How to read new lines from a file as another process is appending them?

Question

So I have ffmpeg writing its progress to a text file, and I need to read the new values (lines) from the said file. How should I approach this using Qt classes in order to minimize the amount of code I have to write?
I don't even have an idea where to start, other than doing ugly things like seeking to the end, storing this pos, then seeking to the end again a bit later and comparing the new pos to the previous one. It's unclear to me if QTextStream can be used here or not, for instance.

Answer 1

I used Win32 API own interface for the file system notification some time ago and that worked 100% reliably. Modern OSes provide us with notifications for the file change. And Qt incorporates such functionality as well. Specifically for the purpose of tracking the file changes I would use QFileSystem::fileChanged signal to start the slot myFileReadNextBuffer() method only in case if the file was changed. But then you would still want to evaluate how many bytes were added by subtracting the previous from the new file length. And there is also relative question here: How to know when and which files are changed in windows filesystem with winapi.

If the file is only growing:

Whether the file is text-based or not I would open it in shared mode and read to the end and read more till the end when the notification received.