National Instruments Drums 320685D-01 ユーザーズマニュアル

Chapter 3

Windows 95/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.

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 the include file method. Do not use export 
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 source code to create the DLL in LabWindows/CVI and any of the 
four compatible external compilers.

•

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

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 
your DLL. 

00ProRef.book : 06chap03.fm  Page 24  Monday, March 9, 1998  3:23 PM