Run an external process without freezing main UI in vb6 - vb6

Question

I have VB6 Application which depends upon another EXE file which is invoked via CreateProcess from lib kernel32 and pipe connection to fetch the output.
How can I invoke it asynchronously in my main form without freezing the UI?
Currently, Form1 Freezes when the external application takes a longer time to respond.
Attribute VB_Name = "CmdOutput"
Option Explicit
''''''''''''''''''''''''''''''''''''''''
' Joacim Andersson, Brixoft Software
' http://www.brixoft.net
''''''''''''''''''''''''''''''''''''''''
' STARTUPINFO flags
Private Const STARTF_USESHOWWINDOW = &H1
Private Const STARTF_USESTDHANDLES = &H100
' ShowWindow flags
Private Const SW_HIDE = 0
' DuplicateHandle flags
Private Const DUPLICATE_CLOSE_SOURCE = &H1
Private Const DUPLICATE_SAME_ACCESS = &H2
' Error codes
Private Const ERROR_BROKEN_PIPE = 109
Private Type SECURITY_ATTRIBUTES
nLength As Long
lpSecurityDescriptor As Long
bInheritHandle As Long
End Type
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 CreatePipe _
Lib "kernel32" ( _
phReadPipe As Long, _
phWritePipe As Long, _
lpPipeAttributes As Any, _
ByVal nSize As Long) As Long
Private Declare Function ReadFile _
Lib "kernel32" ( _
ByVal hFile As Long, _
lpBuffer As Any, _
ByVal nNumberOfBytesToRead As Long, _
lpNumberOfBytesRead As Long, _
lpOverlapped As Any) As Long
Private Declare Function CreateProcess _
Lib "kernel32" Alias "CreateProcessA" ( _
ByVal lpApplicationName As String, _
ByVal lpCommandLine As String, _
lpProcessAttributes As Any, _
lpThreadAttributes As Any, _
ByVal bInheritHandles As Long, _
ByVal dwCreationFlags As Long, _
lpEnvironment As Any, _
ByVal lpCurrentDriectory As String, _
lpStartupInfo As STARTUPINFO, _
lpProcessInformation As PROCESS_INFORMATION) As Long
Private Declare Function GetCurrentProcess _
Lib "kernel32" () As Long
Private Declare Function DuplicateHandle _
Lib "kernel32" ( _
ByVal hSourceProcessHandle As Long, _
ByVal hSourceHandle As Long, _
ByVal hTargetProcessHandle As Long, _
lpTargetHandle As Long, _
ByVal dwDesiredAccess As Long, _
ByVal bInheritHandle As Long, _
ByVal dwOptions As Long) As Long
Private Declare Function CloseHandle _
Lib "kernel32" ( _
ByVal hObject As Long) As Long
Private Declare Function OemToCharBuff _
Lib "user32" Alias "OemToCharBuffA" ( _
lpszSrc As Any, _
ByVal lpszDst As String, _
ByVal cchDstLength As Long) As Long
' Function GetCommandOutput
'
' sCommandLine: [in] Command line to launch
' blnStdOut [in,opt] True (defualt) to capture output to STDOUT
' blnStdErr [in,opt] True to capture output to STDERR. False is default.
' blnOEMConvert: [in,opt] True (default) to convert DOS characters to Windows, False to skip conversion
'
' Returns: String with STDOUT and/or STDERR output
'
Public Function GetCommandOutput( _
sCommandLine As String, _
Optional blnStdOut As Boolean = True, _
Optional blnStdErr As Boolean = False, _
Optional blnOEMConvert As Boolean = True _
) As String
Dim hPipeRead As Long, hPipeWrite1 As Long, hPipeWrite2 As Long
Dim hCurProcess As Long
Dim sa As SECURITY_ATTRIBUTES
Dim si As STARTUPINFO
Dim pi As PROCESS_INFORMATION
Dim baOutput() As Byte
Dim sNewOutput As String
Dim lBytesRead As Long
Dim fTwoHandles As Boolean
Dim lRet As Long
Const BUFSIZE = 1024 ' pipe buffer size
' At least one of them should be True, otherwise there's no point in calling the function
If (Not blnStdOut) And (Not blnStdErr) Then
