vb make api

Posted on 2006-06-25
Medium Priority
Last Modified: 2008-01-09
can i create api enabled dll using vb?
Question by:maratmu
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Hi Dan,

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Creating a Windows DLL with Visual Basic
by Ron Petrusha

As the first rapid application development language, Visual Basic attracted attention for its elegant graphical interface and overall ease of use, which allowed a relatively inexperienced programmer to accomplish in minutes what often took days for advanced programmers using languages like C and C++. As a result, Visual Basic drew millions of new programmers, many of whom might never have even considered programming had it not been for the language's simplicity. Because of this simplicity, and because Visual Basic was attracting a following that the proponents of other languages could only dream of, non-Visual Basic programmers (who were really green with envy) counterattacked by pointing to the inexperience of most Visual Basic programmers and to the problems that stem from Visual Basic's design goal of shielding the developer from the complexities of the underlying operating system. To bolster their contention that Visual Basic is underpowered and underdeveloped, critics liked to point to the many things "real" programmers do that Visual Basic programmers cannot. Perhaps the most common limitation that critics continually point to is Visual Basic's inability to create a standard Windows dynamic link library (DLL).

Certainly it's true that out of the box, Visual Basic doesn't allow you to create a Windows DLL in the same way that you can create other project types, like a Standard EXE or an ActiveX DLL. In this article, we'll go exploring to see how Visual Basic generates its executables. In the process, we'll discover that with a little bit of extra work, we can in fact create Windows DLLs with Visual Basic.
What Is a Windows Dynamic Link Library?

A dynamic link library (DLL) is a library of functions and procedures that can be called from an application or another DLL. Using a library in this way has two major functions:

    * It permits the sharing of code. The same DLL can be used by many other DLLs and applications. The Win32 API, for instance, is implemented as a series of Windows DLLs. Moreover, as long as multiple processes load the same DLL at the same base address, they can share the code in the DLL. In other words, a single DLL in memory can be accessed by multiple processes.
    * It allows for component-based and modular development, which makes development and the upgrade process easier.

Ordinarily, when a static library is used in application development, the library's modules must be linked into the finished application. With dynamic linking, the modules reside in a separate DLL file that is loaded dynamically, either when the application loads or when its member functions are needed.

A dynamic link library may include internal functions, which can be called only from within the DLL. Its main purpose, however, is to provide exported functions--that is, functions that reside in a module of the DLL and can be called from other DLLs and applications. Frequently, a definition (.def) file is used in C/C++ projects to list a DLL's exports.

A DLL also includes an optional entry point, which is called when a process or thread loads or unloads the DLL. Windows calls this entry point when a process loads and unloads the DLL. It also calls the entry point when the process creates or terminates a thread. That allows the DLL to perform any per-process and per-application initialization and cleanup. The syntax of this entry point, which must use the standard-call calling convention (used by default in Visual Basic), is:

Public Function DllMain(hinstDLL As Long, fdwReason As Long,
   lpwReserved As Long) As Boolean

Its parameters are:
hInstDLL, a Long containing the instance handle of the DLL. This is the same as the DLL's module handle.

fdwReason, a constant indicating why the entry point has been called. Possible values are:

      A process is loading the DLL. Any per-process initialization should be performed.
      The process is spawning a new thread. Any per-thread initialization should be performed.
      A thread is exiting. Any per-thread cleanup should be performed.
      A process is detaching the DLL, or the process is exiting. Any per-process cleanup should be performed.
    * lpvReserved
      A Long that provides more information about DLL_PROCESS_ATTACH and DLL_PROCESS_DETACH. (It is unused if fdwReason is DLL_THREAD_ATTACH or DLL_THREAD_DETACH.) If fdwReason is DLL_PROCESS_ATTACH, it is Nothing for libraries loaded dynamically using functions like LoadLibrary and GetProcAddress, and it is not Nothing for libraries loaded statically by providing library stubs at compile time. If fdwReason is DLL_PROCESS_DETACH, it is Nothing if the call has resulted from a call to the Win32 FreeLibrary function, and it is not Nothing if the entry point is called during process termination.

