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

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  • Christian EhrlicherC Offline
    Christian EhrlicherC Offline
    Christian Ehrlicher
    Lifetime Qt Champion
    wrote on last edited by
    #5

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

    I need to redirect everything from my main program.

    Why do you use cout in your own program in the first place? Esp. when it has no console...

    Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
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    1 Reply Last reply
    0
    • E Offline
      E Offline
      eupuser00
      wrote on last edited by
      #6

      No, I am not using cout at all (only qDebug, qWarning,...). But the external tools or the libs use it and they log to stdout and these messages are only visible in Qt app output console(if i run the tool from Qt Creator) and not in my redirected log files.

      1 Reply Last reply
      0
      • hskoglundH Offline
        hskoglundH Offline
        hskoglund
        wrote on last edited by
        #7

        Hi, what happens if you launch your console app with an ancient "DOS"-style redirection, e.g.

        yourconsoleapp.exe > filename.txt
        
        E 1 Reply Last reply
        2
        • hskoglundH hskoglund

          Hi, what happens if you launch your console app with an ancient "DOS"-style redirection, e.g.

          yourconsoleapp.exe > filename.txt
          
          E Offline
          E Offline
          eupuser00
          wrote on last edited by
          #8

          @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'

          JonBJ 1 Reply Last reply
          0
          • hskoglundH Offline
            hskoglundH Offline
            hskoglund
            wrote on last edited by
            #9

            Same if you omit all freopen() calls?

            1 Reply Last reply
            0
            • E 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'

              JonBJ Offline
              JonBJ Offline
              JonB
              wrote on 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
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              • Christian EhrlicherC Offline
                Christian EhrlicherC Offline
                Christian Ehrlicher
                Lifetime Qt Champion
                wrote on 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/
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                1 Reply Last reply
                1
                • JonBJ 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.

                  E Offline
                  E Offline
                  eupuser00
                  wrote on 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.

                  JonBJ D 2 Replies Last reply
                  1
                  • E 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.

                    JonBJ Offline
                    JonBJ Offline
                    JonB
                    wrote on 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 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
                      1
                      • D 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.

                        D Offline
                        D Offline
                        Developer123
                        wrote on last edited by
                        #15

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

                        1 Reply Last reply
                        0
                        • E 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.

                          D Offline
                          D Offline
                          Developer123
                          wrote on last edited by
                          #16

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

                          Christian EhrlicherC D 2 Replies Last reply
                          0
                          • D Developer123

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

                            Christian EhrlicherC Offline
                            Christian EhrlicherC Offline
                            Christian Ehrlicher
                            Lifetime Qt Champion
                            wrote on 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

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

                              D Offline
                              D Offline
                              Developer123
                              wrote on 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

                              JonBJ 1 Reply Last reply
                              0
                              • D 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

                                JonBJ Offline
                                JonBJ Offline
                                JonB
                                wrote on 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
                                0

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