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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
    #1

    I am trying to run a small program written in C# from my Qt application and exchange the data with it using standard input and output. I can create a process, start it and read its output by calling QProcess::readAllStandardOutput() in a slot connected to the 'finished' signal of the process. However, I cannot write to the program's standard input. I have tried to do it from a slot connected to the 'started' signal of the process in the following way:

    qDebug() << process->write("test\n");

    The code executes and the correct number of bytes written is printed to the output, but my C# program does not read any data using Console.Read() or Console.ReadLine() functions. I have tried to write "test\r", "test\r\n", test\n\r" - nothing works. Is it a problem with C# program or am I doing something wrong in my Qt code?

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    • A Offline
      A Offline
      A1exander_Z
      wrote on last edited by
      #2

      I forgrot to mention that the C# program is a Windows console application and it works as expected when run from the console.

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      • SGaistS Offline
        SGaistS Offline
        SGaist
        Lifetime Qt Champion
        wrote on last edited by
        #3

        Hi,

        Does your C# application work like this:

        start

        do something

        end

        ?

        Or is it waiting for an input on stdin ?

        On the former case, you should directly provide the parameters when starting your QProcess

        Hope it helps

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        • A Offline
          A Offline
          A1exander_Z
          wrote on last edited by
          #4

          It does wait for an input from stdin. In the simplest case, it has only two following lines in its Main method:

          string command = Console.ReadLine();
          Console.WriteLine(command);

          When run from the console it works as expected, but when I run it from my Qt application using QProcess, it waits indefinitely. Passing command line parameters works.

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          • SGaistS Offline
            SGaistS Offline
            SGaist
            Lifetime Qt Champion
            wrote on last edited by
            #5

            Then can you show the code where you use QProcess ?

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            • A Offline
              A Offline
              A1exander_Z
              wrote on last edited by
              #6

              Process creation ('QProcess* process' declaration is in a header file):

              @process = new QProcess();
              connect(process, SIGNAL(finished(int, QProcess::ExitStatus)),
              this, SLOT(finished(int, QProcess::ExitStatus)));
              connect(process, SIGNAL(started()), this, SLOT(started()));
              process->setProgram("test.exe");
              process->open();@

              Slots:

              @void MainWindow::started()
              {
              qDebug() << process->write("test\n");
              }@

              @void MainWindow::finished(int exitCode, Process::ExitStatus status)
              {
              qDebug() << "Finished";
              qDebug() << exitCode;
              }@

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              • SGaistS Offline
                SGaistS Offline
                SGaist
                Lifetime Qt Champion
                wrote on last edited by
                #7

                You are using QProcess in an unusual way (at least to me)

                Since its a command line application, I would rather use the synchronous approach described "here":http://qt-project.org/doc/qt-5/qprocess.html#synchronous-process-api

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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 ?

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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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                      • 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 ?

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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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                            • 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# ?

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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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                                    • 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 ?

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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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