Creating a child process with stdout to a file

[Mukiwa]

Is it possible to create a child process (using CreateProcess) so that it's standard output is changed to a specific file ?

Thanks
Dave.

 

[classic773]

Did you find any answers? I'd like to create a process and redirect its standard error stream so that I can read it and show the percentage completion, based on key words. I'm not sure how to do this.

 

[Guest_imported]

The process is quite simple and goes something like:

Use CreateFile to obtain a handle to a file with write access. Use the lpSecurityAttributes parameter to specify a SECURITY_ATTRIBUTES structure. Set the bInheritHandle member of this structure to TRUE.

Set the fInheritHandles parameter of CreateProcess to TRUE.

Use the lpStartupInfo parameter of CreateProcess to specify a STARTUPINFO structure. Set the dwFlags member of this structure to STARTF_USESTDHANDLES. Set the hStdOutput member to the handle obtained previously.

 

[classic773]

Could somebody help me write this portion of code. I'm not to familiar with handles, security attributes, etc. I'm pretty good with c++ but visual is new to me. If somebody could write a portion of code that executes a program and reads its output as a stream in real-time, I would love you forever. Otherwise if you could walk me through it I would appreciate that also.

 

[Andyman35]

I wrote this function a while back which you may find useful:

CString SpawnProcessReturnStdOut( char *command_line )
{
CString result = "";

// Setup default pipe security with the bInheritHandle flag set to
// allow pipe handles to be inherited by the child process.
SECURITY_ATTRIBUTES saAttr;
saAttr.nLength = sizeof(SECURITY_ATTRIBUTES);
saAttr.bInheritHandle = TRUE;
saAttr.lpSecurityDescriptor = NULL;

// Create the anonymous pipe and store read/write handles
HANDLE hPipeInputRead, hPipeOutputWrite;
CreatePipe( &hPipeInputRead,
&hPipeOutputWrite,
&saAttr,
0 );

// Start up the child process
BOOL bRet = FALSE;
STARTUPINFO si = {0};
PROCESS_INFORMATION pi = {0};

// Make child process use the anonymous pipe for stdin and stdout
si.cb = sizeof(si);
si.dwFlags = STARTF_USESTDHANDLES | STARTF_USESHOWWINDOW;
si.wShowWindow = SW_HIDE;
si.hStdInput = hPipeInputRead;
si.hStdOutput = hPipeOutputWrite;

bRet = ::CreateProcess( NULL,
command_line,
NULL,
NULL,
TRUE,
0,
NULL,
NULL,
&si,
&pi );
if (bRet)
{
// Read the contents of the pipe
DWORD dwRead, dwAvail;
CHAR chBuf[256];
BOOL pipe_data = true;
BOOL process_finished;

do
{
process_finished = (WaitForSingleObject (pi.hProcess, 0) == WAIT_OBJECT_0);

// Peek to see if there is any more data in the pipe
PeekNamedPipe( hPipeInputRead,chBuf,255,&dwRead,&dwAvail,NULL);

// Clear buffer to zero
memset(chBuf,0,sizeof(chBuf));

if( dwAvail > 0 )
{
pipe_data = ReadFile( hPipeInputRead, chBuf, 255, &dwRead, NULL);
result += chBuf;
}
}
while( dwAvail > 0 || !process_finished );

// Close the process and pipe handles
CloseHandle (pi.hProcess);
CloseHandle (pi.hThread);
CloseHandle( hPipeInputRead );
CloseHandle( hPipeOutputWrite );
}

return result;
}