<div dir="ltr"><p>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.</p><p dir="ltr">Josh's email is mostly talking about selecting on replies rather than events. I'm pretty sure the concerns expressed about the single-threaded case in my email were unfounded.</p><p dir="ltr">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.<br></p><p>while true<br> select on fd<br> if fd is x connection<br> while xcb_poll_for_event returns an event<br> process the event<br> else<br> handle the fd however<br> </p> <p dir="ltr">I think. Somebody correct me if I'm wrong. </p> <br><div class="gmail_quote"><div dir="ltr">On Fri, Jul 10, 2015, 17:54 Ben Hildred <<a href="mailto:42656e@gmail.com" target="_blank">42656e@gmail.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><br clear="all"><div>First off the description of xcb_get_file_descriptor at <a href="http://xcb.freedesktop.org/PublicApi/#index8h2" target="_blank">http://xcb.freedesktop.org/PublicApi/#index8h2</a> gives the definate impression that it is only valid to call when you already know the result. I t would be useful to clarify if that is indeed the case or if it will work with any connect function which would make more sense. So while researching that (to no good answer) I came across some disturbing comments.</div><div><br></div><div>From <a href="http://lists.freedesktop.org/archives/xcb/2013-April/008224.html" target="_blank">http://lists.freedesktop.org/archives/xcb/2013-April/008224.html</a></div><div><span style="color:rgb(0,0,0);white-space:pre-wrap">>On Mon, Apr 08, 2013 at 07:52:19PM -0400, Micah Nordland wrote:</span></div><div><span style="color:rgb(0,0,0);white-space:pre-wrap">>></span><i style="color:rgb(0,0,0);white-space:pre-wrap"> I believe this is what you are looking for:</i></div><div><span style="color:rgb(0,0,0);white-space:pre-wrap">>></span><i style="color:rgb(0,0,0);white-space:pre-wrap"> </i></div><div><span style="color:rgb(0,0,0);white-space:pre-wrap">>></span><i style="color:rgb(0,0,0);white-space:pre-wrap"> int xcb_get_file_descriptor (xcb_connection_t *c)</i></div><div><span style="color:rgb(0,0,0);white-space:pre-wrap">>></span><i style="color:rgb(0,0,0);white-space:pre-wrap"> Access the file descriptor of the connection. ]</i></div><div><span style="color:rgb(0,0,0);white-space:pre-wrap">></span></div><div><span style="color:rgb(0,0,0);white-space:pre-wrap">>That does get a file descriptor referring to the X connection, but you</span></div><div><span style="color:rgb(0,0,0);white-space:pre-wrap">>can't use that to wait for X responses without quite a bit of care,</span></div><div><span style="color:rgb(0,0,0);white-space:pre-wrap">>because XCB might have some already read off the wire and stored in</span></div><div><span style="color:rgb(0,0,0);white-space:pre-wrap">>internal buffers; thus, if you block on the X connection having data,</span></div><div><span style="color:rgb(0,0,0);white-space:pre-wrap">>you might wait forever while XCB already has data to give you.</span></div><div><span style="color:rgb(0,0,0);white-space:pre-wrap">></span></div><div><span style="color:rgb(0,0,0);white-space:pre-wrap">>- Josh Triplett</span><br></div><div><span style="color:rgb(0,0,0);white-space:pre-wrap"><br></span></div><div><div>From <a href="http://lists.freedesktop.org/archives/xcb/2006-August/001847.html" target="_blank">http://lists.freedesktop.org/archives/xcb/2006-August/001847.html</a></div><div>>The problem with select()ing on the XCB FD is that I'm</div><div>>pretty sure you can get into races with XCB in a</div><div>>multithreaded situation, and even in a single-threaded</div><div>>situation you may experience false wakeups from reply data.</div><div>>If you select(), then XCBPollForEvent() until no more events</div><div>>are available whenever the XCB FD shows readable, it will</div><div>>*probably* work, but I'm a little confused about races</div><div>>between calling XCBPollForEvent(), live data arriving, and</div><div>>falling back into the select(). Even if it works, I'm not</div><div>>convinced that we want to *guarantee* that it will work...</div><div>></div><div>>It seems clear from folks' comments that XCB core or at</div><div>>least Util needs to support some better I/O-multiplexing</div><div>>mechanism for event-driven single-threaded programs. Some</div><div>>of us will chat offline about this RSN, and hopefully there</div><div>>will also be an online conversation about what it should</div><div>>be. Good ideas appreciated.</div><div>></div><div><span style="white-space:pre-wrap">> </span>Bart</div></div><div><br></div><div>Now at this point I am quite concerned as I was planning on using select to monitor three file descriptors in a seyon like program (one of which is the X server) and since this is a practice program before implementing an XCB binding into unthreaded language that relies heavily on select for event loops. My plan was to use a double level event loop. The outer level uses select to wait for input on multiple filehandles. the inner loop (actually one per file handle) would read from each filehandle until a read did not return a complete event. Why would there be any concern about race conditions?</div><div>--</div><div><div>--</div><div><div>Ben Hildred</div><div>Automation Support Services</div></div><div>303 815 6721</div></div> </div> _______________________________________________<br> Xcb mailing list<br> <a href="mailto:Xcb@lists.freedesktop.org" target="_blank">Xcb@lists.freedesktop.org</a><br> <a href="http://lists.freedesktop.org/mailman/listinfo/xcb" rel="noreferrer" target="_blank">http://lists.freedesktop.org/mailman/listinfo/xcb</a></blockquote></div></div>