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TitleMake a standard DLL
DescriptionThis example shows how to make a standard DLL in Visual Basic 6.
KeywordsDLL, ActiveX DLL
CategoriesActiveX, Windows
This examples builds a standard DLL that you can call by using the normal DLL calling conventions. For full details, see the article Creating a Windows DLL with Visual Basic.

Thanks to Luke Emmet for pointing this article out.

The basic staps are:

  1. Hack the linking process.
    1. Make an executable program to call the linker. Reomve the default Form1 and create the following Sub Main.
Public Sub Main()
   Dim SpecialLink As Boolean, fCPL As Boolean, fResource _
       As Boolean
   Dim intPos As Integer
   Dim strCmd As String
   Dim strPath As String
   Dim strFileContents As String
   Dim strDefFile As String, strResFile As String
   Dim oFS As New Scripting.FileSystemObject
   Dim fld As Folder
   Dim fil As File
   Dim ts As TextStream, tsDef As TextStream

   strCmd = Command
   Set ts = oFS.CreateTextFile(App.Path & "\lnklog.txt")
   ts.WriteLine "Beginning execution at " & Date & " " & _
   ts.WriteBlankLines 1
   ts.WriteLine "Command line arguments to LINK call:"
   ts.WriteBlankLines 1
   ts.WriteLine "   " & strCmd
   ts.WriteBlankLines 2
   ' Determine if .DEF file exists
   ' Extract path from first .obj argument
   intPos = InStr(1, strCmd, ".OBJ", vbTextCompare)
   strPath = Mid(strCmd, 2, intPos + 2)
   intPos = InStrRev(strPath, "\")
   strPath = Left(strPath, intPos - 1)
   ' Open folder
   Set fld = oFS.GetFolder(strPath)
   ' Get files in folder
   For Each fil In fld.Files
      If UCase(oFS.GetExtensionName(fil)) = "DEF" Then
         strDefFile = fil
         SpecialLink = True
      End If
      If UCase(oFS.GetExtensionName(fil)) = "RES" Then
         strResFile = fil
         fResource = True
      End If
      If SpecialLink And fResource Then Exit For
   ' Change command line arguments if flag set
   If SpecialLink Then
      ' Determine contents of .DEF file
      Set tsDef = oFS.OpenTextFile(strDefFile)
      strFileContents = tsDef.ReadAll
      If InStr(1, strFileContents, "CplApplet", _
          vbTextCompare) > 0 Then
         fCPL = True
      End If
      ' Add module definition before /DLL switch
      intPos = InStr(1, strCmd, "/DLL", vbTextCompare)
      If intPos > 0 Then
         strCmd = Left(strCmd, intPos - 1) & _
               " /DEF:" & Chr(34) & strDefFile & Chr(34) & _
                   " " & _
               Mid(strCmd, intPos)
      End If
      ' Include .RES file if one exists
      If fResource Then
         intPos = InStr(1, strCmd, "/ENTRY", vbTextCompare)
         strCmd = Left(strCmd, intPos - 1) & Chr(34) & _
             strResFile & _
                  Chr(34) & " " & Mid(strCmd, intPos)
      End If
      ' If Control Panel applet, change "DLL" extension to
      ' "CPL"
      If fCPL Then
         strCmd = Replace(strCmd, ".dll", ".cpl", 1, , _
      End If
      ' Write linker options to output file
      ts.WriteLine "Command line arguments after " & _
      ts.WriteBlankLines 1
      ts.WriteLine "   " & strCmd
      ts.WriteBlankLines 2
   End If
   ts.WriteLine "Calling LINK.EXE linker"
   Shell "linklnk.exe " & strCmd
   If Err.Number <> 0 Then
      ts.WriteLine "Error in calling linker..."
   End If
   ts.WriteBlankLines 1
   ts.WriteLine "Returned from linker call"
End Sub
This program does roughly the same thing that Visual Basic does when it creates a DLL except it adds the /DEF flag to the command.
  • Compile the executable.
  • Rename the normal Visual Basic linker from Link.exe to LinkLnk.exe. On my system, it's at C:\Program Files\Microsoft Visual Studio\VB98.
  • Copy the executable program that you compiled into this directory and name it Link.exe. When Visual Basic links the DLL, it calls this program, which calls the renamed LinkLnk.exe program, adding the new /DEF parameter.
  • Export the DLL's routines.
    1. Create a file named .def where is the name of the DLL. In this example, the DLL is named Fibonacci.dll so this file is called Fibonacci.def.
    2. Add code to this file similar to the following:
    NAME MathLib
    LIBRARY MathMod
    DESCRIPTION "Add-on Library of Mathematical Routines"
    EXPORTS DllMain @1
            Fibo @2
    This tells the linker about the main entry point DllMain and this example's function Fibo, both of which are created shortly.
  • Build the DLL.
    1. Create a new ActiveX DLL project.
    2. Leave the default Class1 class alone. You will not use it but Visual Basic needs it to it has something to compile into the ActiveX DLL.
    3. Add a code module and insert this code:
    Public Const DLL_PROCESS_DETACH = 0
    Public Const DLL_PROCESS_ATTACH = 1
    Public Const DLL_THREAD_ATTACH = 2
    Public Const DLL_THREAD_DETACH = 3
    Public Function DllMain(hInst As Long, fdwReason As Long, _
        lpvReserved As Long) As Boolean
       Select Case fdwReason
             ' No per-process cleanup needed
             DllMain = True
          Case DLL_THREAD_ATTACH
             ' No per-thread initialization needed
          Case DLL_THREAD_DETACH
             ' No per-thread cleanup needed
       End Select
    End Function
    ' Return a Fibonacci number.
    Public Function Fibo(ByVal N As Integer) As Long
        If N <= 1 Then
            Fibo = 1
            Fibo = Fibo(N - 1) + Fibo(N - 2)
        End If
    End Function
    DllMain is the DLL entry point. Fibo is a function that the DLL is exporting.
  • Compile the DLL. This should invoke the new Link.exe you built. If you look in that program's directory, you should see the log file it generates.
  • Build a test program to call the DLL.
    1. Make a standard Visual Basic EXE.
    2. Declare the routine exported by the DLL as in the following code:
    Private Declare Function Fibo Lib _
        "C:\WebSite\HowToSrc\a2\Fibonacci.dll" (ByVal N As _
        Integer) As Long
    Insert the path to the DLL on your computer.
  • Run the program.

    That should do it. Watch out for typos. If the .DEF file doesn't spell the function's name correctly, the DLL won't compile and the error messages are not very good.

    See the article mentioned at the beginning for more detail and some information about how the original author figured all this out.

    Copyright © 1997-2006 Rocky Mountain Computer Consulting, Inc.   All rights reserved.