linux-kernel - Re: gcc fixed size char array initialization bug

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Date:	Fri, 3 Aug 2007 00:09:15 +0100
From:	Al Viro <viro@....linux.org.uk>
To:	Guennadi Liakhovetski <g.liakhovetski@....de>
Cc:	Stefan Richter <stefanr@...6.in-berlin.de>,
	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org
Subject: Re: gcc fixed size char array initialization bug - known?

On Fri, Aug 03, 2007 at 12:51:16AM +0200, Guennadi Liakhovetski wrote:
> On Fri, 3 Aug 2007, Stefan Richter wrote:
> 
> > Guennadi Liakhovetski wrote:
> > > with
> > > 
> > > 	char c[4] = "012345";
> > > 
> > > the compiler warns, but actually allocates a 6-byte long array...
> > 
> > Off-topic here, but:  sizeof c / sizeof *c == 4.
> 
> Don't think it is OT here - kernel depends on gcc. And, what I meant, is, 
> that gcc places all 7 (sorry, not 6 as I said above) characters in the 
> .rodata section of the compiled object file. Of course, it doesn't mean, 
> that c is 7 characters long.

So gcc does that kind of recovery, after having warned you.  Makes sense,
as long as it's for ordinary variables (and not, say it, struct fields) -
you get less likely runtime breakage on the undefined behaviour (e.g.
passing c to string functions).  So gcc has generated some padding between
the global variables, that's all.

It doesn't change the fact that use of c[4] or strlen(c) or strcpy(..., c)
means nasal demon country for you.

Now, if gcc does that for similar situation with struct fields, you'd have
a cause to complain.
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