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Posted 27 Nov 2003

Redirecting an arbitrary Console's Input/Output

, 27 Nov 2003
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Redirecting an arbitrary console's input/output in a simple, graceful way

Sample Image - redir.gif


To redirect the input/output of a console application is interesting and useful. You can display the child's output in a window (just like Visual Studio's output window), or search some keywords in the output string to determine if the child process has completed its work successfully. An old, 'ugly' DOS program could become an useful component of your fancy Win32 GUI program.

My idea is to develop a simple, easy to use redirector class which can redirect an arbitrary console, and won't be affected by the behavior of the child process.


The technique of redirecting the input/output of a console process is very sample: The CreateProcess() API through the STARTUPINFO structure enables us to redirect the standard handles of a child console based process. So we can set these handles to either a pipe handle, file handle, or any handle that we can read and write. The detail of this technique has been described clearly in MSDN: HOWTO: Spawn Console Processes with Redirected Standard Handles.

However, MSDN's sample code has two big problem. First, it assumes the child process will send output at first, then wait for input, then flush the output buffer and exit. If the child process doesn't behave like that, the parent process will be hung up. The reason of this is the ReadFile() function remains blocked untill the child process sends some output, or exits.

Second, It has problem to redirect a 16-bit console (including console based MS-DOS applications.) On Windows 9x, ReadFile remains blocked even after the child process has terminated; On Windows NT/XP, ReadFile always returns FALSE with error code set to ERROR_BROKEN_PIPE if the child process is a DOS application.

Solving the block problem of ReadFile

To prevent the parent process from being blocked by ReadFile, we can simply pass a file handle as stdout to the child process, then monitor this file. A more simple way is to call PeekNamedPipe() function before calling ReadFile(). The PeekNamedPipe function checks information about data in the pipe, then returns immediately. If there's no data available in the pipe, don't call ReadFile.

By calling PeekNamedPipe before ReadFile, we also solve the block problem of redirecting a 16-bit console on Windows 9x.

The class CRedirector creates pipes and launchs the child process at first. then creates a listener thread to monitor the output of the child process. This is the main loop of the listener thread:

for (;;)
    // redirect stdout till there's no more data.
    nRet = pRedir->RedirectStdout();
    if (nRet <= 0)

    // check if the child process has terminated.
    DWORD dwRc = ::WaitForMultipleObjects(
        2, aHandles, FALSE, pRedir->m_dwWaitTime);
    if (WAIT_OBJECT_0 == dwRc)      // the child process ended
    if (WAIT_OBJECT_0+1 == dwRc)    // m_hEvtStop was signalled, exit

This is the main loop of the RedirectStdout() function:

for (;;)
    DWORD dwAvail = 0;
    if (!::PeekNamedPipe(m_hStdoutRead, NULL, 0, NULL,
        &dwAvail, NULL))    // error, the child process might ended

    if (!dwAvail)           // no data available, return
        return 1;

    char szOutput[256];
    DWORD dwRead = 0;
    if (!::ReadFile(m_hStdoutRead, szOutput, min(255, dwAvail),
        &dwRead, NULL) || !dwRead)
             // error, the child process might ended

    szOutput[dwRead] = 0;
    WriteStdOut(szOutput);          // display the output

WriteStdOut is a virtual member function. It does nothing in CRedirector class. However it can be overrided to achieve our specific target, like I did in the demo project:

int nSize = m_pWnd->GetWindowTextLength();
         // m_pWnd points to a multiline Edit control
m_pWnd->SetSel(nSize, nSize);
       // add the message to the end of Edit control

To redirect DOS console based applications on NT/2000/XP

MSDN's solution is to launch an intermediate Win32 Console application as a stub process between the Win32 parent and the 16-bit console based child. In fact the DOS prompt program (on NT/XP it's cmd.exe, on 9x it's is a natural stub process we just need. We can test this in RedirDemo.exe:

  1. Input 'cmd.exe' in Command Editbox, then press Run button.
  2. Input the name of the 16-bit console based application (dosapp.exe for example) in the Input Editbox, then press Input button. Now we can see the output of the 16-bit consol.
  3. Input 'exit' in the Input Editbox, then press Input button to terminate cmd.exe

Apparently this is not a good solution because it's too complicated. A more effective way is to use a batch file as the stub. Edit stub.bat file like this:

%1 %2 %3 %4 %5 %6 %7 %8 %9

Then run a command like 'stub.bat dosapp.exe', then the 16-bit DOS console application runs OK.


