Calling an FTN95 DLL from Visual Basic

An FTN95 DLL that is called by an executable that uses the STDCALL calling convention (e.g. one created using Win32 Visual Basic) must use F_STDCALL in the declaration of all exported subprograms. For example,

F_STDCALL FUNCTION F(X)
INTEGER F,X
F=X
END

Such a function could make calls to Windows API functions provided an interface is provided via, for example, "USE MSWIN" or "INCLUDE <windows.ins>". Alternatively an interface for each API function can be given explicitly. For example,

STDCALL SENDMESSAGE 'SendMessageA' (VAL,VAL,VAL,VAL):INTEGER*4

A call is automatically made to a DLL function called LIBMAIN when the DLL is loaded. If you do not provide a definition of LIBMAIN then SLINK will provide a default for you. LIBMAIN is used to initialise global data. It takes the following form.

F_STDCALL INTEGER FUNCTION LIBMAIN(hInst,ul,lpR)
INTEGER hInst,ul,lpR
!***** Initialise global data here
LIBMAIN=1
END

A simple SLINK script would take the form:

dll
lo f95.obj
exportall
file c:\windows\f95.dll

A Visual Basic program can use calls to the API functions LoadLibrary and FreeLibrary in order to ensure that the DLL does not unload whilst a Visual Basic form is loaded. Here is some sample code for this purpose.

Private Declare Function LoadLibrary Lib "kernel32" Alias "LoadLibraryA" (ByVal s As String) As Long
Private Declare Function FreeLibrary Lib "kernel32" (h As Long) As Long
Dim hLibModule As Long

Private Sub Form_Load()
hLibModule = LoadLibrary("f95.dll")
End Sub

Private Sub Form_Unload(Cancel As Integer)
i& = FreeLibrary(hLibModule)
End Sub

The FTN95 DLL is assumed to reside in the Windows system directory.