[Openmcl-devel] FFI problem

[Gary Byers]

I haven't tried to reproduce this (and this guess may be way off), but
I wonder if the current thread has a current GL context established
(via something like -[NSOpenGLView setOpenGLContext:] or whatever
the equivalent is in AGL or CGL) ?

The unmapped address that the foreign code is trying to indirect
through (#xab0) doesn't look like it has anything to do with the
address that's being passed in the call to #_glGetIntegerv - wherever
that stack-block got allocated, it's unlikely to be that low in memory.
I'd find it easier to believe that some internal data structure related
to the current OpenGL context stores GL_MAX_LIST_NESTING information
#xab0 bytes from the beginning of that structure, and that #_glGetIntegerv
doesn't check to see if the context is NULL before trying to access a
field in a null pointer.)

Note that the notion of "the current GL context" is generally per-thread,
so (for instance) an event-handling thread might have a properly installed
GL context and a listener thread might not.

Like I said, this is mostly a guess.  I was doing a lot of GL stuff a
year or so ago and it sort of rings a bell, but I may be misremembering.

On Mon, 22 May 2006, Phil wrote:

> I've been attempting to get the following FFI call to work without much
> luck...
>
> Call glGetIntegerv(GLenum pname, GLint *params) using the following:
>
> (let ((gl-int-size (ccl::foreign-size :<GL>int :bytes)))
>   (ccl::%stack-block ((return-value gl-int-size))
>     (#_glGetIntegerv #$GL_MAX_LIST_NESTING return-value)
>     (ccl::%get-unsigned-long return-value)))
>
> The result is the kernel debugger:
>
> ? Unhandled exception 11 at 0x92e97ba0, context->regs at #xf0805498
> Read operation to unmapped address 0xab0
>  In foreign code at address 0x92e97ba0
> ? for help
> [28544] OpenMCL kernel debugger:
>
> What am I missing here?
>
> Thanks,
> Phil
>
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