glXGetCurrentDisplay returning NULL even with appropriate visual

[Brian Paul]

tom fogal wrote:
> Hi, I've run into a snag in what seems like glX initialization.
> 
> Through debug versions of client libraries [1], I think I've
> established why my application is segfaulting in XQueryExtension.  The
> display (`dpy' variable) passed is NULL, which seems quite suspect.
> Assuming that is indeed the problem, the root issue appears to be a:
> 
>     glXQueryVersion(glXGetCurrentDisplay(), ...);
> 
> line, where glXGetCurrentDisplay returns NULL.  Quite strangely, I
> can call glXGetCurrentDisplay manually in the debugger, and it gives
> me what seems to be a valid display; certainly not NULL.  My DISPLAY
> seems set correctly; an XOpenDisplay call succeeds; glXChooseVisual
> gives me a pointer instead of NULL.  I've thrown a `system("glxinfo");'
> call in various places, and I get the appropriate output (direct
> rendering is enabled, GL extensions are consistent, list of visuals
> seems reasonable).
> 
> Since I'm using GLEW, I wonder if it is related to:
> 
>     http://www.nabble.com/using-chromium-and-glew--td15953506.html
> 
> As a summary, the user is trying to use Chromium with GLEW, and
> dynamically loading glXGetCurrentDisplay via glXGetProcAddress appears
> not to work when Chromium is thrown into the mix.  I'm not using
> Chromium, however.
> 
> I have tried GLEW 1.3.4 and GLEW 1.5.1, which give little discernible
> difference.  It seems like, in the 1.5.1 case, the Display I get back
> from glXGetCurrentDisplay (when called manually in the debugger) has a
> much larger set of values in the `event_vec' and `wire_vec' fields.
> 
> Attached is the relevant portions of a gdb session.  Any ideas what I
> am (or my dependent libraries are...) doing wrong?

I suspect you haven't yet called glXMakeCurrent().  From the man page:

"""
DESCRIPTION
        glXGetCurrentDisplay returns the display for the current 
context. If no
        context is current, NULL is returned.
"""


-Brian