rcbc@forseti.cs.cornell.edu (Robert Cooper) (03/27/90)
We've run into a rather tricky problem with the way X11 handles
input events and the way Isis handles input on multiple file descriptors.
This problem occurs with the "grid" demo, on X11 releases 3 and 4 (and
possibly earlier releases). grid will often fail to repaint itself when its
window is uncovered.
Actually the problem is not confined to Isis, but will happen in any
program that uses needs to listen to input from multiple sources, including
X. Basically the XFlush routine secretly reads data off the X
file descriptor and puts it in the X input event queue. This
is a real problem when your program does something like the following
sequence:
do some X output
XFlush(display);
do a select on the X file descriptor and some of your own file descs.
if the X file descriptor has input pending then
process X input
The select doesn't see the new X input events because they've already
been read. The "obvious" solution of processing all pending
events after doing a flush is a problem because almost any
input event that needs processing involves output and many
output routines silently call XFlush as they need to. In fact
the XFlush in the above sequence is usually unnecessary to trigger
this behaviour.
Does anyone know a good solution to this problem?
I've fixed "grid" with a kluge that involves checking for events, and
processing them after (almost) every output. This is quite messy.
Perhaps Isis should have a notion of user callable "work_pending"
routines in addition to file descriptors? I'd rather not do this
just to fix a bad citizen like X.
(The question remains of why XFlush does input at all. It does this in
response to EWOULDBLOCK errors on output. Presumably on some systems that X
runs on output blocks when there is pending input on the same descriptor.)
-- Robert Cooper (rcbc@cs.cornell.edu)