National Instruments 320685D-01 Recommendations, Automatic Inclusion of Type Library Resource for Visual Basic

Chapter 3 Windows95/NT Compiler/Linker Issues

LabWindows/CVI Programmer Reference Manual 3-24

National Instruments Corporation

You can solve these problems in several different ways:

• You can avoid exporting variables from DLLs, and thereby eliminate the need to use

import qualifiers. For each variable you want to export, you can create functions to

get and set its value or a function to return a pointer to the variable. You do not have

to use import qualifiers for functions. This is the simplest approach and works in

LabWindows/CVI. However, it does not work if you use an export qualifier in a

function definition and you create the DLL with an external compiler that requires

the declaration to use the same qualifier.

• You can create a separate include file for distribution with the DLL.

• You can use a special macro that resolves to either an import or export qualifier

depending on a conditional compilation flag. In LabWindows/CVI you can set the flag

in your DLL project by using the Compiler Defines command in the Options menu of

the Project window.

Recommendations

To make creating a DLL as simple as possible, adhere to the following recommendations:

•Use the DLLSTDCALL macro in the declaration and definition of all functions you want

to export. Do not export functions with a variable number of arguments.

• Identify the symbols you want to export using th e include file met hod. Do not use expo rt

qualifiers. If you use an external compiler, use the .def file method.

• Do not export variables from the DLL. For each variable you want to export, create

functions to get and set its value or a function to return a pointer to the variable. Do not

use import qualifiers in the include file.

If you follow these recommendations, you reap the following benefits:

• You can distribute with your DLL the same include file that you include in the source files

you use to make the DLL. This is especially useful when you create DLLs from

instrument drivers.

• You can use the same sour ce code to create the DLL in Lab Wind ows/CVI and an y of the

four compatible external compilers.

• You can use your DLL in Microsoft Visual Basic or other non-C environments.

Automatic Inclusion of Type Library Resource for Visual Basic

The Create Dynamic Link Library command gives you the option to automatically create

a Type Library resource and include it in the DLL. When you use this option, Visual Basic

users can call the DLL without having to use a header file that contains Declare statements

for the DLL functions. The command requires that you have a function panel file for

yourDLL.