redirecting stdio for sequential programs in an MFC dialog

Question

redirecting stdio for sequential programs in an MFC dialog

Dennis Foreman 0

My C++ MFC dialog creates 2 child processes sequentially. I redirect STDOUT to separate files for each of the programs (and restore to default after each one runs). STDOUT from program 1 gets sent to its file as expected. Program 2 STDOUT disappears. Both programs return 0. I know Program 2 runs because a MessageBox it creates for debugging appears. To remove any other effects, I have reduced Program 2 to just: std::cout<<"aha"<<std::endl;

Is there anything I should know about doing this? PS. I have no access to the source code for the REAL programs 1 and 2.

David Lowndes 4,381 Reputation points MVP

2023-03-10T13:36:58.6066667+00:00

What happens if you execute them in the opposite order?
RLWA32 30,101 Reputation points

2023-03-10T15:38:31.5066667+00:00

Narrative descriptions don't provide much diagnostic information. Posting a relevant code snippet would be helpful.

Dennis Foreman 0

Reversing the order was interesting. Program 2 (now running as program 1) still does not output anything, but still pops its MessageBox window, so it DOES run. Here is it's code, followed by the code for redirecting STDOUT:

#include <Windows.h>
#include <iostream>

int main(int argc, char** argv)
{
    MessageBox(NULL,TEXT("reached basic app"),TEXT("dbgmsg"),MB_OK);
    std::cout << "Hello World-cout!"<<std::endl;
    std::cerr << "Hello World-cerr!"<<std::endl;
}
// following code is performed by the caller of the above program
stdout_old = _dup(1);	// save fd of original stdout
		err = _wfreopen_s(&fp_exifout, std_outpath, L"w+", stdout); // fp_exifout gets fd_value of new std_outpath, replacing stdout
		if (err != 0)
		{
			MessageBox(_T("***** could not redirect stdout"));
			exit(5);
		}
		else
		{
			if ((fp_exifout!=NULL) &&(-1 == _dup2(_fileno(fp_exifout), 1)))
			{
				perror("Can't _dup2 stdout");
				exit(6);
			}
			MessageBox(_T("****** redirect stdout ok to:") + std_outpath);
		}

Dennis Foreman 0 Reputation points

2023-03-10T16:44:04.35+00:00

The code for setting up and running CreateProcess is identical for both programs, except for the names of the programs and program 2 has no parameters.
David Lowndes 4,381 Reputation points MVP

2023-03-10T17:40:38.3866667+00:00

Show us how you're doing that then.

Dennis Foreman 0

STARTUPINFO si = { sizeof(si) };  // setup needed to start magick

	PROCESS_INFORMATION pi;
	ZeroMemory(&pi, sizeof(pi));

	ZeroMemory(&si, sizeof(si));
	si.cb = sizeof(si);

	si.dwFlags |= STARTF_USESTDHANDLES;
	si.hStdInput = NULL;
	DWORD flags = CREATE_NO_WINDOW;
	// flags = 0;
	**redirect_stdio_magick(inpath_with_lld, redir_stdio_files_begin); // just open the file**

	// (const wchar_t*)
	//mogstring = "magick mogrify -format jpg O:\\r.png"; // for testing
	//std::wcout<<"MOGRIFY mogstring parameter=["<< (const wchar_t*)mogstring <<"]" << std::endl;
	//djdebug5( _T("MOGRIFY mogstring parameter=[") + mogstring  + _T("]"));
	//exit(0);
	std::ostringstream mystream;
	mystream << mogstring.GetString()<<std::endl;
	std::string args = mystream.str();
	//CString cstr(args.c_str());

	USES_CONVERSION_EX;
	//std::string text = args;
	//LPWSTR parms= A2W_EX(args.c_str(), args.length());
	LPWSTR parms = CT2W(mogstring);

	std::wcout << "Mogrify:" <<std::endl;
		//std::wcout << "parms1 = [" << parms << "]" << std::endl;
		//std::wcout << "parms2 = [" << *parms << "]" << std::endl;
		std::wcout << "mogstring = [" << mogstring.GetString() << "]" << std::endl;	// THIS ONE WORKS!!!
		;

