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Writing to the standard input of a process

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  • A Offline
    A Offline
    A1exander_Z
    wrote on last edited by
    #8

    Thank you, it helped - at least now I can get the response. I tried the following code:

    @process = new QProcess();
    connect(process, SIGNAL(finished(int, QProcess::ExitStatus)), this, SLOT(finished(int, QProcess::ExitStatus)));
    process->setProgram("test.exe");
    process->start();
    process->write("test\n");
    process->waitForFinished();
    @

    Slot:

    @void MainWindow::finished(int exitCode, QProcess::ExitStatus status)
    {
    qDebug() << "Finished";
    qDebug() << exitCode;
    qDebug() << process->readAllStandardOutput();
    }@

    The code in the 'finished' slot is executed and prints the output produced by the called program (echoed "test"). The actual difference seems to be in the 'process->waitForFinished()' call. If I remove it, the C# program does not terminate (the created process remains in the list of processes).

    The problem is than I need to execute it asynchronously from my GUI application, with a possibility for user to terminate it, so the GUI should remain responsive.

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    • SGaistS Offline
      SGaistS Offline
      SGaist
      Lifetime Qt Champion
      wrote on last edited by
      #9

      Does it also happen if you add waitForStarted before writing to it ?

      Interested in AI ? www.idiap.ch
      Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

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      • A Offline
        A Offline
        A1exander_Z
        wrote on last edited by
        #10

        Yes, adding process->waitForStarted() before process->write("test\n") does not change anything, the process does not terminate if there is no process->waitForFinished() line in the end. Closing (or not closing) the write channel has no effect in all cases.

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        0
        • SGaistS Offline
          SGaistS Offline
          SGaist
          Lifetime Qt Champion
          wrote on last edited by
          #11

          I just forgot one basic question… What version of Qt are you using on what version of Windows ?

          Interested in AI ? www.idiap.ch
          Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

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          • A Offline
            A Offline
            A1exander_Z
            wrote on last edited by
            #12

            Qt 5.2.0 on Windows 7 Ultimate 64 bit, Service Pack 1.

            I have tried to build my program using two compilers, MinGW 32 bit and MSVC2012 64 bit. The behavior is the same in both cases.

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            • SGaistS Offline
              SGaistS Offline
              SGaist
              Lifetime Qt Champion
              wrote on last edited by
              #13

              Does it also happen if you call a standard cmd command ?

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              Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

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              • A Offline
                A Offline
                A1exander_Z
                wrote on last edited by
                #14

                I am not sure how to call a standard cmd command from a Qt application (I have tried process->setProgram("dir"), it runs but produces no output to stdout), but I did further research.

                I wrote a simple program in C++ and compiled it with MinGW:

                @#include <iostream>
                #include <string>

                int main(int argc, char *argv[])
                {
                std::string s;
                std::cin >> s;
                std::cout << s << std::endl;
                }@

                It works both with and without process->waitForFinished() call, my Qt application can write to its standard input and retreive echoed string from its output. Writing from a slot connected to the 'QProsess::started' signal also works. It seems that the problem is with my C# program after all.

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                • SGaistS Offline
                  SGaistS Offline
                  SGaist
                  Lifetime Qt Champion
                  wrote on last edited by
                  #15

                  Did you try with a similar minimal application written in C# ?

                  Interested in AI ? www.idiap.ch
                  Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

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                  • A Offline
                    A Offline
                    A1exander_Z
                    wrote on last edited by
                    #16

                    Yes, I have tried this:

                    @class Program
                    {
                    static int Main(string[] args)
                    {
                    string s;
                    s = Console.ReadLine();
                    Console.WriteLine(s);
                    return 0;
                    }
                    }@

                    Works if I place waitForFinished() call after writing to the standard input of the process, but never terminates without it. Changing configuration of the C# program from Debug to Release in Visual Studio does not help.

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                    • SGaistS Offline
                      SGaistS Offline
                      SGaist
                      Lifetime Qt Champion
                      wrote on last edited by
                      #17

                      Maybe a silly question (I don't know much about C#) but aren't you missing a "\n" here ?

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                      Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

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                      • A Offline
                        A Offline
                        A1exander_Z
                        wrote on last edited by
                        #18

                        No, Console.WriteLine() method inserts it automatically. And with waitForFinished() call it works even when I change Console.WriteLine() to Console.Write(), which does not insert a newline symbol.

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                        • SGaistS Offline
                          SGaistS Offline
                          SGaist
                          Lifetime Qt Champion
                          wrote on last edited by
                          #19

                          Could there be some subtle difference on how the C# runtime uses stdin/stdout ?

                          Interested in AI ? www.idiap.ch
                          Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

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                          • A Offline
                            A Offline
                            A1exander_Z
                            wrote on last edited by
                            #20

                            Yes, it seems that there is some difference, but I am not a C# expert and I have no idea what it is. I will try a simple program compiled using Mono but for now, I think it is easier to send all data as command line parameters.

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                            • SGaistS Offline
                              SGaistS Offline
                              SGaist
                              Lifetime Qt Champion
                              wrote on last edited by
                              #21

                              As long as it's clean and works, do what is best.

                              Happy Coding !

                              Interested in AI ? www.idiap.ch
                              Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

                              M 1 Reply Last reply
                              0
                              • SGaistS SGaist

                                As long as it's clean and works, do what is best.

                                Happy Coding !

                                M Offline
                                M Offline
                                mhmdkanj
                                wrote on last edited by
                                #22

                                @SGaist I am currently having the same issue for C# executables and writing to their standard input. Assuming that I cannot change the C# code, how would I fix this in Qt in case you can enlighten me after a few years.. Many thanks!

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                                • SGaistS Offline
                                  SGaistS Offline
                                  SGaist
                                  Lifetime Qt Champion
                                  wrote on last edited by
                                  #23

                                  hi,

                                  Did you apply the same technique as @A1exander_Z did ?

                                  Interested in AI ? www.idiap.ch
                                  Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

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