lkml.org 
[lkml]   [2009]   [Nov]   [13]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: sizeof vs strlen (was Re: [PATCH 4/4] fs/qnx4: decrement sizeof size in strncmp)
From
Date
On Thu, 2009-11-12 at 16:33 +0100, Julia Lawall wrote: 
> On Thu, 12 Nov 2009, Bernd Petrovitsch wrote:
>
> > On Thu, 2009-11-12 at 08:49 +0100, Julia Lawall wrote:
> > > From: Julia Lawall <julia@diku.dk>
> > >
> > > As observed by Joe Perches, sizeof of a constant string includes the
> > > trailing 0. If what is wanted is to check the initial characters of
> > > another string, this trailing 0 should not be taken into account. If an
> > > exact match is wanted, strcmp should be used instead.
> > [...]
> > > strncmp(foo, abc,
> > > - sizeof(abc)
> > > + sizeof(abc)-1
> > > )
> > > // </smpl>
> > Am I the only one who find "strlen()" instead of "sizeof()-1" here much
> > more readable (and to the point).
> >
> > As for run-time, it shouldn't make a difference for static/constant
> > strings as gcc should be able calculate the length at compile time. And
> > if the string is not constant, sizeof() is probably wrong anyways.
>
> Does gcc have access to the definition of strlen? It does not seem to be
> an inlined function, eg in lib/string.c.
Since "strlen()" is defined in the C-Standard C-compilers could rely on
the defined behaviour (but I don't know exactly how gcc behaves with
-ffreestanding for all supported versions).
Then there is __builin_strlen() (see also
http://gcc.gnu.org/onlinedocs/gcc-4.4.2/gcc/Other-Builtins.html#Other-Builtins).

Stepping a quite small abstraction layer higher:
include/linux/string.h has at teh end:
---- snip ----
static inline bool strstarts(const char *str, const char *prefix)
{
return strncmp(str, prefix, strlen(prefix)) == 0;
}
---- snip ----
seems to be what most uses of strnmcp() actually are: check if one
string is a prefix of another.
Bernd
--
Firmix Software GmbH http://www.firmix.at/
mobil: +43 664 4416156 fax: +43 1 7890849-55
Embedded Linux Development and Services




\
 
 \ /
  Last update: 2009-11-13 17:45    [from the cache]
©2003-2011 Jasper Spaans