		LPWSTR bparm = _T(" C:\\1-lcl-bats\\typeit.bat this is the cmdline");

	/*LPWSTR parms = CW2W(mogstring); */
	
	si.hStdError = fp_magickerr;				// indicate redirected std files
	si.hStdOutput = fp_magickout;
	si.hStdInput = NULL;
	CString testparm= _T(" aha");
	LPWSTR aha = _T(" \"aha\" ");
	mret=CreateProcess(
		// _T("C:\\Program Files\\ImageMagick-7.0.10-Q16-HDRI\\magick.exe"),	// app nanme
		//_T("C:\\Windows\\System32\\xcopy.exe" ),	// this pair runs 
		//_T(" nosuch.bin out.bin"),
		// following ran, but no output produced in redirected files
		_T("E:\\BU-onedrive\\OneDrive - Binghamton University\\VS-projects\\basic_console_app\\Debug\\basic_console_app.exe"),
		NULL,
		//_T("C:\\Windows\\System32\\cmd.exe"), // need shellexec to do this
		//_T("C:\\bats\\typeit.bat" ),
		//(LPWSTR)CT2W(testparm),	// need (LPWSTR) from CString NOT LPWCSTR
		//_T(" aha"),        // this is NOT an LPWSTR, but an LPWCSTR
		//aha,
		//parms,     // (LPWSTR) from CString
		//bparm,
		NULL,			// process sec attribs
		NULL,			// thread sec attribs
		TRUE,			// bool, inherit handles (yes, no) 
		flags,			// creation flags
		NULL,			// Use parent's environment
		double_all_bslashes(inpath_with_lld),			// maybe needs an extra 0x0  ??
		//NULL,			// Use parent's curr dir
		&si,			// -> startup infostruct
		&pi	 
		);
	if(mret)		//successfull creation of process (Magick)
		{	 std::ostringstream myestream;
			WaitForSingleObject(pi.hProcess, INFINITE);
			
			BOOL fcnresult = GetExitCodeProcess(pi.hProcess, &exitcode); // exitcode is the actual process end value
			//  convert DWORD to a string for output in messages
			myestream << exitcode;
			
			std::string res = myestream.str();
			std::cerr<<"mogrify_it ended. Actual rc=" << myestream.str() << std::endl;
			std::cout << "mogrify_it ended. Actual rc=" << res << std::endl;

			fflush(fp_magickout);
			fflush(fp_magickerr);
			//retcode 1= Incorrect function. Indicates that Action has attempted to execute 
			//a non-recognized command in Windows command prompt cmd.exe.
			CString cstr(res.c_str()); // can't use c_str inside messagebox, use this conversion to CString 1st
			MessageBox(_T("mogrify success. Actual exitcode=[")+cstr+ _T("]")); 

			CloseHandle(pi.hProcess);
			CloseHandle(pi.hThread);
			myexitcode = 0;
		}

RLWA32 30,101 Reputation points

2023-03-10T19:52:47.63+00:00

I don't see how a child process can get a redirected handle from the parent since it does not appear that an inheritable file handle was created by the parent.
Dennis Foreman 0 Reputation points

2023-03-10T22:04:44.3833333+00:00

The handle was created by the function: redirect_stdio_magick near the top of my pasted code. The child then inherits this redirection. A copy of that function is used for Program 1 of my original description, with the only difference being the name of the file to be written. I will attempt to edit my post and format that line.

