FILE: How Visual Basic 4.0 Calls C and Fortran DLLsLast reviewed: January 16, 1998Article ID: Q161825 |
The information in this article applies to:
SUMMARYThis 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. The following file is available for download from the Microsoft Software Library:
~ Vbstring.exe (size: 25888 bytes)For more information about downloading files from the Microsoft Software Library, please see the following article in the Microsoft Knowledge Base:
ARTICLE-ID: Q119591 TITLE : How to Obtain Microsoft Support Files from Online Services MORE INFORMATIONVisual 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 a:
Sample Files
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 DLLsThis sample assumes you are using the following development environments:
Notes on Arrays in Different LanguagesGiven 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. Keywords : APrgOther CLIss FL32Iss FORTLngIss vbwin kbsample Technology : kbole Version : WINDOWS:4.0 Platform : NT WINDOWS |
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