linux-kernel - Re: [PATCH 0/1] Constify struct address_space

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Message-ID: <20091215181403.GB24406@elf.ucw.cz>
Date:	Tue, 15 Dec 2009 19:14:03 +0100
From:	Pavel Machek <pavel@....cz>
To:	Emese Revfy <re.emese@...il.com>
Cc:	Arjan van de Ven <arjan@...radead.org>,
	Paul Mundt <lethal@...ux-sh.org>,
	Matthew Wilcox <matthew@....cx>, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org, viro@...iv.linux.org.uk,
	akpm@...ux-foundation.org
Subject: Re: [PATCH 0/1] Constify struct address_space_operations for
 2.6.32-git-053fe57ac v2

Hi!

> Arjan van de Ven wrote:
> > On Mon, 14 Dec 2009 22:25:26 +0100
> > Pavel Machek <pavel@....cz> wrote:
> > 
> >> On Mon 2009-12-14 08:00:49, Arjan van de Ven wrote:
> >>> On Mon, 14 Dec 2009 12:26:56 +0100
> >>> Pavel Machek <pavel@....cz> wrote:
> >> I certainly object "constify ops... as much as possible". If it
> >> uglifies the code, it should not be done. If it is as simple as adding
> >> const to few lines, its probably ok.
> >>
> >> But .... the patch contained huge load of 
> >>
> >> -	int (* resume)()
> >> +	int (* const resume)()
> >>
> >> What is that?
> > 
> > the ops stuct instantiation itself should be const.
> > the members not so much; that makes no sense.
> 
> Consitfying the structure fields prevents direct modifications of runtime
> allocated ops structures therefore it gives a strong signal to the programmer
> that he's trying to do something undesired (this approach is in fact already
> used in the kernel, see: iwl_ops).

One const in structure declaration seems to be just enough, see:

const struct a {
	void (* f)(void);
	void (* const g)(void);
} s;

void h(void)
{
	struct a *p = &s;
	s.f = 0;
	s.g = 0;
	p->f = 0;
	p->g = 0;
}


delme.c: In function 'h':
delme.c:8: warning: initialization discards qualifiers from pointer target type
delme.c:9: error: assignment of read-only variable 's'
delme.c:10: error: assignment of read-only variable 's'
delme.c:12: error: assignment of read-only member 'g'

You get clean-enough warnings.
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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