windows - How to wait for a shell process to finish before executing further code in VB6

Question

I have a small VB6 app in which I use the Shell command to execute a program. I am storing the output of the program in a file. I am then reading this file and putting the output on the screen using a msgbox in VB6.

This is what my code looks like now:

sCommand = "evaluate.exe<test.txt "
Shell ("cmd.exe /c" & App.Path & sCommand)

MsgBox Text2String(App.Path & "experiments" & genname & "freq")

The problem is that the output which the VB program is printing using the msgbox is the old state of the file. Is there some way to hold the execution of the VB code until my shell command program finishes so that I get the correct state of the output file and not a previous state?

Answer 1

The secret sauce needed to do this is the WaitForSingleObject function, which blocks execution of your application's process until the specified process completes (or times out). It's part of the Windows API, easily called from a VB 6 application after adding the appropriate declaration to your code.

That declaration would look something like this:

Private Declare Function WaitForSingleObject Lib "kernel32" (ByVal hHandle _
                            As Long, ByVal dwMilliseconds As Long) As Long

It takes two parameters: a handle to the process that you want to wait on, and the time-out interval (in milliseconds) that indicates the maximum amount of time that you want to wait. If you do not specify a time-out interval (a value of zero), the function does not wait and returns immediately. If you specify an infinite time-out interval, the function returns only when the process signals that it has completed.

Armed with that knowledge, the only task that remains is figuring out how to get a handle to the process that you started. That turns out to be pretty simple, and can be accomplished a number of different ways:

1. One possibility (and the way I'd do it) is by using the ShellExecuteEx function, also from the Windows API, as a drop-in replacement for the Shell function that is built into VB 6. This version is far more versatile and powerful, yet just as easily called using the appropriate declaration.

   It returns a handle to the process that it creates. All you have to do is pass that handle to the WaitForSingleObject function as the hHandle parameter, and you're in business. Execution of your application will be blocked (suspended) until the process that you've called terminates.

2. Another possibility is to use the CreateProcess function (once again, from the Windows API). This function creates a new process and its primary thread in the same security context as the calling process (i.e., your VB 6 application).

   Microsoft has published a knowledge base article detailing this approach that even provides a complete sample implementation. You can find that article here: How To Use a 32-Bit Application to Determine When a Shelled Process Ends.

3. Finally, perhaps the simplest approach yet is to take advantage of the fact that the built-in Shell function's return value is an application task ID. This is a unique number that identifies the program you started, and it can be passed to the OpenProcess function to obtain a process handle that can be passed to the WaitForSingleObject function.

   However, the simplicity of this approach does come at a cost. A very significant disadvantage is that it will cause your VB 6 application to become completely unresponsive. Because it will not be processing Windows messages, it will not respond to user interaction or even redraw the screen.

   The good folks over at VBnet have made complete sample code available in the following article: WaitForSingleObject: Determine when a Shelled App has Ended.
   _{I'd love to be able to reproduce the code here to help stave off link rot (VB 6 is getting up there in years now; there's no guarantee that these resources will be around forever), but the distribution license in the code itself appears to explicitly forbid that.}