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?

Don't want to waste resources on the extra thread and have to deal with locking not if I can avoid it. I also don't want to waste cpu cycles when my app is idle. — megazord, Mar 16 '12 at 17:10
Sounds to me like you're trying to optimize your code before seeing whether or not it actually needs to be. I'd say the thread approach is probably too complex, but polling periodically with a timer is pretty dang cheap. — Chris, Mar 16 '12 at 18:53

score 3 · Accepted Answer · edited Sep 26 '13 at 20:29

3

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.

edited Sep 26 '13 at 20:29

László Papp

51,870
39
111
135

answered Jun 27 '12 at 04:45

Eric des Courtis

5,135
6
24
37

score 3 · Answer 2 · edited Apr 04 '22 at 22:17

3

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

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

edited Apr 04 '22 at 22:17

Tustin2121

2,058
2
25
38

answered Mar 17 '12 at 12:21

je4d

7,628
32
46

score 2 · Answer 3 · answered Sep 21 '18 at 11:18

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();
}

score 2 · Answer 4 · edited May 23 '17 at 11:51

2

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.

edited May 23 '17 at 11:51

Community

1
1

answered Mar 16 '12 at 19:13

Sam Miller

23,808
4
67
87

score 2 · Answer 5 · answered Mar 17 '12 at 12:06

2

Try using QSocketNotifier

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

answered Mar 17 '12 at 12:06

Kamil Klimek

12,884
2
43
58

I didn't try it, found this solution somwhere – Kamil Klimek Mar 27 '12 at 13:00
It does actually work.... on Linux. Simplified your testcase to use Qt5 syntax: http://www.kdab.com/~dfaure/2019/stdin_socketnotifier.cpp – David Faure Jul 04 '19 at 15:05
For Windows, one needs to use QWinEventNotifier, as done here https://github.com/juangburgos/QConsoleListener/blob/master/src/qconsolelistener.cpp – David Faure Jul 04 '19 at 15:07

score 1 · Answer 6 · answered Mar 16 '12 at 18:14

1

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.

answered Mar 16 '12 at 18:14

Karlson

2,958
1
21
48

This sounds like a good idea until you look at the impact on battery life for mobiles and laptops. Not to mention you introduce an artificial latency that makes your application seem sluggish. – Eric des Courtis Sep 26 '17 at 19:54
@ericdescourtis Given this answer is over 5 years old there are much better options now. – Karlson Sep 26 '17 at 20:15
‮?degnahc sah yltcaxe tahw ot sa su nethgilne uoy dluoC !taerG – Eric des Courtis Sep 27 '17 at 14:05

Reading asynchronously from stdin with Qt

6 Answers6