How is my code Receiving Signals before the Qt Eventloop is Started

Question

I'm trying to understand why putting the a.exec() call in the following Qt 4.8 code does not need to happen before my QProcess waitForFinished() and waitForStarted() calls can work. I understand that a.exec() starts the event loop, and in my mind the waitFor* slots need to receive a signal ( i.e 'started()' or 'finished()' ) before moving on with execution. How can this happen if the event loop has not been started?

Documentation for waitForStarted():

Blocks until the process has started and the started() signal has been emitted, or until msecs milliseconds have passed.

Code:

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

    QCoreApplication a(argc, argv);

    // Exec the i2c command to get the power button status
    QProcess i2cprocess;
    i2cprocess.start("/usr/bin/i2cget -f -y 1 0x4b 0x45");

    // Wait for it to start
    if(!i2cprocess.waitForStarted())
    {
        qDebug() << "Could not start QProcess to check power button status.";
        exit(-1);
    }

    // Wait for it to finish
    bool returnValue = i2cprocess.waitForFinished();
    if ( returnValue )
    {
        QByteArray status = i2cprocess.readAllStandardOutput().trimmed();
        bool ok;
        quint16 hexValue = status.toUInt(&ok, 16);
        qDebug() << "Power button status: " << status << hexValue << (hexValue & 0x01);

        // We want LSB
        exit(hexValue & 0x01);

    }
    else
    {
        qDebug() << "Error, process never completed to check power button status.";
        exit(-1);
    }

    return a.exec();

}

Answer 1

The Qt documentation for QProcess states: -

QProcess provides a set of functions which allow it to be used without an event loop, by suspending the calling thread until certain signals are emitted:

waitForStarted() blocks until the process has started.

waitForReadyRead() blocks until new data is available for reading on the current read channel.

waitForBytesWritten() blocks until one payload of data has been written to the process.

waitForFinished() blocks until the process has finished.

Calling these functions from the main thread (the thread that calls QApplication::exec()) may cause your user interface to freeze.