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

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Date:	Mon, 14 Dec 2009 23:41:24 +0100
From:	Emese Revfy <re.emese@...il.com>
To:	Arjan van de Ven <arjan@...radead.org>
CC:	Pavel Machek <pavel@....cz>, 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

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).

There is another benefit in that static but non-const ops structures cannot be
directly modified either, therefore it will be easier to make them const later.

Example:

 1 struct a {                                                                                                                                                 
 2  void (* f)(void);
 3  void (* const g)(void);
 4 } s;
 5 
 6 void h(void)
 7 {
 8  struct a *p = &s;
 9  s.f = 0;
10  s.g = 0;
11  p->f = 0;
12  p->g = 0;
13 }

gcc -c a.c
a.c: In function 'h':
a.c:10: error: assignment of read-only member 'g'
a.c:12: error: assignment of read-only member 'g'

--
Emese
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