Reading asynchronously from stdin with Qt

Question

I would like to read asynchronously from stdin with Qt. I don't want to use a separate thread or have to setup a timer to periodically check if the file descriptor has data. How can I do this?

Answer 1

If you want to integrate stdin/stdout/stderr I/O with the QT event loop, you can either:

1. Use a QSocketNotifier and do the I/O yourself with read(2) and write(2), or
2. Get a QFile object and call bool QFile::open ( int fd, OpenMode mode ) to do Qt-style I/O with it.

Answer 2

Maybe this works for you:

https://github.com/juangburgos/QConsoleListener

Works like this:

#include <QCoreApplication>
#include <QDebug>

#include <QConsoleListener>

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    // listen to console input
    QConsoleListener console;
    QObject::connect(&console, &QConsoleListener::newLine, &a, [&a](const QString &strNewLine) {
        qDebug() << "Echo :" << strNewLine;
        // quit
        if (strNewLine.compare("q", Qt::CaseInsensitive) == 0)
        {
            qDebug() << "Goodbye";
            a.quit();
        }
    });

    qDebug() << "Listening to console input:";
    return a.exec();
}

Answer 3

Try using QSocketNotifier

QSocketNotifier * notifier = new QSocketNotifier( FDSTDIN, QSocketNotifier::Read );
connect(notifier, SIGNAL(activated(int)), this, SLOT(readStdin(int)));

Answer 4

If you read the Qt documentation, it says you cannot do this because it is not portable. Why not use a TCP socket that should work assuming you have control over the other end. Worst case you can make a proxy application.

Answer 5

If you are open to using boost, you could use the Asio library. A posix::stream_descriptor assigned to STDIN_FILENO works quite well. See also this answer.

Answer 6

As Chris pointed out the best way would be to have a separate thread that would poll from the stdin and populate data for the display or processing thread to process.

Now you can certainly set up QTimer and set up a handler for the timeout() signal to read from stdin as well. The method of implementing is entirely up to you.

And for the second method you can take a look at QT's timer class documentation for an example on how to do this. One thing to remember would be to actually restart the timer once your processing is completed.