Using QTextStream to read a file that is being written to?

Question

I currently read data from a file line by line like so

readFile(QFile *file)
{
    QTextStream in(file);
    while(!in.atEnd())
    {
        // Do stuff
    }
}

I need to extend this to handle files that are currently being written to. The while loop should only exit if nothing has been written to the file for N seconds.

This should be easily implemented by adding in a timer and a sleep/wait loop, continuously checking if we are still atEnd() . However, the documentation is not very clear whether the return value of atEnd() will change if new data has been appended to the file.

If I use a QTextStream to read from a file that is being written to, will the return value of atEnd() accurately reflect the change in file contents?

Answer 1

will the return value of atEnd() accurately reflect the change in file contents?

Yes

You can test this by executing the following function:

void FileReader::_appendFileTest()
{    
    QFile file("/home/cklein/appendFile");
    file.open(QIODevice::ReadOnly);

    QTextStream in(&file);

    while(true)
    {
        while(!in.atEnd())
        {
            QString line = in.readLine();

            qDebug() << line;
        }
        SleepThread::msleep(1000);
    }
}

_{Code for SleepThread taken from here . It just exposes the protected QThread::msleep() function.}

And while the application is running, type this in the terminal

~ $ echo "foo" >> appendFile
~ $ echo "bar" >> appendFile

You can see that without resetting or re-opening the file, the atEnd() function returns a false when the file is appended to.