Redirect Output of CMD.EXE to a Pipe

Environment: Visual C++ 6.0 SP2, Windows NT 4.0 SP4

I have the thought of redirecting output of WIN32 console programs to a pipe or a file long ago, but I don't know to do at all at first, finally I searched through Microsoft MSDN and found a way, the code pasted below was copied from MSDN sample code except some changes in function CreateChildProcess(), where I stressed in the code.

In the demo source, I redirect the stdout of CMD.exe to a pipe and start a thread to read from this pipe, then all that CMD.EXE output was redirected to the pipe where I am reading.



BOOL CShellView::CreateShellRedirect()
{
SECURITY_ATTRIBUTES saAttr;
BOOL fSuccess;

// Set the bInheritHandle flag so pipe handles are inherited.
saAttr.nLength = sizeof(SECURITY_ATTRIBUTES);
saAttr.bInheritHandle = TRUE;
saAttr.lpSecurityDescriptor = NULL;

// The steps for redirecting child process's STDOUT:
// 1. Save current STDOUT, to be restored later.
// 2. Create anonymous pipe to be STDOUT for child process.
// 3. Set STDOUT of the parent process to be write handle to
// the pipe, so it is inherited by the child process.
// 4. Create a noninheritable duplicate of the read handle and
// close the inheritable read handle.

// Save the handle to the current STDOUT.
hSaveStdout = GetStdHandle(STD_OUTPUT_HANDLE);

// Create a pipe for the child process's STDOUT.
if( !CreatePipe( &hChildStdoutRd, &hChildStdoutWr, &saAttr, 0) )
{
TRACE0( _T("Stdout pipe creation failed\n") );
return FALSE;
}

// Set a write handle to the pipe to be STDOUT.
if( !SetStdHandle(STD_OUTPUT_HANDLE, hChildStdoutWr) )
{
TRACE0( _T("Redirecting STDOUT failed\n") );
return FALSE;
}

// Create noninheritable read handle and close the inheritable read handle.
fSuccess = DuplicateHandle( GetCurrentProcess(), hChildStdoutRd,
GetCurrentProcess(), &hChildStdoutRdDup ,
0, FALSE,
DUPLICATE_SAME_ACCESS );
if( !fSuccess )
{
TRACE0( _T("DuplicateHandle failed\n") );
return FALSE;
}
CloseHandle( hChildStdoutRd );

// The steps for redirecting child process's STDIN:
// 1. Save current STDIN, to be restored later.
// 2. Create anonymous pipe to be STDIN for child process.
// 3. Set STDIN of the parent to be the read handle to the
// pipe, so it is inherited by the child process.
// 4. Create a noninheritable duplicate of the write handle,
// and close the inheritable write handle.

// Save the handle to the current STDIN.
hSaveStdin = GetStdHandle(STD_INPUT_HANDLE);

// Create a pipe for the child process's STDIN.
if( !CreatePipe(&hChildStdinRd, &hChildStdinWr, &saAttr, 0) )
{
TRACE0( _T("Stdin pipe creation failed\n") );
return FALSE;
}
// Set a read handle to the pipe to be STDIN.
if( !SetStdHandle(STD_INPUT_HANDLE, hChildStdinRd) )
{
TRACE0( _T("Redirecting Stdin failed\n") );
return FALSE;
}
// Duplicate the write handle to the pipe so it is not inherited.
fSuccess = DuplicateHandle(GetCurrentProcess(), hChildStdinWr,
GetCurrentProcess(), &hChildStdinWrDup,
0, FALSE, // not inherited
DUPLICATE_SAME_ACCESS );
if( !fSuccess )
{
TRACE0( _T("DuplicateHandle failed\n") );
return FALSE;
}
CloseHandle(hChildStdinWr);

// Now create the child process.
if( !CreateChildProcess(dwProcessId) )
{
TRACE0( _T("CreateChildProcess failed\n") );
return FALSE;
}
// After process creation, restore the saved STDIN and STDOUT.
if( !SetStdHandle(STD_INPUT_HANDLE, hSaveStdin) )
{
TRACE0( _T("Re-redirecting Stdin failed\n") );
return FALSE;
}
if( !SetStdHandle(STD_OUTPUT_HANDLE, hSaveStdout) )
{
TRACE0( _T("Re-redirecting Stdout failed\n") );
return FALSE;
}
m_pReadThread =
fxBeginThread( (AFX_THREADPROC)ReadPipeThreadProc,(LPVOID)this );
if( !m_pReadThread )
{
TRACE0( _T("Cannot start read-redirect thread!\n") );
return FALSE;
}
return TRUE;
}

BOOL CShellView::CreateChildProcess(DWORD& dwProcessId)
{
PROCESS_INFORMATION piProcInfo;
STARTUPINFO siStartInfo;

// Set up members of STARTUPINFO structure.
ZeroMemory( &siStartInfo, sizeof(STARTUPINFO) );
siStartInfo.cb = sizeof(STARTUPINFO);
/////////////////////////////////////////
/*
Note here the
dwFlags should be STARTF_USESTDHANDLES, which is descripted in WIN32 API
document, but this was
omitted in MSDN sample
*/
siStartInfo.dwFlags = STARTF_USESTDHANDLES;
siStartInfo.hStdInput = hChildStdinRd;
siStartInfo.hStdOutput = hChildStdoutWr;
siStartInfo.hStdError = hChildStdoutWr;

///////////////////////////////////////////////////
TCHAR shellCmd[_MAX_PATH];
if( !GetEnvironmentVariable(_T("ComSpec"), shellCmd, _MAX_PATH) )
return FALSE;
#ifdef _UNICODE
_tcscat( shellCmd, _T(" /U") );
#else
_tcscat( shellCmd, _T(" /A") );
#endif
// Create the child process.
BOOL ret = CreateProcess( NULL,
shellCmd, // applicatin name
NULL, // process security attributes
NULL, // primary thread security attributes
TRUE, // handles are inherited
DETACHED_PROCESS, // creation flags
NULL, // use parent's environment
NULL, // use parent's current directory
&siStartInfo, // STARTUPINFO pointer
&piProcInfo); // receives PROCESS_INFORMATION
if( ret )
dwProcessId = piProcInfo.dwProcessId;
return ret;
}

Download source - 5 KB

Date Last Updated: April 4, 1999