linux-kernel - Re: [PATCH] NTFS error messages: replace static char pointers by static char arrays

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Message-ID: <20071009220331.GD11579@Ahmed>
Date:	Wed, 10 Oct 2007 00:03:31 +0200
From:	"Ahmed S. Darwish" <darwish.07@...il.com>
To:	Philipp Matthias Hahn <pmhahn@...an.lahn.de>
Cc:	Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] NTFS error messages: replace static char pointers by static char arrays

On Tue, Oct 09, 2007 at 08:33:59PM +0200, Philipp Matthias Hahn wrote:
> Hello!
> 
> On Tue, Oct 09, 2007 at 02:40:35PM +0200, Ahmed S. Darwish wrote:
> > On Tue, Oct 09, 2007 at 01:55:42AM +0400, Dmitri Vorobiev wrote:
> > > The patch below contains a small code clean-up for the NTFS driver: all
> > > static char pointers to error message strings have been replaced by 
> > > static char arrays.
> 
>       char *       a = "a"; // pointer and content can be changed

Only the pointer can be changed here. AFAIK "a" is a const string.

> const char *       b = "b"; // the thing pointed to is const

The "const" here is redundant (just useful for forcing the compiler to
prevent us from shooting our feet). The "b" string is already constant.

>       char * const c = "c"; // the pointer is const
> const char * const d = "d"; // pointer and content can't be changed
> 
> void foo(void) {
>         *a = 'A';

This will segfault.

>         a++;
>         *b = 'B'; // error: assignment of read-only location
>         b++;
>         *c = 'C';

Last line will segfault too.

>         c++;      // error: increment  of read-only variable 'c'
>         *d = 'D'; // error: assignment of read-only location
>         d++;      // error: increment  of read-only variable 'd'
> }
> 

Please continue below.

> > Isn't the only difference between char *c = "msg" and char c[] = "msg" is 
> > that the first is a non-const pointer to a const char array while the second 
> > is a modifiable char array ?
> 
> $ cat [ab].c
> const char *a = "a";
> const char b[] = "b";
> $ gcc -c [ab].c
> $ size [ab].o
>    text    data     bss     dec     hex filename
>       2       4       0       6       6 a.o
>       2       0       0       2       2 b.o
> 
> 'a' has two entries: one for the named read-writeable pointer, and one for the
>     anonymous read-only string, the pointer points to.
> 'b' has a single entry: just the named read-only string.
> 

Got the point, Thanks!.

-- 
Ahmed S. Darwish
HomePage: http://darwish.07.googlepages.com
Blog: http://darwish-07.blogspot.com
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