Err.Raise 5 ' Invalid Procedure call or Argument
End If
' If both are true, we need two write handles. If not, one is enough.
fTwoHandles = blnStdOut And blnStdErr
ReDim baOutput(BUFSIZE - 1) As Byte
With sa
.nLength = Len(sa)
.bInheritHandle = 1 ' get inheritable pipe handles
End With
If CreatePipe(hPipeRead, hPipeWrite1, sa, BUFSIZE) = 0 Then
Exit Function
End If
hCurProcess = GetCurrentProcess()
' Replace our inheritable read handle with an non-inheritable. Not that it
' seems to be necessary in this case, but the docs say we should.
Call DuplicateHandle(hCurProcess, hPipeRead, hCurProcess, hPipeRead, 0&, _
0&, DUPLICATE_SAME_ACCESS Or DUPLICATE_CLOSE_SOURCE)
' If both STDOUT and STDERR should be redirected, get an extra handle.
If fTwoHandles Then
Call DuplicateHandle(hCurProcess, hPipeWrite1, hCurProcess, hPipeWrite2, 0&, _
1&, DUPLICATE_SAME_ACCESS)
End If
With si
.cb = Len(si)
.dwFlags = STARTF_USESHOWWINDOW Or STARTF_USESTDHANDLES
.wShowWindow = SW_HIDE ' hide the window
If fTwoHandles Then
.hStdOutput = hPipeWrite1
.hStdError = hPipeWrite2
ElseIf blnStdOut Then
.hStdOutput = hPipeWrite1
Else
.hStdError = hPipeWrite1
End If
End With
If CreateProcess(vbNullString, sCommandLine, ByVal 0&, ByVal 0&, 1, 0&, _
ByVal 0&, vbNullString, si, pi) Then
' Close thread handle - we don't need it
Call CloseHandle(pi.hThread)
' Also close our handle(s) to the write end of the pipe. This is important, since
' ReadFile will *not* return until all write handles are closed or the buffer is full.
Call CloseHandle(hPipeWrite1)
hPipeWrite1 = 0
If hPipeWrite2 Then
Call CloseHandle(hPipeWrite2)
hPipeWrite2 = 0
End If
Do
' Add a DoEvents to allow more data to be written to the buffer for each call.
' This results in fewer, larger chunks to be read.
'DoEvents
If ReadFile(hPipeRead, baOutput(0), BUFSIZE, lBytesRead, ByVal 0&) = 0 Then
Exit Do
End If
If blnOEMConvert Then
' convert from "DOS" to "Windows" characters
sNewOutput = String$(lBytesRead, 0)
Call OemToCharBuff(baOutput(0), sNewOutput, lBytesRead)
Else
' perform no conversion (except to Unicode)
sNewOutput = Left$(StrConv(baOutput(), vbUnicode), lBytesRead)
End If
GetCommandOutput = GetCommandOutput & sNewOutput
' If you are executing an application that outputs data during a long time,
' and don't want to lock up your application, it might be a better idea to
' wrap this code in a class module in an ActiveX EXE and execute it asynchronously.
' Then you can raise an event here each time more data is available.
'RaiseEvent OutputAvailabele(sNewOutput)
Loop
' When the process terminates successfully, Err.LastDllError will be
' ERROR_BROKEN_PIPE (109). Other values indicates an error.
Call CloseHandle(pi.hProcess)
Else
GetCommandOutput = "Failed to create process, check the path of the command line."
End If
' clean up
Call CloseHandle(hPipeRead)
If hPipeWrite1 Then
Call CloseHandle(hPipeWrite1)
End If
If hPipeWrite2 Then
Call CloseHandle(hPipeWrite2)
End If
End Function

Answer 1

As the code comment in the code you linked suggests, you could pull this out-of-process into an ActiveX exe to wrap this.
To keep it in-process, you would need to use MsgWaitForMultipleObjects (https://learn.microsoft.com/en-us/windows/desktop/api/winuser/nf-winuser-msgwaitformultipleobjects).
Google located a VB6 sample: http://www.freevbcode.com/ShowCode.asp?ID=4322
The idea here is that the function will wait until either data is available on the handle OR there is a UI event (in which case DoEvents is used to dispatch).