How Visual Basic 4.0 Calls C and Fortran DLLs

ID: Q161825

The information in this article applies to:

Microsoft Visual Basic Standard, Professional, and Enterprise Editions for Windows, version 4.0
Microsoft Visual C++, 32-bit Editions, versions 2.0, 2.1, 2.2, 4.0, 4.1, 4.2, 6.0
Microsoft Fortran Powerstation 32 for Windows NT

SUMMARY

This sample demonstrates how a Visual Basic 4.0 application calls Fortran and Visual C DLL's. Specifically, it shows how to pass fixed length strings to and from those DLL's.

MORE INFORMATION

The following file is available for download from the Microsoft Download Center. Click the file name below to download the file:

Vbstring.exe

Release Date: Jan-01=1997

For more information about how to download files from the Microsoft Download Center, please visit the Download Center at the following Web address

http://www.microsoft.com/downloads/search.asp

and then click How to use the Microsoft Download Center.

Visual Basic 4.0 replaced the string management system used by previous versions of Visual Basic with a more robust string management system. Visual Basic 4.0 relies on the OLE automation data types, such as BSTR and safe arrays, to manage string usage. However, passing arrays of numeric types, integers, or reals were not changed. For instance, before passing an array of strings to a DLL, the strings must be converted to an array of bytes, and then converted back to strings on return from the DLL. Please, refer to VB4DLL.TXT that ships with Visual Basic 4.0 for more information.

All new C DLL's should use the syntax described in the VB4DLL.TXT file because the API calls that are employed allow for improved error handling. The programmer can do something in the event of an error, such as display a message box and abort the operation.

Please note that the new Fortran DLL's cannot use the syntax described in VB4DLL.TXT because they have no direct access to the correct API functions.

The sample also demonstrates different methods you can use to resolve the function symbol within the DLL. A mismatch in the function symbol results in a Visual Basic runtime error, "Specified DLL function not found (Error 453)." The sample includes two methods to resolve C function symbols and three methods to resolve Fortran function symbols.

Each source file contains more information pertinent to that language.

This sample also enables you to choose whether the data is passed to either the C or Fortran DLL.

This sample shows how to pass the following:

Two dimensional array of 4-byte integers.
Two dimensional array of 8-byte floating point numbers.
String.
One dimensional array of strings.
Two dimensional array of strings.
One dimensional array of 4-byte integers, a one dimensional array of 8-byte floating point numbers, and a one dimensional array of strings.

Sample Files



    FileName             Description
    -------------------------------------------------------

    cdll.c               C source file to build 32-bit DLL
    fordll.for           Fortran source file for FPS NT 1.0 reference
    fordll32.f90         Fortran source file for the 32-bit DLL
    vbstring.vbp         Visual Basic 4.0 project file
    vbstring.frm         Visual Basic form including event handlers

Building DLLs

This sample assumes you are using the following development environments:

Microsoft Visual C++, version 4.0
Microsoft FORTRAN PowerStation for Windows 95 and Windows NT, version 4.0

You will need to setup the environment before running BUILDDLL.BAT.

At an MS-DOS command-line prompt type:


 C:\MSDEV\BIN\VCVARS32.BAT

Build the C DLL. At the command-line prompt type:


 BUILDDLL C

Build the Fortran DLL. At the same prompt type:


 BUILDDLL Fortran

Start Visual Basic 4.0 and open the project file VBSTRING.VBP.

Run the Visual Basic application by creating an executable file (.exe)or running the program from within Visual Basic.

IMPORTANT: The DLLs must be in the \WINDOWS\SYSTEM directory, the directory containing the Visual Basic executable file, or one of the environment file paths.

Notes on Arrays in Different Languages

Given an upper bound of n, note that in Visual Basic all of the array ranges are specified as "1 to n". When a lower bound is not specified, Visual Basic, by default, assumes 0<=index<=n where "n" is inclusive. C array subscripts are assumed to be in the range 0<=index<=n-1. Fortran array ranges are 1<=index<=n. You need to be careful when you assign the index ranges, and make sure that they match. If you exceed them, you will probably get an Access Violation, an Unhandled Exception, or a random run-time error in your Visual Basic Application.



Language     Declaration                Range         # Items in Array
--------     -----------                -----         ----------------

Basic        Dim X(10) As Integer       0<=index<=10         11
Basic*       Dim X(1 to 10) As Integer  1<=index<=10         10
C            int X[10];                 0<=index<=9          10
Fortran      integer X(10)              1<=index<=10         10
Fortran*     integer X(0:9)             0<=index<=9          10

* With lower bound specified. C is always zero-based.

REFERENCES

For information on passing Strings from Visual Basic 5.0 or Visual Basic 6.0 to C DLLs, please see the following article in the Microsoft Knowledge Base:

Q187912 HOWTO: Pass a String Between Visual Basic and Your C DLL

Additional query words: kbstream

Keywords : kbfile kbsample kbCompiler kbFL32 kbFortranPS kbLangFortran vbwin
Version : WINDOWS:4.0; winnt:2.0,2.1,2.2,4.0,4.1,4.2,6.0; :
Platform : WINDOWS winnt
Issue type : kbinfo