refer to the Win32 Programmer's Reference for DllEntryPoint() and the Visual C++ documentation for DllMain(). Note that DllMainCRTStartup() calls CRT_INIT() and CRT_INIT() will call your application's DllMain(), if it exists.
If you wish to use the second method and call the CRT initialization code yourself, instead of using DllMainCRTStartup() and DllMain(), there are two techniques:
1. if there is no entry function which performs initialization code, simply
specify CRT_INIT() as the entry point of the DLL. Assuming that you've
included NTWIN32.MAK, which defines DLLENTRY as "@12", add the following
option to the DLL's link line:
-entry:_CRT_INIT$(DLLENTRY)
- or -
2. if you *do* have your own DLL entry point, do the following in the entry
point:
a. Use this prototype for CRT_INIT():
BOOL WINAPI _CRT_INIT(HINSTANCE hinstDLL, DWORD fdwReason,
LPVOID lpReserved);
For information on CRT_INIT() return values, see the documentation
DllEntryPoint; the same values are returned.
b. On DLL_PROCESS_ATTACH and DLL_THREAD_ATTACH (see "DllEntryPoint" in
the Win32 API reference for more information on these flags), call
CRT_INIT(), first, before any C Run-time functions are called or any
floating-point operations are performed.
c. Call your own process/thread initialization/termination code.
d. On DLL_PROCESS_DETACH and DLL_THREAD_DETACH, call CRT_INIT() last,
after all C Run-time functions have been called and all floating-
point operations are completed.
Be sure to pass on to CRT_INIT() all of the parameters of the entry
point; CRT_INIT() expects those parameters, so things may not work
reliably if they are omitted (in particular, fdwReason is required to
determine whether process initialization or termination is needed).
Below is a skeleton sample entry point function that shows when and how
to make these calls to CRT_INIT() in the DLL entry point:
BOOL WINAPI DllEntryPoint(HINSTANCE hinstDLL, DWORD fdwReason,
LPVOID lpReserved)
{
if (fdwReason == DLL_PROCESS_ATTACH || fdwReason == DLL_THREAD_ATTACH)
if (!_CRT_INIT(hinstDLL, fdwReason, lpReserved))
return(FALSE);
if (fdwReason == DLL_PROCESS_DETACH || fdwReason == DLL_THREAD_DETACH)
if (!_CRT_INIT(hinstDLL, fdwReason, lpReserved))
return(FALSE);
return(TRUE);
}
NOTE that this is *not* necessary if you are using DllMain() and
-entry:_DllMainCRTStartup@12.
There are macros defined in NTWIN32.MAK that can be used to simplify your makefiles and to ensure that they are properly built to avoid conflicts. For this reason, Microsoft highly recommends using NTWIN32.MAK and the macros therein.
For compilation, use:
$(cvarsdll) for apps/DLLs using CRT in a DLL
For linking, use one of the following:
$(conlibsdll) for console apps/DLLs using CRT in a DLL
$(guilibsdll) for GUI apps using CRT in a DLL
If an application that makes C Run-time calls links to a DLL that also makes C Run-time calls, be aware that if they are both linked with one of the statically-linked C Run-time libraries (LIBC.LIB or LIBCMT.LIB), the .EXE and DLL will have separate copies of all C Run-time functions and global variables. This means that C Run-time data cannot be shared between the .EXE and the DLL. Some of the problems that can occur as a result are:
You can link your DLL with CRTDLL.LIB/MSVCRT.LIB regardless of what your .EXE is linked with if you avoid mixing CRT data structures and passing CRT file handles or CRT FILE* pointers to other modules.
When mixing library types adhere to the following:
- .EXE is linked with MSVCRT.LIB
- DLL A is linked with LIBCMT.LIB
- DLL B is linked with CRTDLL.LIB
If the .EXE creates a CRT file handle using _create() or _open(), this
file handle may only be passed to _lseek(), _read(), _write(), _close(),
etc. in the .EXE file. Do not pass this CRT file handle to either DLL.
Do not pass a CRT file handle obtained from either DLL to the other DLL
or to the .EXE.
If DLL A allocates a block of memory with malloc(), only DLL A may call
free(), _expand(), or realloc() to operate on that block. You cannot
call malloc() from DLL A and try to free that block from the .EXE or
from DLL B.
NOTE: If all three modules were linked with CRTDLL.LIB or all three were
linked with MSVCRT.LIb, these restrictions would not apply.
When linking DLLs with LIBC.LIB, be aware that if there is a possibility
that such a DLL will be called by a multithreaded program, the DLL will
not support multiple threads running in the DLL at the same time, which can
cause major problems. If there is a possibility that the DLL will be called
by multithreaded programs, be sure to link it with one of the libraries
that support multithreaded programs (LIBCMT.LIB, CRTDLL.LIB or MSVCRT.LIB).
Additional query words:
Last Reviewed: October 1, 1998