Tried to format it as bold, but couldn't get it to stay. So there is a ** in front and after that line. IT's the 10th line of the post (not counting empty lines).
RLWA32 30,101 Reputation points

2023-03-10T23:39:54.3466667+00:00

si.hStdOutput = fp_magickout;

Is fp_magickout a FILE* or a Win32 HANDLE?
Dennis Foreman 0 Reputation points

2023-03-11T02:09:13.1833333+00:00

FILE* and global
Dennis Foreman 0 Reputation points

2023-03-11T02:09:54.26+00:00

and it is global
Minxin Yu 3,936 Reputation points Microsoft Vendor

2023-03-21T08:48:25.2833333+00:00

Was your problem solved? Is there new errors?

David Lowndes 4,381 Reputation points MVP

2023-03-10T13:36:58.6066667+00:00

What happens if you execute them in the opposite order?
RLWA32 30,101 Reputation points

2023-03-10T15:38:31.5066667+00:00

Narrative descriptions don't provide much diagnostic information. Posting a relevant code snippet would be helpful.
Dennis Foreman 0 Reputation points

2023-03-10T16:40:34.3566667+00:00

Reversing the order was interesting. Program 2 (now running as program 1) still does not output anything, but still pops its MessageBox window, so it DOES run. Here is it's code, followed by the code for redirecting STDOUT:

#include <Windows.h> #include <iostream> int main(int argc, char** argv) { MessageBox(NULL,TEXT("reached basic app"),TEXT("dbgmsg"),MB_OK); std::cout << "Hello World-cout!"<<std::endl; std::cerr << "Hello World-cerr!"<<std::endl; } // following code is performed by the caller of the above program stdout_old = _dup(1); // save fd of original stdout err = _wfreopen_s(&fp_exifout, std_outpath, L"w+", stdout); // fp_exifout gets fd_value of new std_outpath, replacing stdout if (err != 0) { MessageBox(_T("***** could not redirect stdout")); exit(5); } else { if ((fp_exifout!=NULL) &&(-1 == _dup2(_fileno(fp_exifout), 1))) { perror("Can't _dup2 stdout"); exit(6); } MessageBox(_T("****** redirect stdout ok to:") + std_outpath); }
Dennis Foreman 0 Reputation points

2023-03-10T16:44:04.35+00:00

The code for setting up and running CreateProcess is identical for both programs, except for the names of the programs and program 2 has no parameters.
David Lowndes 4,381 Reputation points MVP

2023-03-10T17:40:38.3866667+00:00

Show us how you're doing that then.
RLWA32 30,101 Reputation points

2023-03-10T19:52:47.63+00:00

I don't see how a child process can get a redirected handle from the parent since it does not appear that an inheritable file handle was created by the parent.
Dennis Foreman 0 Reputation points

2023-03-10T22:04:44.3833333+00:00

The handle was created by the function: redirect_stdio_magick near the top of my pasted code. The child then inherits this redirection. A copy of that function is used for Program 1 of my original description, with the only difference being the name of the file to be written. I will attempt to edit my post and format that line.

Tried to format it as bold, but couldn't get it to stay. So there is a ** in front and after that line. IT's the 10th line of the post (not counting empty lines).
RLWA32 30,101 Reputation points

2023-03-10T23:39:54.3466667+00:00

si.hStdOutput = fp_magickout;

Is fp_magickout a FILE* or a Win32 HANDLE?
Dennis Foreman 0 Reputation points

2023-03-11T02:09:13.1833333+00:00

FILE* and global
Dennis Foreman 0 Reputation points

2023-03-11T02:09:54.26+00:00

and it is global
Minxin Yu 3,936 Reputation points Microsoft Vendor

2023-03-21T08:48:25.2833333+00:00

Was your problem solved? Is there new errors?

Answer 1

RLWA32 30,101

Although the CRT will create inheritable files, the STARTUPINFO struct uses Win32 HANDLE types, not FILE* pointers returned from CRT functions.

