[Xcb] best practice for non-threaded multiple file descriptor event loop.

Ben Hildred 42656e at gmail.com
Sun Jul 12 13:07:09 PDT 2015

On Sun, Jul 12, 2015 at 12:32 PM, Josh Triplett <josh at joshtriplett.org>

> On Sun, Jul 12, 2015 at 05:27:38PM +0000, Bart Massey wrote:
> > xcb_get_file_descriptor() does what you think it does. The description in
> > the documentation is based on the fact that xcb_connect_to_fd() is always
> > called to open a connection: the "you" is a bit confusing there and
> should
> > probably be fixed.
> ok so fine for my use, we just need to fix the docs. If you have a pointer
to the source, I will write a diff.

> > Josh's email is mostly talking about selecting on replies rather than
> > events.
> I was talking about either replies or events; in either case, it's
> possible for XCB to have already read them off the wire and buffered
> them, so the fd no longer has data, but you shouldn't block.
> > X mixes replies and events on a single TCP stream, and XCB demuxes and
> > queues them. You don't really have access to the event queue like in Xlib
> > because that is error-prone and expensive. You will have to be kind of
> > careful with the selects and use the non-blocking event grabber
> > xcb_poll_for_event() to get this to work. It will be fiddly to get right,
> > but as far as I can remember it should work. If you are careful, the only
> > time XCB should block is waiting for a server response when you force a
> > cookie early. XCB doesn't really care if you block in your own code.
> >
> > while true
> >     select on fd
> >     if fd is x connection
> >         while xcb_poll_for_event returns an event
> >             process the event
> >     else
> >         handle the fd however
> This indeed basically was my plan (select at the bottom of the loop
instead of the top, but same idea)

> That *should* work, assuming no other thread touches XCB, and assuming
> you check for outstanding replies in addition to events whenever you
> have request cookies that you don't have the replies for yet.
> - Josh Triplett

Single threaded no no worries about another thread, but
if xcb_poll_for_event does not check for replies what is the non-blocking
way to check for replies? or do I just check if an event is a reply event?

Ben Hildred
Automation Support Services
303 815 6721
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