General Information Regarding Dynamic-Link Libraries

The information in this article applies to:

• Microsoft Windows Software Development Kit (SDK) for Windows versions 3.0 and 3.1

SUMMARY

Dynamic-link libraries (DLLs) are very useful tools in Windows programming. Many applications can use the code in a DLL, meaning that only one copy of the code is resident in the system. It is possible to update a DLL without changing applications that use the DLL, as long as the interface to the functions in the DLL does not change.

This article provides some general information regarding DLLs.

MORE INFORMATION

The small and medium memory models are the preferred memory models for Windows 3.0 applications and DLLs. By default, the compact and large memory models create multiple data segments. These data segments are fixed in the memory space and cause difficulties for all applications running in Windows. Fixed segments in standard and enhanced modes can cause fragmentation of the heap. Because of a problem in Windows 3.0, all fixed segments are also page-locked, and can overburden the enhanced-mode virtual memory manager.

In Windows 3.1 with the Microsoft C/C++ and later compilers, the large model is the preferred method since the problem with Windows 3.0 has been fixed in Windows 3.1.

If an application requires the large memory model, a method can be used to unpage-lock the extra data segments of the DLL. For additional information on large-model programming, query on the following words in this Knowledge Base:

   large and model and protected

When compiling a DLL with the Microsoft C Compiler, specify the -Aw switch so that the code generator does not assume that the application's stack segment is equal to its data segment (SS != DS). The generated code will not automatically load DS at each module entry point. The -An switch should be used to specify near data, and the -As or -Al switch to specify near or far code, respectively.

When writing code in DLLs, it is important to remember that SS != DS. This is an issue when an application function passes a pointer to one of its local variables to the DLL function. In Microsoft C, all near pointers are assumed to be relative to DS. The way to avoid problems is to either:

• Use a function prototype declaring the parameter to be a FAR pointer.
• Explicitly cast the parameter to FAR.

The -Gw compiler switch generates the Windows prolog and epilog code. When a function is called, the prolog saves the current data segment on the stack. When memory moves in real mode, Windows "walks" the stack and changes the stored DS values to reflect the memory motion.

The prolog code is not required when a DLL will be used only in protected mode. There are two restrictions when choosing not to use the -Gw switch in protected mode:

1. Windows debuggers will not be able to properly provide stack trace information.

2. Exported functions must include the _loadds attribute in the declaration of the function. The following is an example declaration:

         BOOL FAR PASCAL _loadds ExportedFunction(VOID)
   

When exporting _cdecl functions in the module definition (DEF) file, one must maintain the case of the function name and use an underscore prefix. For example:

EXPORTS

   WEP                  @1  RESIDENTNAME
   _ExportedFunction    @2

DLLs can maintain data by using the local or global memory allocation routines. In most cases, memory allocated by GlobalAlloc is owned by the calling task. Global memory allocated by DLLs with GMEM_DDESHARE specified is owned by the DLL. When using local memory management functions, allocations are performed within the current data segment. Therefore, the handles returned from these calls are only valid when processed by the DLL.

Naming the segments within the DLL, keeping segment size less than 16K and specifying the segment options PRELOAD, LOADONCALL, MOVEABLE, and DISCARDABLE, as appropriate, will ease memory requirements and simplify the task of the memory manager.

There are three methods to access the functions in the DLL:

1. List the names of the functions in the IMPORTS section in the application's DEF file.

2. Link the application to an import library generated from the DLL's DEF file using the IMPLIB utility.

3. Use the LoadLibrary and GetProcAddress functions to load the library and retrieve a pointer to the function, respectively. This dynamic-dynamic linking method is the most powerful.

For additional information regarding linking DLL routines, please see page 178 of the Microsoft Visual C++ "C Language Reference".