Hi,
I recently came across this issue in viewperf (from SPEC):
Viewperf defines a local glGenBuffers function pointer, initializes to
NULL, and then decides since it isn't using VBOs to never initialize to
a real function. Thus with PBOs on by default in VGL now, VGL does
readback, calls glGenBuffers() and gets a SEGV because the pointer is NULL.
The most simple patch is add a "&& glGenBuffers" before pbdrawable uses
glGenBuffers) but means the rest of the code can't rely on the
fconfig.readback status.
The still simple but slightly cleaner (if you'll excuse the type abuse)
fix is to disable PBO readback if glGenBuffers is NULL (check in
fakerconfig.cpp). Patch attached. (I wasn't sure if you cared about
requiring libGL for everything that links with fakerconfig.cpp or not,
hence the dlsym rather than including gl.h)
An alternate fix, that has the benefit of allowing PBOs with viewperf,
would be to check if the symbol was NULL and dlsym(RTLD_NEXT,
glGenBuffers), plus all the other symbols viewperf NULLs.
Patch applies to trunk with:
patch -p1 < 0001-Disable-PBO-use-if-glGenBuffers-is-NULL.patch
Cheers,
-Nathan
P.S. Of course this error points out that the same issue could happen
with *any* symbol that VGL calls directly but the app redefined. I
think the "dlsym all GL functions VGL uses" step is only something to
consider if we see it in practice, but something I'm keeping in the back
of my mind is that google suggests that GLEW in particular seems to have
this paradigm of all extension functions are pointers, and app needs to
initialize after a context is created. Any call that invokes VGL usage
(e.g. glxSwapBuffers) before that GLEW initialization is done in the app
will easily end up the the same SEGV situation.
Index: server/fakerconfig.cpp
===================================================================
--- server/fakerconfig.cpp (revision 3514)
+++ server/fakerconfig.cpp (working copy)
@@ -26,6 +26,7 @@
#include <sys/ipc.h>
#include <sys/types.h>
#endif
+#include <dlfcn.h>
#include "vglutil.h"
#include <X11/Xatom.h>
#ifdef USEXV
@@ -407,6 +408,20 @@
if(readback>=0 && (!fconfig_envset || fconfig_env.readback!=readback))
fconfig.readback=fconfig_env.readback=readback;
}
+ if (RRREAD_PBO==fconfig.readback)
+ {
+ int *gen=(int *)dlsym(RTLD_DEFAULT, "glGenBuffers");
+ if (!gen || !*gen)
+ {
+ static int printed=0;
+ fconfig.readback=fcenv.readback=RRREAD_SYNC;
+ if (!printed)
+ {
+ rrout.println("[VGL] Forcing synchronous readback due to NULL glGenBuffers\n");
+ printed=1;
+ }
+ }
+ }
fetchenv_dbl("VGL_REFRESHRATE", refreshrate, 0.0, 1000000.0);
fetchenv_int("VGL_SAMPLES", samples, 0, 64);
fetchenv_bool("VGL_SPOIL", spoil);
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