Knowledge Base

How To Create a Process for Reading and Writing to a Pipe

Article ID: 173085

Article Last Modified on 6/29/2004

APPLIES TO

Microsoft Visual Basic 6.0 Professional Edition
Microsoft Visual Basic 6.0 Enterprise Edition
Microsoft Visual Basic 5.0 Control Creation Edition
Microsoft Visual Basic 5.0 Enterprise Edition
Microsoft Visual Basic 4.0 Professional Edition
Microsoft Visual Basic 4.0 32-Bit Enterprise Edition

This article was previously published under Q173085

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

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

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

Add a CommandButton (Command1) to Form1.

Add the following code to the Command1_Click event:

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

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Microsoft KB Archive/173085

How To Create a Process for Reading and Writing to a Pipe

SUMMARY

MORE INFORMATION