National Instruments 320685D-01 Preparing Source Code for Use in a DLL, Calling Convention for Exported Functions

Chapter 3 Windows95/NT Compiler/Linker Issues

National Instruments Corporation 3-21 LabWindows/CVI Programmer Reference Manual

To customize an import library, you must have an include file that contains the declarations

of all the functions you want to export from the DLL. Load the include file into a Source

window, and execute the Generate DLL Import Source command in the Options menu.

After you have generated the import source, you can modify it, including making calls to

functions in other source files. Create a new project that contains the import source file and

any other files it refers to. Select Static Library from the submenu attached to the Target

command in the Build menu of the Project window. Execute the Create Static Library

command.

Note This import source code does not operate in the same way as a normal DLL import

library. When you link a normal DLL import library into an executable, the

operating system attempts to load the DLL as soon as the progr am starts. The

import source code LabWindows/CVI generates does not load the DLL until you

call one of the functions it exports.

Preparing Source Code for Use in a DLL

When you create a DLL, you must address the following issues that can affect your source

code and include file:

• The calling convention you use to declare the functions you want to export

• How you specify which DLL functions and variables you want to export

• Marking imported symbols in the DLL include file you distribute

This section discusses how you can address these issues when you create your DLL in

LabWindows/CVI. If you create your DLL in an external compiler, the approach is very

similar. The external compilers, however, do not agree in all aspects. This chapter also

discusses these differences.

Some of the information in this section is very technical and complex. Recommendations on

the best approaches to these issues are at the end of the section. These recommendations are

intended to make creating the DLL as simple as possible, and to make it easy to use the same

source code in LabWindows/CVI and the external compilers.

Calling Convention for Exported Functions

If you intend for only C or C++ programs to use your DLL, you can use the __cdecl or

Watcom stack-based calling convention to declare the functions you want to export. If,

however, you want your DLL to be callable from environments such as Microsoft Visual

Basic, you must declare the functions you want to export with the __stdcall calling

convention.

You must do this by explicitly defining the functions with the __stdcall keyword. This is

true whether or not you choose to make __stdcall the default calling convention for your