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Date: Thu, 11 Oct 2012 16:11:18 +0300
From: Andriy Gapon <avg@FreeBSD.org>
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To: Erik Cederstrand <erik@cederstrand.dk>
Subject: Re: curcpu false positive?
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on 11/10/2012 16:06 Erik Cederstrand said the following:
> Hello,
> 
> I'm looking at some Clang Static Analyzer reports in the kernel, and a lot of them point back to a null pointer dereference in __pcpu_type (sys/amd64/include/pcpu.h:102) which is defined as:
> 
> 102	 /*
> 103	 * Evaluates to the type of the per-cpu variable name.
> 104	 */
> 105	#define __pcpu_type(name)                                               \
> 106	        __typeof(((struct pcpu *)0)->name)
> 
> 
> Which indeed looks like a NULL pointer dereference. Looking at the latest commit message there, I'm sure the code is correct, but I'm unsure why the null pointer is OK. I'd appreciate an explanation :-)

Read about __typeof [1].
It's evaluated at compile time, so actual value of an expression does not matter
at all.

[1] http://gcc.gnu.org/onlinedocs/gcc/Typeof.html
-- 
Andriy Gapon