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

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

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  • E eupuser00
    14 Feb 2020, 16:17

    @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'

    J Online
    J Online
    JonB
    wrote on 14 Feb 2020, 16:33 last edited by
    #10

    @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.

    E 1 Reply Last reply 17 Feb 2020, 19:28
    0
    • C Offline
      C Offline
      Christian Ehrlicher
      Lifetime Qt Champion
      wrote on 14 Feb 2020, 16:52 last edited by
      #11

      @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?

      Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
      Visit the Qt Academy at https://academy.qt.io/catalog

      1 Reply Last reply
      1
      • J JonB
        14 Feb 2020, 16:33

        @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.

        E Offline
        E Offline
        eupuser00
        wrote on 17 Feb 2020, 19:28 last edited by
        #12

        @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.

        J D 2 Replies Last reply 17 Feb 2020, 19:40
        1
        • E eupuser00
          17 Feb 2020, 19:28

          @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.

          J Online
          J Online
          JonB
          wrote on 17 Feb 2020, 19:40 last edited by
          #13

          @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.

          1 Reply Last reply
          0
          • D Offline
            D Offline
            David K. Hess
            wrote on 7 Oct 2021, 23:45 last edited by
            #14

            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.

            D 1 Reply Last reply 21 Nov 2022, 15:58
            1
            • D David K. Hess
              7 Oct 2021, 23:45

              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.

              D Offline
              D Offline
              Developer123
              wrote on 21 Nov 2022, 15:58 last edited by
              #15

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

              1 Reply Last reply
              0
              • E eupuser00
                17 Feb 2020, 19:28

                @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.

                D Offline
                D Offline
                Developer123
                wrote on 21 Nov 2022, 16:03 last edited by
                #16

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

                C D 2 Replies Last reply 21 Nov 2022, 16:08
                0
                • D Developer123
                  21 Nov 2022, 16:03

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

                  C Offline
                  C Offline
                  Christian Ehrlicher
                  Lifetime Qt Champion
                  wrote on 21 Nov 2022, 16:08 last edited by
                  #17

                  @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

                  Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
                  Visit the Qt Academy at https://academy.qt.io/catalog

                  1 Reply Last reply
                  1
                  • D Developer123
                    21 Nov 2022, 16:03

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

                    D Offline
                    D Offline
                    Developer123
                    wrote on 22 Nov 2022, 11:48 last edited by
                    #18

                    @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

                    J 1 Reply Last reply 22 Nov 2022, 11:50
                    0
                    • D Developer123
                      22 Nov 2022, 11:48

                      @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

                      J Online
                      J Online
                      JonB
                      wrote on 22 Nov 2022, 11:50 last edited by
                      #19

                      @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)".

                      1 Reply Last reply
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