For example, to obtain the low-level HANDLE for use with STARTUPINFO from a CRT FILE* pointer -

HANDLE hFile{ INVALID_HANDLE_VALUE };
FILE *fp = _tfopen(_T("Created by ConsoleApplication1.txt"), _T("w"));
if (fp)
    hFile = reinterpret_cast<HANDLE>(_get_osfhandle(_fileno(fp)));
if (hFile != INVALID_HANDLE_VALUE)
{
//   CreateProcess code using hFile in STARTUPINFO
}

RLWA32 30,101 Reputation points

2023-03-11T08:21:41.9433333+00:00

@Dennis Foreman By the way, If you close files using the CRT functions then do not call CloseHandle on hFile.
Dennis Foreman 0 Reputation points

2023-03-11T12:17:53.8833333+00:00
I had to do a minor change to use _wfopen_s instead of _tfopen like this:

err= _wfopen_s(&fp_o,std_outpath_out.GetString(), _T("w")); hFile_out_magick = reinterpret_cast<HANDLE>(_get_osfhandle(_fileno(fp_o)));

My new code successfully created the handle , but when I used it here: si.hStdOutput = hFile_out_magick;

I got a "debug assertion failed" error when Program 2 ran. I pasted a screenshot of the error message but it disappeared.

RLWA32 30,101

I can't diagnose problems in code I haven't seen so you can look at the code that I used in my own example.

MFC Dialog Application Functions -

void ShowLastError(DWORD code = GetLastError())
{
    LPTSTR pszMsg{};
    FormatMessage(FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS,
        nullptr, code, 0, (LPTSTR)&pszMsg, 0, nullptr);
    AfxMessageBox(pszMsg, MB_ICONERROR);
    LocalFree(pszMsg);
}

// Assumes that target application is in the same file system folder as the MFC Dialog application.
// File created by target application will be created in the MFC Dialog application working directory
void CMFCApplication1Dlg::OnBnClickedButton1()
{
    STARTUPINFOEX si{ sizeof si };
    SIZE_T sz{};
    PROCESS_INFORMATION pi{};
    SECURITY_ATTRIBUTES sa{ sizeof sa, nullptr, TRUE};
    CString strPath;
    TCHAR szCmdLine[1024]{};

    AfxGetModuleFileName(NULL, strPath);

    CPath exePath(strPath);
    exePath.RemoveFileSpec();
    exePath.Append(_T("ConsoleApplication1.exe"));
    exePath.QuoteSpaces();
    _tcscpy_s(szCmdLine, (LPCTSTR) exePath);
    
    HANDLE hFile{ INVALID_HANDLE_VALUE };
    FILE* fp{};
    errno_t err = _tfopen_s(&fp, _T("Created by ConsoleApplication1.txt"), _T("w"));
    if (!err)
        hFile = reinterpret_cast<HANDLE>(_get_osfhandle(_fileno(fp)));

    if (hFile != INVALID_HANDLE_VALUE)
    {
        // Use attribute list to ensure that only the intended file handle is inherited by the child process
        InitializeProcThreadAttributeList(si.lpAttributeList, 1, 0, &sz);
        si.lpAttributeList = static_cast<LPPROC_THREAD_ATTRIBUTE_LIST>(HeapAlloc(GetProcessHeap(), HEAP_ZERO_MEMORY, sz));
        if (InitializeProcThreadAttributeList(si.lpAttributeList, 1, 0, &sz) &&
            UpdateProcThreadAttribute(si.lpAttributeList, 0, PROC_THREAD_ATTRIBUTE_HANDLE_LIST, &hFile, sizeof hFile, NULL, NULL))
        {
            si.StartupInfo.dwFlags = STARTF_USESTDHANDLES | STARTF_USESHOWWINDOW;
            si.StartupInfo.hStdOutput = hFile;
            si.StartupInfo.wShowWindow = SW_SHOW;

            if (CreateProcess(nullptr, szCmdLine, nullptr, nullptr, TRUE, EXTENDED_STARTUPINFO_PRESENT,
                nullptr, nullptr, reinterpret_cast<LPSTARTUPINFO>(&si), &pi))
            {
                CloseHandle(pi.hThread);
                CloseHandle(pi.hProcess);
            }
            else
                ShowLastError();

