test_threaded_socketnotifier

example_output.txt

Thread: started!
Thread: writing: Write 0
 ready to read!
 read: Write 0
Thread: writing: Write 1
 ready to read!
 read: Write 1
Thread: writing: Write 2
 ready to read!
 read: Write 2
Thread: writing: Write 3
 ready to read!
 read: Write 3
Thread: writing: Write 4
Thread: exiting
 ready to read!
 read: Write 4

test_threaded_socketnotifier.cpp

#include <QCoreApplication>
#include <QSocketNotifier>
#include <QObject>
#include <QThread>
#include <QDebug>

#include <stdlib.h> // EXIT_FAILURE
#include <unistd.h> // read, write, close, pipe
#include <limits.h> // PIPE_BUF

#define READ_END 0
#define WRITE_END 1

/**
 * @brief The ThreadedWriter class
 * Yes, I know: deriving from QThread is bad practice.
 * But good enough for this short example.
 */
class ThreadedWriter: public QThread
{
private:
    int write_fd = 0;

public:
    explicit ThreadedWriter(int wfd, QObject *parent = nullptr)
        : QThread(parent)
        , write_fd(wfd)
    {
    }

    virtual void run() override {
        int i;
        char buf[32];

        qDebug() << "Thread: started!";

        for (i = 0; i < 5; i++) {
            QThread::msleep(1000);
            snprintf(buf, sizeof(buf) - 1, "Write %d", i);
            qDebug() << "Thread: writing:" << buf;
            ::write(write_fd, buf, sizeof(buf));
        }

        qDebug() << "Thread: exiting";
        quit();
    }
};

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

    int fd[2] = {0, 0};

    if (::pipe(fd) == -1) {
        return EXIT_FAILURE;
    }

    ThreadedWriter tw(fd[WRITE_END]);
    QObject::connect(&tw, &QThread::finished, &app, &QCoreApplication::quit);

    QSocketNotifier sn(fd[READ_END], QSocketNotifier::Read);
    QMetaObject::Connection conn = QObject::connect(&sn, &QSocketNotifier::activated, &sn, [](int sock) {
        Q_UNUSED(sock)
        qDebug() << " ready to read!";
        char buf[PIPE_BUF] = {0};
        ::read(sock, buf, sizeof(buf) - 1);
        qDebug() << " read:" << buf;
    });

    tw.start();

    // { MAIN EVENT LOOP
    const int mainret = app.exec();
    // } MAIN EVENT LOOP

    QObject::disconnect(conn);
    ::close(fd[READ_END]);
    ::close(fd[WRITE_END]);
    return mainret;
}