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Subject:	Re: XWin Server starts but terminates shortly after
Reply-To:	cygwin AT cygwin DOT com
References:	<CA+rGx5fU2q-6Wqsv2VBgJ_HaAOOSFkm32B60_bBLP45x5RbtKg AT mail DOT gmail DOT com> <5644A543 DOT 4040508 AT dronecode DOT org DOT uk> <564B442E DOT 6060009 AT dronecode DOT org DOT uk>
To:	cygwin AT cygwin DOT com
From:	Jon Turney <jon DOT turney AT dronecode DOT org DOT uk>
Cc:	Thomas Schweikle <tschweikle AT gmail DOT com>
Message-ID:	<564E2F54.1000803@dronecode.org.uk>
Date:	Thu, 19 Nov 2015 20:21:40 +0000
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In-Reply-To:	<564B442E.6060009@dronecode.org.uk>

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On 17/11/2015 15:13, Jon Turney wrote:
> On 13/11/2015 15:13, Thomas Schweikle wrote:
>> Adding "-nowgl" does the trick. XWin is running again.
>
> It seems that this problem manifests itself when running on a Windows
> guest under VMWare, with their SVGA driver.
>
> This seems to be caused by a c0000374 (STATUS_HEAP_CORRUPTION) exception
> raised whilst loading the VMWare OpenGL driver.

This is easy to reduce to just the code that XWin uses to probe the 
capabilities of the native OpenGL renderer (attached).

$ gcc -Wall xwin-gl-probe.c -lgdi32 -lopengl32 -o xwin-gl-probe.exe

$ strace ./xwin-gl-probe.exe
[...]
--- Process 2356, exception c0000374 at 0000000077B64102
[...]

If I add a checking with HeapValidate() before the crashing call to 
ChoosePixelFormat(), that doesn't report any problems, so that seems to 
rule out the heap corruption being introduced by this code.

Compiling the same code with VS 2013 works without problems on my test 
VM (VMWare Player 12.0.1 + W7 x64 + VMWare SVGA driver)

This doesn't really get me any further forward though.  Does this crash 
loading vm3dgl64 because of a bug in vm3dgl64 which is only exposed in 
Cygwin? or because the Cygwin environment doesn't satisfy some 
requirement of vm3dgl64 that it should?

This isn't the first report of a crash in this probe with various 
graphics drivers (although typically the exception is c0000005, which we 
can catch and fallback to software rendering), so while it's tempting to 
assume this is a problem in the graphics driver, it's possible that 
something systematic is wrong.

-- 
Jon TURNEY
Volunteer Cygwin/X X Server maintainer

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