11. General Limitations and Known Bugs

11.1. OpenGL

The following OpenGL features are not supported at this time: overlays, stereo, hardware-accelerated indirect rendering.

OpenGL-like functionality is provided with the Mesa library. XFree86 4.0.1 uses Mesa 3.3. Subsequent releases of XFree86 will use newer versions of Mesa. When newer versions of Mesa are available, the 3D drivers can be updated without reinstalling XFree86 or libGL.so.

11.2. GLX

The GLX 1.3 API is exported but none of the new 1.3 functions are operational.

The new glXGetProcAddressARB function is fully supported.

GLXPixmap rendering is only supported for indirect rendering contexts. This is a common OpenGL limitation. Attempting to use a direct rendering context with a GLXPixmap will result in an X protocol error.

11.3. Debugging

Debugging DRI drivers with gdb can be difficult because of the locking involved. When debugging OpenGL applications, you should avoid stepping inside the GL functions. If you're trying to debug a DRI driver it's recommended that you do so remotely, from a second system.

11.4. Scheduling

When you run multiple GL applications at once you may notice poor time slicing. This is due to an interaction problem with the Linux scheduler which will be addressed in the future.

11.5. libGL.so and dlopen()

A number of popular OpenGL applications on Linux (such as Quake3, HereticII, Heavy Gear 2, etc) dynamically open the libGL.so library at runtime with dlopen(), rather than linking with -lGL at compile/link time.

If dynamic loading of libGL.so is not implemented carefully, there can be a number of serious problems. Here are the things to be careful of in your application:

• Specify the RTLD_GLOBAL flag to dlopen(). If you don't do this then you'll likely see a runtime error message complaining that _glapi_Context is undefined when libGL.so tries to open a hardware-specific driver. Without this flag, nested opening of dynamic libraries does not work.
• Do not close the library with dlclose() until after XCloseDisplay() has been called. When libGL.so initializes itself it registers several callbacks functions with Xlib. When XCloseDisplay() is called those callback functions are called. If libGL.so has already been unloaded with dlclose() this will cause a segmentation fault.
• Your application should link with -lpthread. On Linux, libGL.so uses the pthreads library in order to provide thread safety. There is apparently a bug in the dlopen()/dlclose() code which causes crashes if the library uses pthreads but the parent application doesn't. The only known work-around is to link the application with -lpthread.

Some applications don't yet incorporate these procedures and may fail. For example, changing the graphics settings in some video games will expose this problem. The DRI developers are working with game vendors to prevent this problem in the future.

11.6. Bug Database

The DRI bug database which includes bugs related to specific drivers is at the SourceForge DRI Bug Database

Please scan both the open and closed bug lists to determine if your problem has already been reported and perhaps fixed.