The return value of the function is meaningful only if fdwReason is DLL_PROCESS_ATTACH. If initialization succeeds, the function should return True; otherwise, it should return False. Note that because the function is an entry point called by Windows, the values of the arguments passed to the function are provided by Windows. Also, the entry point is not called when a thread is terminated using the Win32 TerminateThread function, nor is it called when a process is terminated using the Win32 TerminateProcess function.
The DLL Code

In attempting to develop a Windows DLL, we'll create a very simple library of math functions. The following is the DLL's code, which we'll store in a code module (a .bas file) named MathLib:

Option Explicit

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
         ' No per-thread initialization needed
         ' No per-thread cleanup needed
   End Select
End Function

Public Function Increment(var As Integer) As Integer
   If Not IsNumeric(var) Then Err.Raise 5
   Increment = var + 1
End Function

Public Function Decrement(var As Integer) As Integer
   If Not IsNumeric(var) Then Err.Raise 5
   Decrement = var - 1
End Function

Public Function Square(var As Long) As Long
   If Not IsNumeric(var) Then Err.Raise 5
   Square = var ^ 2
End Function

Several characteristics about the code are worth mentioning. The first is that although it includes a DllMain procedure, no per-process or per-thread initialization needs to be performed. So DllMain simply returns True if it is called with the fdwReason argument set to DLL_PROCESS_ATTACH.

Second, the point of providing a Windows DLL is to allow other languages to call it. To ensure interoperability, we want to confine ourselves to language features that the Win32 API supports, so that our DLL can be called from as many development environments and platforms as possible. We could have made each of our three math functions more flexible, for example, by defining both the incoming argument and the return value as Variants. That would have allowed the function to determine the data type it should interpret the incoming data as, in addition to the data type it should return. But the Variant is a data type defined by COM, Microsoft's Component Object Model, and is not a data type Win32 API recognizes. So instead, the code uses standard Win32 API data types.

We'll also need a test program to tell us whether our Windows DLL is working properly. For that purpose, we can create a Standard EXE project with one form and one code module. The code module simply consists of the Declare statements that define the functions found in the DLL:

Public Declare Function Increment Lib "MathLib.dll" (var As Integer) As Integer

Public Declare Function Decrement Lib "MathLib.dll" (var As Integer) As Integer

Public Declare Function Square Lib "MathLib.dll" (var As Long) As Long

Rather than simply specifying the name of the DLL in the Lib clause, you also should add the full path to the directory that contains the DLL.

The form's code performs the calls to the DLL functions:

Option Explicit

Dim incr As Integer
Dim decr As Integer
Dim sqr As Long

Private Sub cmdDecrement_Click()
   decr = Increment(decr)
   cmdDecrement.Caption = "x = " & CStr(decr)
End Sub

Private Sub cmdIncrement_Click()
   incr = Increment(incr)
   cmdIncrement.Caption = "x = " & CStr(incr)
End Sub

Private Sub cmdSquare_Click()
   sqr = Square(srr)
   cmdSquare.Caption = "x = " & CStr(sqr)
End Sub

Private Sub Form_Load()
   incr = 1
   decr = 100
   sqr = 2
End Sub


The ActiveX DLL Project Type

Let's begin by creating an ActiveX DLL project and seeing what happens if we try to call it as if it were a standard Windows DLL. When you create an ActiveX DLL project, Visual Basic automatically adds a class module (a .cls file) to it. You can rename this if you want, but don't include any code. Instead, add a code module (a .bas file) to the project, add the DLL's code, and then compile the DLL. When you run the DLL test application, the error message dialog shown in Figure 1 appears. The error message indicates that although the DLL was found, the specific called function (Increment) was not.

Figure 1
Figure 1. Error when accessing an ActiveX DLL as a Windows DLL
The most likely cause of this error is that the function is not actually exported by the DLL. We can use the DumpBin utility to examine a DLL's exports by using the syntax

Dumpbin <path and name of dll> /exports

If we run DumpBin using this syntax, we see the following output:

Microsoft (R) COFF Binary File Dumper Version 6.00.8447
Copyright (C) Microsoft Corp 1992-1998. All rights reserved.

Dump of file mathlib.dll

File Type: DLL

  Section contains the following exports for MathLib.dll

           0 characteristics
    41B9E52C time date stamp Fri Dec 10 10:04:28 2004
        0.00 version
           1 ordinal base
           4 number of functions
           4 number of names

    ordinal hint RVA      name

          1    0 0000192E DllCanUnloadNow
          2    1 00001902 DllGetClassObject
          3    2 00001918 DllRegisterServer
          4    3 000018EC DllUnregisterServer


        1000 .data
        1000 .reloc
        1000 .rsrc
        1000 .text

Our DLL exports four functions, all of which are utility functions that support COM. Clearly we need to export DllMain and our three math functions. But how? Visual Basic does not appear to allow you to export DLL functions from ActiveX DLLs, thus effectively preventing you from using Visual Basic to create a standard Windows DLL.

