Microsoft KB Archive/173085

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

Article ID: 173085

Article Last Modified on 6/29/2004

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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

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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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Keywords: kbhowto KB173085

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