ACC: How to Determine When a Shelled Process Ends (178116)

SUMMARY

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.

MORE INFORMATION

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.

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.

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.

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

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&

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

Type "Testing" (without the quotation marks) in the Debug window, and then press Enter.

Note that Notepad starts.
On the File menu, click Exit.

Note that "Process Finished" appears in a message box in Microsoft Access.