This difficulty, however, is not insurmountable. When we select the File -> Make <filename>.dll menu option to create an ActiveX DLL, it appears that Visual Basic is seamlessly taking our source code and outputting an ActiveX DLL. But if we examine the subdirectory in which Visual Basic was installed, it appears that the process is not quite so seamless. Along with VB6.EXE, the Visual Basic executable that defines the Visual Basic environment, we can also find C2.EXE and LINK.EXE, which are a compiler and a linker, respectively. Their presence in this directory suggests that VB6.EXE itself does not handle the generation of a DLL file, but that at some point in the compilation process, it calls these programs.

We can find out how Visual Basic is using the compiler and linker more precisely by renaming them and creating wrapper executables named C2 and LINK that in turn call the real compiler and linker. The following is the source code for a new version of a console-mode C2.EXE that calls the "real" C2 compiler, which we've renamed C2comp.exe:

Public Sub Main()

On Error Resume Next

   Dim strCmd As String, strPath As String
   Dim oFS As New Scripting.FileSystemObject
   Dim ts As TextStream

   strCmd = Command
   strPath = App.Path
   Set ts = oFS.CreateTextFile(strPath & "\c2log.txt")
   ts.WriteLine "Beginning execution at " & Date & " " & Time()
   ts.WriteBlankLines 1
   ts.WriteLine "Command line parameters to c2 call:"
   ts.WriteLine "   " & strCmd
   ts.WriteBlankLines 1
   ts.WriteLine "Calling C2 compiler"
   Shell "c2comp.exe " & strCmd
   If Err.Number <> 0 Then
      ts.WriteLine "Error in calling C2 compiler..."
   End If
   ts.WriteBlankLines 1
   ts.WriteLine "Returned from c2 compiler call"
End Sub

The process of compiling an ActiveX DLL produces the following output in our log file:

Beginning execution at 12/10/2004 12:44:22 PM

Command line parameters to c2 call:
   -il "C:\DOCUME~1\Ron\LOCALS~1\Temp\VB277103" -f "C:\VB Projects\
   MathLib\MathMod.bas" -W 3 -Gy -G5 -Gs4096 -dos -Zl -Fo"C:\
   VB Projects\MathLib\MathMod.OBJ" -QIfdiv -ML -basic

Calling C2 compiler

Returned from c2 compiler call

These are fairly standard command-line arguments to produce object files that in turn are supplied to the linker. That means that to determine how to produce a Windows DLL, we'll have to intercept the call to the linker so that we can see what arguments Visual Basic passes to it. The following code does that:

Public Sub Main()

On Error Resume Next

   Dim strCmd As String, strPath As String
   Dim oFS As New Scripting.FileSystemObject
   Dim ts As TextStream

   strCmd = Command
   strPath = App.Path
   Set ts = oFS.CreateTextFile(strPath & "\lnklog.txt")
   ts.WriteLine "Beginning execution at " & Date & " " & Time()
   ts.WriteBlankLines 1
   ts.WriteLine "Command line parameters to LINK call:"
   ts.WriteLine "   " & strCmd
   ts.WriteBlankLines 1
   ts.WriteLine "Calling LINK 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

It requires that we rename the linker LinkLnk.exe and name our link wrapper Link.exe.

When we attempt to compile an ActiveX DLL project, our linker log file contains the following output:

Beginning execution at 12/11/2004 12:44:33 PM

Command line parameters to LINK call:
   "C:\Program Files\Microsoft Visual Studio\VB98\Class1.OBJ"
   "C:\Program Files\Microsoft Visual Studio\VB98\Project1.OBJ"
   "C:\Program Files\Microsoft Visual Studio\VB98\VBAEXE6.LIB"
   /OUT:"C:\Program Files\Microsoft Visual Studio\VB98\Project1.dll"
   /INCREMENTAL:NO /OPT:REF /MERGE:.rdata=.text /IGNORE:4078

Calling LINK linker

Returned from linker call

If we compare these command-line arguments with the syntax required to link the object files for a DLL using either C or C++, an omission becomes immediately apparent. Although the /DLL switch is supplied to create a standard DLL, there is no /DEF switch to define a module definition (.def) file that lists the functions exported by the DLL. (If we were programming in C or C++, we could use statements within our code to define our exports. Visual Basic doesn't support this, however, making the .def file the sole means of defining a library's exports.) Moreover, if we examine the files generated for an ActiveX DLL project by the Visual Basic environment, we'll also find that Visual Basic itself has not generated a .def file.


