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

Asked 2011-04-16T10:46:41.580
13

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?

windows winapi vb6 process waitforsingleobject
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1 Answer

14

There is no need to resort to the extra effort of calling CreateProcess(), etc. This more or less duplicates the old Randy Birch code though it wasn't based on his example. There are only so many ways to skin a cat.

Here we have a prepackaged Function for handy use, which also returns the exit code. Drop it into a static (.BAS) module or include it inline in a Form or Class.

Option Explicit

Private Const INFINITE = &HFFFFFFFF&
Private Const SYNCHRONIZE = &H100000
Private Const PROCESS_QUERY_INFORMATION = &H400&

Private Declare Function CloseHandle Lib "kernel32" ( _
    ByVal hObject As Long) As Long

Private Declare Function GetExitCodeProcess Lib "kernel32" ( _
    ByVal hProcess As Long, _
    lpExitCode As Long) As Long

Private Declare Function OpenProcess Lib "kernel32" ( _
    ByVal dwDesiredAccess As Long, _
    ByVal bInheritHandle As Long, _
    ByVal dwProcessId As Long) As Long

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

Public Function ShellSync( _
    ByVal PathName As String, _
    ByVal WindowStyle As VbAppWinStyle) As Long
    'Shell and wait.  Return exit code result, raise an
    'exception on any error.
    Dim lngPid As Long
    Dim lngHandle As Long
    Dim lngExitCode As Long

    lngPid = Shell(PathName, WindowStyle)
    If lngPid <> 0 Then
        lngHandle = OpenProcess(SYNCHRONIZE _
                             Or PROCESS_QUERY_INFORMATION, 0, lngPid)
        If lngHandle <> 0 Then
            WaitForSingleObject lngHandle, INFINITE
            If GetExitCodeProcess(lngHandle, lngExitCode) <> 0 Then
                ShellSync = lngExitCode
                CloseHandle lngHandle
            Else
                CloseHandle lngHandle
                Err.Raise &H8004AA00, "ShellSync", _
                          "Failed to retrieve exit code, error " _
                        &
answered 2011-04-16T13:00:52.163

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