The information in this article applies to:
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.
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.
large and model and protectedHowever, if the language compiler used supports the large memory model with a single data segment, this method is not required. 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:
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:
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:
A DLL is limited to one instance. The DATA SINGLE or DATA NONE
statements can be used in the DEF file to specify that the DLL should
have a data segment of its own or not. Because a DLL does not have a
stack of its own, automatic variables are allocated from the stack of
the calling application.
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:
For additional information regarding linking DLL routines, please see page 178 of the Microsoft Visual C++ "C Language Reference". Additional query words: 3.00 3.10
Keywords : kb16bitonly |
Last Reviewed: November 2, 1999 © 2000 Microsoft Corporation. All rights reserved. Terms of Use. |