Using the C Run-Time

WINDOWS NT     | WINDOWS

The information in this article applies to:

• Microsoft Win32 Software Development Kit (SDK), versions 3.1, 3.5, 3.51, and 4.0

SUMMARY

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

MORE INFORMATION

Section 1: Three Forms of C Run-Time (CRT) Libraries Are Available

There are three forms of the C Run-time library provided with the Win32 SDK:

• LIBC.LIB is a statically linked library for single-threaded programs.
• LIBCMT.LIB is a statically linked library that supports multithreaded programs.
• CRTDLL.LIB is an import library for CRTDLL.DLL that also supports multithreaded programs. CRTDLL.DLL itself is part of Windows NT.

Section 2: Using the CRT Libraries When Building a DLL

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

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.
   
   

Section 3: Using NTWIN32.MAK to Simplify the Build Process

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

   $(conlibsdll)        for console apps/DLLs using CRT in a DLL
   $(guilibsdll)        for GUI apps using CRT in a DLL

Section 4: Problems Encountered When Using Multiple CRT Libraries

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:

• Passing buffered stream handles from the .EXE/DLL to the other module
• Allocating memory with a C Run-time call in the .EXE/DLL and reallocating or freeing it in the other module
• Checking or setting the value of the global errno variable in the .EXE/DLL and expecting it to be the same in the other module. A related problem is calling perror() in the opposite module from where the C Run- time error occurred, since perror() uses errno.

Section 5: Mixing Library Types

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:

• CRT file handles may only be operated on by the CRT module that created them.
• CRT FILE* pointers may only be operated on by the CRT module that created them.
• Memory allocated with the CRT function malloc() may only be freed or reallocated by the CRT module that allocated it.

   - .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.