How to redirect stdout & stderr to a file in a Qt console App (QCoreApplication) ?

[eupuser00]

@hskoglund

Not output at all. freopen is not logging anything when i use:
freopen("log_file.txt", "w", stdout);
or
freopen("log_file.txt", "w", (FILE*)1);

Also Qt keeps complaining for freopen :

main.cpp:16:1: warning: 'freopen' is deprecated: This function or variable may be unsafe. Consider using freopen_s instead. To disable deprecation, use _CRT_SECURE_NO_WARNINGS. See online help for details.
stdio.h:242:20: note: 'freopen' has been explicitly marked deprecated here
vcruntime.h:255:55: note: expanded from macro '_CRT_INSECURE_DEPRECATE'
vcruntime.h:245:47: note: expanded from macro '_CRT_DEPRECATE_TEXT'

[hskoglund]

Same if you omit all freopen() calls?

[JonB]

@eupuser00
Is there any reason you don't want to look at low-level e.g. fdopen() when I've already explained that you need to do that to achieve what you want, and why stdout does not work for you? Makes me wonder what the point of answering was.

[Christian Ehrlicher]

@eupuser00 said in How to redirect stdout & stderr to a file in a Qt console App (QCoreApplication) ?:

But the external tools or the libs use it and they log to stdout and these messages

So how do you call them when not with QProcess? And when you use something else - why not use QProcess?

[eupuser00]

@hskoglund
Command line style redirection didn't work when I used "2>&1 > file" as one of the arguments to the QProcess::start().
With "freopen()" even the qDebug messages which were logged through my custom msg handler was failing. (Without "freopen()" the qDebug messages were logged.)

@Christian-Ehrlicher
I was calling the external programs through the static function in "QProcess::execute()". But now switched to use the "QProcess::start()" so that I can use "QProcess::setProcessChannelMode()" as shown below.

@JonB
I was able to get it working with File Handles and dup(). Here is the code snippet:

// Global scope
// Setting same file for stdout & stderr (and qDebug from custom msg handler 
// func, "customMessageOutput")
QFile outFile("log_file.txt");

execProg(){
  ...
  QProcess extTool;
  extTool.setProcessChannelMode(QProcess::ForwardedChannels);
  extTool.start(program, arg);
  extTool.waitForFinished();
  ...
}  

redirect_stdout_stderr() {
   outFile.open(QIODevice::WriteOnly | QIODevice::Append)
  fout = outFile.handle();
 
  // redirect stdout 
  _dup2(fout, _fileno(stdout))

  // redirect stderr 
  _dup2(_fileno(stdout), _fileno(stderr))
}

main(){
  redirect_stdout_stderr();    
  // Installing a custom message handler
  // Note: Definition for "customMessageOutput" is not shown here.
  qInstallMessageHandler(customMessageOutput);
  ...
  execProg();
  ...
}

• For brevity, I have not included error handling.

• I had trouble to redirect both stdout and stderr to the same file. I was opening the same file twice and trying to use 2 file descriptors/ handles. This got solved when I used:
  _dup2(_fileno(stdout), _fileno(stderr))

Thank you all for the suggestions and pointing me towards the right direction to achieve a solution.

I would like to know whether there is a better/elegant solution in Qt? If yes, then I would like to adapt to it.

PS: I will mark this thread as "solved" once I get some or no suggestions.

[JonB]

@eupuser00
Well done! For heeding the advice, and for getting the code right :)

Yes, dup/dup2 is for merging stdout & stderr.

From a GUI I did something like the following during start up in old code

close(0);
close(1);
close(2);
open("/dev/null", O_RDONLY);
open(logfile, O_WRONLY);
dup(1);

I like having stdin (0) opened from a null device, so any attempt to read from it delivers EOF.

Qt isn't going to provide a better solution. This operates at the lowest level.

You might have to check your code on Windows. Hopefully it works, I suspect I did that stuff via native calls as their will be some situation where it's better.

[David K. Hess]

Late to the party, but figured out some more of this situation that may help others.

The straight duping to fd 1 or 2 and/or use of _fileno() with stdout and stderr don't work when the app is launched normally within Windows by say a double-click in File Explorer (and thus has no console). The reason why is in that situation stdout and stderr both have an fd of -2 which I suppose indicates they are open for purposes of not breaking calling code but are not open in terms of actually doing anything when used. In other words, sort of a CRT /dev/null setup.

More info on that situation here: https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/fileno?view=msvc-160

So, this general pattern worked for me:

        FILE *loggingStreamStdout;
        FILE *loggingStreamStderr;
        QFile loggingFile("app.log");

        loggingFile.open(QIODevice::WriteOnly | QIODevice::Append | QIODevice::Unbuffered);
        int fd = loggingFile.handle();

        freopen_s(&loggingStreamStdout, "\\\\.\\NUL", "w", stdout);
        setvbuf(loggingStreamStdout, NULL, _IONBF, 0);
        freopen_s(&loggingStreamStderr, "\\\\.\\NUL", "w", stderr);
        setvbuf(loggingStreamStderr, NULL, _IONBF, 0);
        _dup2(fd, _fileno(stdout));
        _dup2(fd, _fileno(stderr));

        qInstallMessageHandler(messageOutput);

That uses a QFile to handle your log file and then freopens stdout and stderr on Window's version of /dev/null. This gives them valid file descriptors (instead of the -2). Then you can _dup2() the log file fd to them. I also like to set the streams as non-buffered so that tools like Baretail.exe show real-time output from the exe.

[Developer123]

@David-K-Hess It says identifier "_dup2" is undefined

[Developer123]

@eupuser00 it's almost useless for other people if you paste incomplete code...

[Christian Ehrlicher]

@Developer123 said in How to redirect stdout & stderr to a file in a Qt console App (QCoreApplication) ?:

it's almost useless for other people if you paste incomplete code...

_dup2() is a function, a function is defined in a header (as all functions not defined in the same source). Since the op did not post the includes you have to search the correct one by yourself which shouldn't be too hard. First google link: https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/dup-dup2?view=msvc-170

[Developer123]

@Developer123 I found a solution to print everything to a file, if anyone else needs, works out of the box

https://stackoverflow.com/a/11202102/2076973

[JonB]

@Developer123
The code answers there all implement sending Qt error messages within an application to a file. Perfectly useful in itself, but nothing to do with the topic title here of "How to redirect stdout & stderr to a file in a Qt console App (QCoreApplication)".