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

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Message-Id: <f5dda736b43865bb40583992bafe7a6b@kernel.crashing.org>
Date:	Thu, 28 Jun 2007 11:08:25 +0200
From:	Segher Boessenkool <segher@...nel.crashing.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Neil Booth <neil@...kokuya.co.uk>, linux-kernel@...r.kernel.org,
	Al Viro <viro@....linux.org.uk>,
	Josh Triplett <josh@...edesktop.org>,
	linux-sparse@...r.kernel.org
Subject: Re: [PATCH 16/16] fix handling of integer constant expressions

>> Here are three independently invalid non-ICEs that sparse doesn't
>> diagnose.
>>
>> extern int f(void);
>> enum { cast_to_ptr = (int) (void *) 0 };
>> enum { cast_to_float = (int) (double) 1 };
>
> Those two *really* shouldn't fail. I don't care if the C standard says 
> so,
> that is *fine*.

GCC doesn't guarantee you this, either.

> In particular, "offsetof()" should be portably able to basically be the
> standard #define, which involves an integer cast from a constant 
> pointer.
> That had *better* be a valid constant integer expression, because it's
> very useful.

Yes it's useful.  That's why GCC gives you __builtin_offsetof()
for this purpose.

> And I think standards can go screw themselves, and you can make it an
> error with some "--standard-pedantic" switch or similar.
>
> Standards are just random pieces of paper, for crying out loud! They 
> have
> zero relevance in the end.

Sure, as long as you don't care about compatibility across
compilers, what matters is what the compilers you _do_ use
actually implement.  And note that GCC doesn't guarantee
you much over what the C standard does.  Almost everything
it allows extra is just an implementation side effect.


Segher

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