National Instruments 320685D-01 Recognizing Windows Messages Passed from a DLL , Creating 16-bit DLLs with Microsoft Visual C++ 1.5

Chapter 4 Windows3.1 Compiler/Linker Issues

National Instruments Corporation 4-21 LabWindows/CVI Programmer Reference Manual

Recognizing Windows Messages Passed from a DLL

The normal way of communicating with a DLL is to call functions in the DLL. However,

cases exist where other communication methods are necessary. The most typical case is that

of an interrupt service routine in a DLL that must notify the application that the interrupt

occurred. In cases like this, you must communicate with the DLL through a callback function.

LabWindows/CVI recognizes messages posted by a DLL through the Windows SDK

functionPostMessage, and can initiate a user callback function. This method is useful

forhardware interrupts, but it is subject to the latency associated with Windows messaging.

LabWindows/CVI uses RegisterWinMsgCallback, UnRegisterWinMsgCallback, and

GetCVIWindowHandle to recognize Windows messages from a DLL. You can call these

functions from a module compiled in Watcom or from source code.

For complete information on these functions, refer to the function descriptions in Chapter 4,

User Interface Library Reference, of the LabWindows/CVI User Interface Reference Manual.

To use these functions, call RegisterWinMsgCallback and GetCVIWindowHandle. Pass

their return values, the message number and the window handle, to the DLL. When the DLL

sends a message, it calls PostMessage with these values. When LabWindows/CVI receives

the message, it calls the callback function.

Note LabWindows/CVI can receive the message only when it is processing events.

LabWindows/CVI processes events when it is waiting for user input. If the

program you run in LabWindows/CVI does not call RunUserInterface,

GetUserEvent, or scanf, or if it does not return from a User Interface Library

callback, events will not be processed. You can remedy this in the program by

periodically calling the User Interface Library function ProcessSystemEvents.

Creating 16-bit DLLs with Microsoft Visual C++ 1.5

Be sure to consider the following issues or project options when you create a DLL with

Microsoft Visual C++ 1.5:

• Every function you call fr om out side the DLL must be far, exported, and must load the

data segment into the DS register. The function must load the DS register if you want to

use any non-local variables in a function.

• Use the large or huge memory model. The savings you gain by using smaller memory

models is not worth having to use the far keyword throughout your code. This project

option is in Compiler»Memory Model»Segment Setup.

• You can make the compiler load the data segment into the DS register automatically by

using the SS!=DS, DS loaded on function entry project option in Compiler»Memory

Model»Segment Setup.

• If you try to use the optimize entry code option (/GD), by selecting

Compiler»Windows»Prolog/Epilog»Generate Prolog/Epilog For, it conflicts with the