On Wed, Sep 08, 1999 at 10:25:08PM -0400, Brian Paul wrote:
> Issues
>
> * Is this extension properly named?
I'm semi-convinced this should actually sit in its own namespace,
and then initially we put it in GLX by analogy to the WGL equivalent.
> * Is void * the appropriate return type?
void (*)(...) seems more appropriate now. Or maybe a GL typedef
(glFunctionPointerEXT?)
> * Should GetProcAddressEXT allow querying of itself?
>
> * There's a recursion problem with this feature. The purpose of
> GetProcAddressEXT is to return pointers to extension functions
> and GetProcAddressEXT is itself such a function! This presents
> a puzzle to the application developer.
GetProcAddressEXT must be exported from the libGL described by the
standard. The only extensions which *have* to be queried for in this
fashion are those added to a loadable driver module after the base libGL
it's loaded by is shipped. Exactly how we draw the line is unclear. In
the Windows world, only a handful of extensions are supplied in
OPENGL32.DLL, and everything else needs to be queried.
> Additions to Chapter 6 of the OpenGL 1.2 Specification (State and State
> Requests)
>
> Add a new section numbered 6.1.13:
>
> 6.1.13 Obtaining Extension Function Pointers
>
> The GL extensions which are available to a client application can
> only be determined at runtime. Therefore, the address of extension
> functions should be queried at runtime.
We need to be crystal clear on exactly which GL (and GLX) functions
can be queried, or implementations will make bad choices. An easy way to
lock in apps might be for an OpenGL implementation to expose a critical
extension in libGL but *not* via the query; an app calling this
extension directly would not run on another implementation exporting it
only dynamically. Instead it would fail catastrophically when calling
the unresolved entry point, which could then be blamed on the driver
although the driver would be doing exactly what it should. Both ISVs and
GL implementors will need to be sensitive to this.
Also, an emerging use of wglGetProcAddress() is to return pointers
to OpenGL 1.2 core functions. While this exact scenario is unlikely to
arise on Linux, we should still be very clear up front about whether
non-EXT functions can be queried.
> The function
> void *GetProcAddressEXT(const ubyte *procName);
> returns the address of the extension function named by procName.
> NULL is returned if the named function is not available.
>
> The pointer returned by GetProcAddressEXT should be cast to a
> function type which matches the extension function's definition.
>
> Functions which are extensions to the GL window system interface
> (such as GLX and WGL) are also obtainable with GetProcAddressEXT.
Also note that the returned pointer is only valid when the current
context is the same as the context to which the GL was connected when
the pointer was obtained.
Nice job!
Jon Leech
SGI
