HOW TO: Determine When a Shelled Process Ends in Access 2000 (209876)

SUMMARY

Advanced: Requires expert coding, interoperability, and multiuser skills.



When you run the Shell() function in a Visual Basic for Applications procedure, it starts an executable program asynchronously and returns control to the procedure. This shelled program continues to run independently of your procedure until you close it.

If your procedure needs to wait for the shelled process to end, you can use the Windows API to poll the status of the application, but this is not very efficient. This article shows you a more efficient method.

Microsoft provides programming examples for illustration only, without warranty either expressed or implied. This includes, but is not limited to, the implied warranties of merchantability or fitness for a particular purpose. This article assumes that you are familiar with the programming language that is being demonstrated and with the tools that are used to create and to debug procedures. Microsoft support engineers can help explain the functionality of a particular procedure, but they will not modify these examples to provide added functionality or construct procedures to meet your specific requirements.

The Win32 API has integrated functionality that enables your application to wait until a shelled process has completed. To use these functions, you need to have a handle to the shelled process. To accomplish this, you need to use the CreateProcess() function instead of the Shell() function to begin your shelled program.

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Creating the Shelled Process

To create an addressable process, use the CreateProcess() function to start your shelled application. The CreateProcess() function gives your program the process handle of the shelled process via one of its passed parameters.

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Waiting for the Shelled Process to End

Once you use the CreateProcess() function to get a process handle, you can pass that handle to the WaitForSingleObject() function. This causes your Visual Basic for Applications procedure to suspend execution until the shelled process ends.

Below are the steps necessary to build a Visual Basic for Applications procedure that uses the CreateProcess() function to run the Windows Notepad (NOTEPAD.EXE) application. This code shows by example how to use the Windows API CreateProcess() and WaitForSingleObject() functions to wait until a shelled process ends before resuming execution.

The syntax of the CreateProcess() function is complex, so in the example code, it is encapsulated into a function called ExecCmd(). ExecCmd() takes one parameter, the command line of the application to execute.

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Step-by-Step Instructions on How to Create the CreateProcess() Procedure

1. Create a module and type the following lines in the Declarations section:

   Option Explicit
   
   Private Type STARTUPINFO
      cb As Long
      lpReserved As String
      lpDesktop As String
      lpTitle As String
      dwX As Long
      dwY As Long
      dwXSize As Long
      dwYSize As Long
      dwXCountChars As Long
      dwYCountChars As Long
      dwFillAttribute As Long
      dwFlags As Long
      wShowWindow As Integer
      cbReserved2 As Integer
      lpReserved2 As Long
      hStdInput As Long
      hStdOutput As Long
      hStdError As Long
   End Type
   
   Private Type PROCESS_INFORMATION
      hProcess As Long
      hThread As Long
      dwProcessID As Long
      dwThreadID As Long
   End Type
   
   Private Declare Function WaitForSingleObject Lib "kernel32" (ByVal _
      hHandle As Long, ByVal dwMilliseconds As Long) As Long
   
   Private Declare Function CreateProcessA Lib "kernel32" (ByVal _
      lpApplicationName As Long, ByVal lpCommandLine As String, ByVal _
      lpProcessAttributes As Long, ByVal lpThreadAttributes As Long, _
      ByVal bInheritHandles As Long, ByVal dwCreationFlags As Long, _
      ByVal lpEnvironment As Long, ByVal lpCurrentDirectory As Long, _
      lpStartupInfo As STARTUPINFO, lpProcessInformation As _
      PROCESS_INFORMATION) As Long
   
   Private Declare Function CloseHandle Lib "kernel32" (ByVal _
      hObject As Long) As Long
   
   Private Const NORMAL_PRIORITY_CLASS = &H20&
   Private Const INFINITE = -1&
   
   					

2. Type the following two procedures in the module:

   Public Sub ExecCmd(cmdline$)
      Dim proc As PROCESS_INFORMATION
      Dim start As STARTUPINFO
      Dim ReturnValue As Integer
   
      ' Initialize the STARTUPINFO structure:
      start.cb = Len(start)
   
      ' Start the shelled application:
      ReturnValue = CreateProcessA(0&, cmdline$, 0&, 0&, 1&, _
         NORMAL_PRIORITY_CLASS, 0&, 0&, start, proc)
   
      ' Wait for the shelled application to finish:
      Do
         ReturnValue = WaitForSingleObject(proc.hProcess, 0)
         DoEvents
         Loop Until ReturnValue <> 258
   
      ReturnValue = CloseHandle(proc.hProcess)
   End Sub
   
   Sub Testing()
      ExecCmd "NOTEPAD.EXE"
      MsgBox "Process Finished"
   End Sub
   					

3. Type Testing in the Debug window, and then press ENTER.
   
   Note that Notepad starts.
4. On the File menu, click Exit.
   
   Note that "Process Finished" appears in a message box in Microsoft Access.

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