Fortran/Visual Basic Calling Conventions

You establish Fortran subroutines and functions in Visual Basic forms and the Fortran routines are then invoked from a Basic module. A Fortran routine has to be a DLL (dynamic-link library) to be called from Basic. For more information on DLLs, see Fortran Dynamic-Link Library Projects and Creating Fortran DLLs.

The calling-convention ALIAS directive (or ATTRIBUTES property ALIAS) is needed if mixed-case names are to be preserved (by default Fortran translates names to all uppercase). However, two special cases require different treatment:

• If a varying number of arguments are to be passed, the C and VARYING properties are needed in the Fortran procedure definition and the CDECL keyword needed in the Basic DECLARE statement in order to establish the C calling and naming convention.
• When passing character arguments, the Fortran routine must not pass the hidden length of character arguments, such as by using the ATTRIBUTES property STDCALL for the routine and specifying the REFERENCE property for any character arguments. Since STDCALL also lowercases Fortran names, the Fortran subprogram name should be referenced in lowercase from the Visual Basic program.

The following Fortran and Visual Basic statements establish an example Fortran function to be called from Basic:

!  Fortran Subprogram establishing Fortran function.
       INTERFACE
          DOUBLE PRECISION FUNCTION GetFVal (r1)
             !DEC$ ATTRIBUTES ALIAS:'GetFVal' :: GetFVal
             !DEC$ ATTRIBUTES VALUE :: r1
             REAL r1
          END FUNCTION
       END INTERFACE

'FORM.FRM Basic Form to establish Fortran function
Declare Function GetFVal Lib "C:\f90\FVAL.DLL" (ByVal r1 As Single) As Double