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

• Microsoft Visual Basic Control Creation and Enterprise Editions for Windows, version 5.0
• Microsoft Visual Basic Professional and Enterprise Editions, 32-bit only, for Windows, version 4.0

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
   

Keywords          : vb432 VB4WIN vb5all
Technology        : kbInetDev
Version           : WINDOWS:4.0,5.0
Platform          : WINDOWS
Issue type        : kbhowto