Creating the Windows DLL

So, after examining an ActiveX DLL's export table, intercepting Visual Basic's call to the compiler, intercepting Visual Basic's call to the linker, and comparing the arguments passed to the linker with those required by a C/C++ compiler to generate a Windows DLL, we've finally identified why we aren't able to successfully create a Windows DLL with Visual Basic. And fortunately, we can work around that restriction. We should be able to create a standard Windows DLL if we do the following:


      Create a .def file for our project. We can specify our exported functions in the .def file in several ways, but it's best to keep it simple:

      NAME MathLib
      LIBRARY MathMod
      DESCRIPTION "Add-on Library of Mathematical Routines"
      EXPORTS DllMain @1
              Increment @2
              Decrement @3
              Square @4

      The NAME statement defines the name of the DLL. The LIBRARY statement must either precede the list of exported functions or appear on the same line as the first function. The .def file should also list the ordinal position of each exported function preceded by an @ symbol.

      Decide how we want to intercept the call to the linker. Two major techniques are available to do this:

            Patching the Import Address Table (IAT), which requires that we build a Visual Basic add-in that modifies the IAT in order to intercept particular calls by Visual Basic to the Win32 API. Although it's certainly the most elegant method, its complexity makes it a worthy subject for a separate article.

            Building a proxy linker that intercepts the call to the real linker, modifies the command-line arguments to be passed to the linker, and then calls the linker with the correct command-line arguments. This is the approach we used to discover what arguments Visual Basic was passing to the compiler and linker, and it's the approach we'll adopt to create a Windows DLL.

      In building our proxy linker, we want a sufficiently flexible design so that we can generate other kinds of files, if need be.

      Modify the arguments to the linker to add the /DEF switch along with the path and filename of our .def file. To do this, you must create a Visual Basic Standard EXE project, add a reference to the Microsoft Scripting Runtime Library, remove the form from the project, and add a code module. The source code for the proxy linker is as follows:

      Option Explicit

      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 & " " & Time()
         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, , vbTextCompare)
            End If
            ' Write linker options to output file
            ts.WriteLine "Command line arguments after modification:"
            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 proxy linker modifies only the command-line arguments passed to the linker if a .def file is present in the directory that contains the Visual Basic project; otherwise it simply passes the command-line arguments on to the linker unchanged. If a .def file is present, it adds a /DEF switch to the command line. It also determines whether any resource files are to be added to the linked file list. Finally, it examines the export table to determine if a function named CplApplet is present; if it is, it changes the output file's extension from .dll to .cpl.

      To install the proxy linker, rename the original Visual Basic linker LinkLnk.exe, copy the proxy linker to the Visual Basic directory, and name it Link.exe.

Once we create our proxy linker, we can reload our MathLib project and compile it into a DLL by selecting the Make MathLib.exe option from the File menu.
Testing the DLL

Once we create our Windows DLL, the final step is to test it to make sure that it works. To do this, create a new Standard EXE project (let's call it MathLibTest) and add a code module. To make sure that code in our project can access the functions exported by the DLL, we use the standard Visual Basic Declare statement. We declare our three exported math routines in the code module as follows:

Option Explicit

Public Declare Function Increment Lib "C:\VBProjects\MathLib\mathlib.dll" ( _
                        value As Integer) As Integer
Public Declare Function Decrement Lib "C:\VBProjects\MathLib\mathlib.dll" ( _
                        value As Integer) As Integer
Public Declare Function Square Lib "C:\VBProjects\MathLib\mathlib.dll" ( _
                        value As Long) As Long

We can then use the following code in the form module to call the routines in the DLL:

Option Explicit

Private Sub cmdDecrement_Click()
   txtDecrement.Text = Decrement(CInt(txtDecrement.Text))
End Sub

Private Sub cmdIncrement_Click()
   txtIncrement.Text = Increment(CInt(txtIncrement.Text))
End Sub

Private Sub cmdSquare_Click()
   txtSquare.Text = Square(CLng(txtSquare.Text))
End Sub

Private Sub Form_Load()
   txtIncrement.Text = 0
   txtDecrement.Text = 100
   txtSquare.Text = 2
End Sub

When we call each of the MathLib functions, the application window might appear as it does in Figure 2, confirming that the calls to the MathLib routines work as expected.

Figure 2
Figure 2: Testing calls to MathLib.dll

Ron Petrusha is the author and coauthor of many books, including "VBScript in a Nutshell."


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