Q173085: HOWTO: Create a Process for Reading and Writing to a Pipe

Article: Q173085
Product(s): Microsoft Visual Basic for Windows
Version(s): WINDOWS:4.0,5.0,6.0
Operating System(s): 
Keyword(s): 
Last Modified: 11-JAN-2001

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The information in this article applies to:

- Microsoft Visual Basic Professional Edition for Windows, version 6.0 
- Microsoft Visual Basic Enterprise Edition for Windows, version 6.0 
- Microsoft Visual Basic Control Creation Edition for Windows, version 5.0 
- Microsoft Visual Basic Enterprise Edition for Windows, version 5.0 
- Microsoft Visual Basic Professional Edition, 32-bit, for Windows, version 4.0 
- Microsoft Visual Basic Enterprise Edition, 32-bit, for Windows, version 4.0 
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SUMMARY
=======

This example illustrates a Visual Basic application starting another process
with the purpose of redirecting that process's standard IO handles. The Visual
Basic application redirects the created process's standard output handle to an
anonymous pipe, then proceeds to read the output through the pipe. This sample
just redirects STDOUT of the new process. To redirect other handles (STDIN and
STDERR), create a pipe for each handle for which redirection is desired. The
Visual Basic application would read from the read ends of the pipes for the
redirected STDOUT and STDERR. If STDIN redirection was desired, the Visual Basic
application would write to the write end of the appropriate pipe. An example
follows:

     'A pipe for redirection of STDOUT
     CreatePipe(hReadPipe1, hWritePipe1, sa, 0)

     'A pipe for redirection of STDERR
     CreatePipe(hReadPipe2, hWritePipe2, sa, 0)

     'A pipe for redirection of STDIN
     CreatePipe(hReadPipe3, hWritePipe3, sa, 0)

     'Preparing to start the process with redirected handles
     start.hStdOutput = hWritePipe1
     start.hStdError = hWritePipe2
     start.hStdInput = hReadPipe3

     'Reading output from the started process's STDOUT
     ReadFile(hReadPipe1, mybuff1, 100, bytesread, ByVal 0&)

     'Reading output from the started process's STDERR
     ReadFile(hReadPipe2, mybuff2, 100, bytesread, ByVal 0&)

     'Writing to the started process's STDIN
     WriteFile(hWritePipe3, mybuff3, 100, byteswritten, ByVal 0&)

MORE INFORMATION
================

1. Start a new project in Visual Basic. Form1 is created by default.

2. Add the following code to the General Declarations section of Form1:

        Option Explicit

        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, _
            ByVal lpBuffer As String, _
            ByVal nNumberOfBytesToRead As Long, _
            lpNumberOfBytesRead As Long, _
            byval lpOverlapped As Any) As Long

        Private Type SECURITY_ATTRIBUTES
            nLength As Long
            lpSecurityDescriptor As Long
            bInheritHandle As Long
        End Type

        Private Type STARTUPINFO
           cb As Long
           lpReserved As Long
           lpDesktop As Long
           lpTitle As Long
           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 CreateProcessA Lib "kernel32" (ByVal _
           lpApplicationName As Long, ByVal lpCommandLine As String, _
           lpProcessAttributes As Any, lpThreadAttributes As Any, _
           ByVal bInheritHandles As Long, ByVal dwCreationFlags As Long, _
           ByVal lpEnvironment As Long, ByVal lpCurrentDirectory As Long, _
           lpStartupInfo As Any, lpProcessInformation As Any) As Long

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

        Private Const NORMAL_PRIORITY_CLASS = &H20&
        Private Const STARTF_USESTDHANDLES = &H100&

        Private Sub ExecCmd(cmdline$)
            Dim proc As PROCESS_INFORMATION, ret As Long, bSuccess As Long
            Dim start As STARTUPINFO
            Dim sa As SECURITY_ATTRIBUTES, hReadPipe As Long, hWritePipe _
            As Long
            Dim bytesread As Long, mybuff As String
            Dim i As Integer

            mybuff = String(256, Chr$(65))

            sa.nLength = Len(sa)
            sa.bInheritHandle = 1&
            sa.lpSecurityDescriptor = 0&

            ret = CreatePipe(hReadPipe, hWritePipe, sa, 0)
            If ret = 0 Then
                MsgBox "CreatePipe failed. Error: " & Err.LastDllError
                Exit Sub
            End If

         start.cb = Len(start)
         start.dwFlags = STARTF_USESTDHANDLES
         start.hStdOutput = hWritePipe

         ' Start the shelled application:
         ret& = CreateProcessA(0&, cmdline$, sa, sa, 1&, _
         NORMAL_PRIORITY_CLASS, 0&, 0&, start, proc)
         If ret <> 1 Then
             MsgBox "CreateProcess failed. Error: " & Err.LastDllError
         End If

         bSuccess = ReadFile(hReadPipe, mybuff, 100, bytesread, 0&)
         If bSuccess = 1 Then
             List1.AddItem Left(mybuff, bytesread)
         Else
             MsgBox "ReadFile failed. Error: " & Err.LastDllError
         End If

            ret& = CloseHandle(proc.hProcess)
            ret& = CloseHandle(proc.hThread)
            ret& = CloseHandle(hReadPipe)
            ret& = CloseHandle(hWritePipe)
        End Sub

3. Add a CommandButton (Command1) to Form1.

4. Add the following code to the Command1_Click event:

        Private Sub Command1_Click()
                ExecCmd ("C:\Console\Debug\console.exe")
        End Sub

Additional query words: kbInetdev kbInternet kbVBp400 kbVBp500 kbVBp600 kbdsi kbDSupport kbVBp

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Keywords          :  
Technology        : kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB500Search kbVB600Search kbVBA500Search kbVBA500 kbVBA600 kbVB500 kbVB600 kbVB400Search kbVB400 kbZNotKeyword3
Version           : WINDOWS:4.0,5.0,6.0
Issue type        : kbhowto

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