On 04/18/2014 05:07 PM, Jamey Sharp wrote: > On Fri, Apr 18, 2014 at 4:49 PM, Ian Romanick <i...@freedesktop.org> wrote: >> On 04/18/2014 03:37 PM, Sarah Sharp wrote: >>> One open question: >>> >>> I've used the normal (error checked) version of xcb_dri2_get_msc. The >>> GLX implementation of glXGetSyncValuesOML uses the unchecked version, >>> but I'm not convinced it's necessary. >>> >>> I talked to Jamey Sharp, and he doesn't understand why most of Mesa >>> calls the unchecked versions of XCB functions. He thought most >>> developers would want to use the normal (checked) versions, since the >>> unchecked versions may segfault on errors. Mesa can always call the >>> checked version, but ignore the error that's set, so it doesn't have to >>> use the unchecked version. >> >> When I added some XCB code to Mesa a couple years ago, it wasn't >> completely clear to me what the difference was between the checked and >> unchecked versions. See 6ccda72 and 588042a. My recollection was that >> the _checked versions returned a cookie that you had to do something >> with. For a bunch of the cases, I think we don't want to do the extra >> bookkeeping (or we don't want the extra opportunity for a memory) leak. >> >> Is there more difference than I know? > > The key difference, in the case of requests that expect a reply, is > what happens to an X error if the server sends you one of those > instead of the reply you were expecting. In the checked case, XCB > returns that error to you when you ask for the reply, through an > optional out-parameter in the reply function. If you use the unchecked > variant of the request, then the error is placed in the event queue; > if you're using Xlib, then by default it will be abort()ing your > client as soon as it pulls the error out of the event queue.
Unless the client has an error handler, right? > There's no opportunity for a memory leak from using the default (that > is, checked) version of requests that expect a reply. If you don't > want to look at the details of the X error, just pass NULL for the > third argument of the reply function, in which case XCB will free the > error struct for you. > > For requests where you do not expect to get a reply, XCB defaults to > unchecked behavior, and you have to use the _checked variant and an > xcb_request_check call if you want to keep the error out of the event > queue. Since xcb_request_check may have to do a round-trip to confirm > that no error occurred, you probably don't usually want checked > behavior for these requests. But that's why the default depends on > whether the request was going to have a reply anyway. Yeah... that's starting to ring some bells. Did I mention that I wrote that XCB code in 2011? :) > So I *think* you'll find life is much better in error cases if you > just s/_unchecked//g throughout Mesa. You'll get better error > reporting where it's free, without adding extra round-trips or > opportunities for memory leaks where it's more complicated. But there > could be something about Mesa's use case I still don't understand. > > Jamey _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev
