[Xcb] [Bug 78140] The tutorial should have a section on error handling

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=78140

--- Comment #3 from Uli Schlachter <psychon at znc.in> ---
(In reply to comment #2)
> I should be able to put together at least a prototype myself but I need to
> ask a few more questions.

Thanks a lot!

> Do I need to call xcb_connection_has_error after absolutely every function
> that gets passed a connection?

Nope. Nothing bad happens when you use an xcb_connection_t that is an error
state. Must functions will just return e.g. NULL (however, no one ever expects
e.g. xcb_get_setup() to return NULL, so other code can still crash...).

> Is there a function to get an error string for each error (keep in mind that
> one may want to sometimes localize error messages so this is not as easy as
> it seems).

Nope, and I wouldn't want to add one exactly because of things like
localization. I guess you are expected to write your own function that
translates the ~5 error codes that are currently in-use into a string and that
returns something like "unknown error 42" for other values.

E.g. qt does the following:
https://qt.gitorious.org/qt/qtbase/source/d52b00e1d3cc60c81b54a89d6da488dc4bbce384:src/plugins/platforms/xcb/qxcbconnection.cpp#L95-126

> How do I check for errors with the xcb_disconnect function (I know that
> free(3) does not give errors but what about close(2))?

There just are no errors. :-)
Also, there aren't many sensible things that can be done if close(),
pthread_mutex_destroy() or pthread_cond_destroy() fail (AFAIR on some systems
close() can fail and still have closed the FD already, so retrying would be
unsafe in multi-threaded programs anyway).

So if any errors occur during xcb_disconnect(), you'll get a memory / file
descriptor leak.

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