National Instruments 320685D-01 Marking Imported Symbols in Include File Distributed with DLL

Chapter 3 Windows95/NT Compiler/Linker Issues

National Instruments Corporation 3-23 LabWindows/CVI Programmer Reference Manual

macro to resolve differences among compilers and platforms. The following are examples of

using the DLLEXPORT macro:

int DLLEXPORT DLLSTDCALL MyFunc (int parm) {}

int DLLEXPORT myVar = 0;

If the type of your variable or function requires an asterisk (*) in the syntax, put the qualifier

after the asterisk, as in the following example:

char * DLLEXPORT myVar = NULL;

Note Borland C/C++ version 4.5x, requires that you place the qualifier before the

asterisk. In Borland C/C++ 5.0, you can place the qualifier on either side of the

asterisk.

When LabWindows/CVI creates a DLL, it exports all symbols for which export qualifiers

appear in either the definition or the declaration. If you use an export qualifier on the

definition and an import qualifier on the declaration, LabWindows/CVI exports the symbol.

The external compilers differ widely in their behavior on this point. Some require that the

declaration and definition agree.

Note If you include in your DLL project an object or library file that defines exported

symbols, LabWindows/CVI cannot correctly create import libraries for each of the

external compilers. This problem does not arise if you use only source code files

in your DLL project.

Marking Imported Symbols in Include File Distributed with DLL

If your DLL might be used in a C or C++ environment, you must distribute an include file

with your DLL. The include file must declare all the symbols the DLL exports. If any of these

symbols are variables, you must mark them with an import qualifier. Variable declarations

require import qualifiers so that the compiler can generate the correct code for accessing the

variables.

You can use import qualifiers on function declarations, b ut the y are not necessary. When you

use an import qualifier on a function declaration, external compilers can generate slightly

more efficient code for calling the function.

Using import qualifiers in the include file you distribute with your DLL can cause problems

if you use the same include file in the DLL source code:

• If you mark variable declarations in the include file with import qualifiers and you use

the include file in a source file other than the one in which you define the variable,

LabWindows/CVI and the external compilers treat the variable as if it were imported

from another DLL and generate incorrect code as a result.

• If you use export qualifiers in the definition of symbols and the include file contains

import qualifiers on the same symbols, some external compilers report an error.