在Qt中读取连续QProcess的stdoutput

Question

I am having some problem regarding QProcess using Qt. I have connected the following function with the onClick event of a push button. Basically, I want to execute another file when this button is clicked, and get its output on my Qt program. This file calculator executes, displays some output, and then waits for an input from the user.

void runPushButtonClicked() {
    QProcess myprocess;
    myprocess.start("./calculator")
    myprocess.waitForFinished();
    QString outputData= myprocess.readStandardOutput();
    qDebug() << outputData;
}

In a scenario, when the calculator is such a file that only outputs some results and terminates eventually, this works perfect. But, in case when the calculator waits for some further input from the user after outputting some results, I get nothing in my outputData . In fact, waitForFinished() would time out, but even when I remove waitForFinished() , the outputData would still be empty.

I have already tried some of the solutions available here on SO, but have been unable to handle this case. Any guidance will be much appreciated.

Answer 1

I would suggest, that you setup a signal handler, that gets called when the subprocess produces output. Eg you have to connect to readyReadStandardOutput .

Then you can identify, when subprocess demands input and send the input you want. This will be done in readSubProcess() .

main.cpp

#include <QtCore>
#include "Foo.h"

int main(int argc, char **argv) {
   QCoreApplication app(argc, argv);

   Foo foo;

   qDebug() << "Starting main loop";
   app.exec();
}

In the following, the subprocess is started and the input checked. When the calculator program finishes, the main also exits.

Foo.h

#include <QtCore>

class Foo : public QObject {
   Q_OBJECT
   QProcess myprocess;
   QString output;

public:
   Foo() : QObject() {
      myprocess.start("./calculator");

      // probably nothing here yet
      qDebug() << "Output right after start:"
               << myprocess.readAllStandardOutput();

      // get informed when data is ready
      connect(&myprocess, SIGNAL(readyReadStandardOutput()),
              this, SLOT(readSubProcess()));
   };

private slots:
   // here we check what was received (called everytime new data is readable)
   void readSubProcess(void) {
      output.append(myprocess.readAllStandardOutput());
      qDebug() << "complete output: " << output;

      // check if input is expected
      if (output.endsWith("type\n")) {
         qDebug() << "ready to receive input";

         // write something to subprocess, if the user has provided input,
         // you need to (read it and) forward it here.
         myprocess.write("hallo back!\n");
         // reset outputbuffer
         output = "";
      }

      // subprocess indicates it finished
      if (output.endsWith("Bye!\n")) {
         // wait for subprocess and exit
         myprocess.waitForFinished();
         QCoreApplication::exit();
      }
   };
};

For the subprocess calculator a simple script is used. You can see the where output is generated and where input is expected.

#/bin/bash

echo "Sub: Im calculator!"

# some processing here with occasionally feedback
sleep 3
echo "Sub: hallo"

sleep 1

echo "Sub: type"
# here the script blocks until some input with '\n' at the end comes via stdin
read BAR

# just echo what we got from input
echo "Sub: you typed: ${BAR}"

sleep 1
echo "Sub: Bye!"

If you do not need to do anything else in your main process (eg show a GUI, manage other threads/processes....), the easiest would be to just in a sleep in a loop after subprocess creation and then something like readSubprocess .