[PyQT] QSocketNotifier problem

[Axel Mittendorf]

Hi, I have a pyqt app which reads data from a fifo ( created with mkfifo)
and displays it
in a text field. I use PyQT 3.8.1 and QT 3.2.2 on SuSE Linux 7.0. I open the
fifo with os.open and create a QSocketNotifier (for reading). Other
processes write into
the fifo (I can do 'echo "hello" > /tmp/myfifo' for example), the activated
event of the QSocketNotifier comes and the data is written out. This
works without problems. BUT when the last process that writes into that
fifo exits, the activated event seems to come "always", about 10000 times
per second. What causes this behavior?

Is it a bad idea to use a SocketNotifier for a fifo?
BTW I used a QTimer before but I was ordered to use a SocketNotifier
instead (not my idea;-).

TIA, Axel

[Jeff Epler]

Re: [PyQT] QSocketNotifier problem

I suspect that QSocketNotifier is built on top of poll or select. The
documentation I have for poll is too terse for me to be sure, but I
suspect POLLIN behaves like the readfs set in select. The documentation
for select makes it clear that "readable" means that reading will not
block---so an end-of-file condition counts.

I'm not sure how you're really supposed to deal with this in a way that
doesn't block, doesn't spin, and doesn't involve doing something based
off a timer. Here's one idea that does involve a timer:

When reading the FIFO indicates an end-of-file condition, close the
fifo. Then, at intervals, attempt to reopen it. When opened with
O_NONBLOCK, the open fails with ENXIO if there is no writer.

Ensuring that there's always a writer for the FIFO might work too,
because then poll/select will not detect an end-of-file condition.

Jeff