            DeleteProcThreadAttributeList(si.lpAttributeList);
        }

        fclose(fp);
        HeapFree(GetProcessHeap(), 0, si.lpAttributeList);
    }
    else
        ShowLastError();
}

Console Application -

#include <iostream>
#include <conio.h>

int main(int argc, char* argv[])
{
    std::cerr << "Error output was not redirected to a file\n";
    std::cout << "Hello World from ConsoleApplication1!\n";
    for (int i = 0; i < argc; ++i)
        std::cout << argv[i] << std::endl;

    std::cerr << "Hit a key to exit\n";
    _getch();
    return 0;
}

And the results of running the code -

Redirect

Dennis Foreman 0 Reputation points

2023-03-11T15:58:20.6533333+00:00

I see what you are doing for ONE handle, but I cannot find any reference online for a PROC-THREAD-ATTRIBUTE- HANDLE-LIST (had to use dashes, because this system wouldn't let me use underscores for the keyword). I need to override both SYSOUT and SYSERR. SYSIN will not get used, so needs to be NULL. Can you advise me on that. I want to try your method. But, this doesn't explain why my method works for my Program 1.
RLWA32 30,101 Reputation points

2023-03-11T17:00:55.3933333+00:00

The different attributes that can be used are documented at UpdateProcThreadAttribute function. To use more than one HANDLE put the handles that you want the child process to inherit in an array and pass the address and size to this function.

Redirecting standard handles within a process with _wfreopen is completely different from passing an inheritable Win32 HANDLE through the STARTUPINFO struct to a child process.
Dennis Foreman 0 Reputation points

2023-03-11T22:02:51.09+00:00

thanks. I'll read your reference and try building the array. If I get it to work, I will use it for both programs. Your source code is very helpful.

RLWA32 30,101

Passing two inheritable handles -

FILE *fp{}, *fp2{};
errno_t err = _tfopen_s(&fp, _T("Redirected stdout for ConsoleApplication1.txt"), _T("w"));
errno_t err2 = _tfopen_s(&fp2, _T("Redirected stderr for ConsoleApplication1.txt"), _T("w"));


if (!err && !err2)
{
    HANDLE rgHandles[2]{};
    rgHandles[0] = reinterpret_cast<HANDLE>(_get_osfhandle(_fileno(fp)));
    rgHandles[1] = reinterpret_cast<HANDLE>(_get_osfhandle(_fileno(fp2)));

    // Use attribute list to ensure that only the intended file handle is inherited by the child process
    InitializeProcThreadAttributeList(si.lpAttributeList, 1, 0, &sz);
    si.lpAttributeList = static_cast<LPPROC_THREAD_ATTRIBUTE_LIST>(HeapAlloc(GetProcessHeap(), HEAP_ZERO_MEMORY, sz));
    if (InitializeProcThreadAttributeList(si.lpAttributeList, 1, 0, &sz) &&
        UpdateProcThreadAttribute(si.lpAttributeList, 0, PROC_THREAD_ATTRIBUTE_HANDLE_LIST, rgHandles, sizeof rgHandles, NULL, NULL))
    {
        si.StartupInfo.dwFlags = STARTF_USESTDHANDLES | STARTF_USESHOWWINDOW;
        si.StartupInfo.hStdOutput = rgHandles[0];
        si.StartupInfo.hStdError = rgHandles[1];
        si.StartupInfo.wShowWindow = SW_SHOW;
// Remainder of code omitted

RLWA32 30,101 Reputation points

2023-03-14T07:16:03.65+00:00

@Dennis Foreman if my suggestions and sample code for using HANDLEs in STARTUPINFO has resolved the child process inherited file output issue kindly accept the answer to close the question.

redirecting stdio for sequential programs in an MFC dialog

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