This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

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About the Author

Web Developer
Canada Canada
Nick Adams is one of my favorite figures in Hemingway's stories. I use it because Jeff Lee has been occupied on Codeproject.

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Comments and Discussions

GeneralSuperb execution... but have a simple question... Pin
Hurty9-Jul-09 22:52
memberHurty9-Jul-09 22:52 
Hy nick. This program is superb. I could say it is the only one I found to work with child processes fine. But I have a favor to ask you... as I'm not familiar with C++ Sniff | :^) could you provide an example for VB.NET. I appreciate it in advance. Thanks.
GeneralWriteStdError shouldn't be called with _T Pin
sashoalm17-Apr-09 22:55
membersashoalm17-Apr-09 22:55 
Generaltelnet.exe Pin
suiram4024-Feb-09 18:01
membersuiram4024-Feb-09 18:01 
GeneralCaptureConsole.DLL - A universal Console Output Redirector for all Compilers Pin
Elmue3-Feb-09 6:02
memberElmue3-Feb-09 6:02 
GeneralStrange problem Pin
Mr. S27-Nov-08 0:30
memberMr. S27-Nov-08 0:30 
GeneralTelnet.exe Pin
tptshepo11-Nov-08 19:55
membertptshepo11-Nov-08 19:55 
Questionhow to send a CTRL+C??? Pin
Motorcure16-Sep-08 23:08
memberMotorcure16-Sep-08 23:08 
AnswerRe: how to send a CTRL+C??? Pin
varandas7918-Mar-11 6:02
membervarandas7918-Mar-11 6:02 
QuestionHow to you know when the process has ended? Pin
mimosa21-Apr-08 13:28
membermimosa21-Apr-08 13:28 
GeneralAbout Redirecting Debug IO Pin
yonken6-Apr-08 0:26
memberyonken6-Apr-08 0:26 
QuestionHow to change the bounds of bytes to output data at a time. Pin
chol92112-Nov-07 22:19
memberchol92112-Nov-07 22:19 port Pin
neolode16-Sep-07 6:38
memberneolode16-Sep-07 6:38 
GeneralOverlapped structure Pin
charian09203-May-07 17:28
membercharian09203-May-07 17:28 
QuestionAlso works for DLLs? Pin
roel hermans3-May-07 1:28
memberroel hermans3-May-07 1:28 
QuestionWhat about advanced key events? Pin
torch#27-Mar-07 22:49
membertorch#27-Mar-07 22:49 
QuestionAdapting To Windows Powershell? Pin
Robert T.25-Oct-06 9:33
memberRobert T.25-Oct-06 9:33 
AnswerRe: Adapting To Windows Powershell? Pin
Anne Jan Beeks31-Jan-07 3:22
memberAnne Jan Beeks31-Jan-07 3:22 
GeneralRe: Adapting To Windows Powershell? Pin
Robert T.26-Mar-07 6:07
memberRobert T.26-Mar-07 6:07 
GeneralRe: Adapting To Windows Powershell? Pin
Anne Jan Beeks26-Mar-07 6:32
memberAnne Jan Beeks26-Mar-07 6:32 
GeneralRe: Adapting To Windows Powershell? Pin
Jake G16-Jan-17 22:11
memberJake G16-Jan-17 22:11 
QuestionHow to avoid using fflush(); ? Pin
q.sa1-Oct-06 8:10
memberq.sa1-Oct-06 8:10 
QuestionAsking for help, Unexpected behavior with my client app Pin
gemex9-Aug-06 22:18
membergemex9-Aug-06 22:18 
GeneralCalling CRedirector::Close() in output thread problem ! Pin
Reivax7214-Jun-06 0:26
memberReivax7214-Jun-06 0:26 
GeneralPlease Help me VB Pin
WolverineSoft2-May-06 13:15
memberWolverineSoft2-May-06 13:15 
General::SetConsoleCtrlHandler(CRedirector::CtrlHandler, TRUE); Pin
Tcpip200525-Apr-06 22:06
memberTcpip200525-Apr-06 22:06 

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