linux-kernel - Re: [PATCH 16/16] fix handling of integer constant expressions

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Message-ID: <20070627122628.GR7590@daikokuya.co.uk>
Date:	Wed, 27 Jun 2007 21:26:28 +0900
From:	Neil Booth <neil@...kokuya.co.uk>
To:	Al Viro <viro@....linux.org.uk>
Cc:	Josh Triplett <josh@...edesktop.org>,
	Segher Boessenkool <segher@...nel.crashing.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-sparse@...r.kernel.org
Subject: Re: [PATCH 16/16] fix handling of integer constant expressions

Al Viro wrote:-

> Egads...  After rereading that...  What a mess.
> 
> int foo(void)
> {
> 	static int a[1][0,2];
> }
> 
> is, AFAICS, allowed.  Reason:
> 	int a[0,2]
> is a VLA due to 6.7.5.2[4] (0,2 is not an ICE).  However, due to the language
> in the same section,
> 	int a[1][0,2]
> is *not* a VLA, since (a) 2 is an ICE and (b) its element type "has a known
> constant size" (it does - the value of 0,2 is certainly guaranteed to be 2).
> I.e., it's VM type, but not a VLA.  I.e. only the first part of 6.7.5.2[2]
> applies and we are actually fine.
> 
> 	So we can have a static single-element array of int [0,2], but
> not a plain static int [0,2].  Lovely, that...

DR 312 clarified the meaning of "known constant size"

   http://www.open-std.org/jtc1/sc22/wg14/www/docs/dr_312.htm

in the sensible way, thankfully, so your example is actually invalid.

Neil.
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