linux-kernel - Re: sizeof vs strlen (was Re: [PATCH 4/4] fs/qnx4: decrement sizeof size in strncmp)

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Message-ID: <Pine.LNX.4.64.0911121137410.19779@ask.diku.dk>
Date:	Thu, 12 Nov 2009 11:37:55 +0100 (CET)
From:	Julia Lawall <julia@...u.dk>
To:	Bernd Petrovitsch <bernd@...mix.at>
Cc:	al@...rsen.net, linux-kernel@...r.kernel.org,
	kernel-janitors@...r.kernel.org
Subject: Re: sizeof vs strlen (was Re: [PATCH 4/4] fs/qnx4: decrement sizeof
 size in strncmp)

On Thu, 12 Nov 2009, Bernd Petrovitsch wrote:

> On Thu, 2009-11-12 at 08:49 +0100, Julia Lawall wrote:
> > From: Julia Lawall <julia@...u.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.

The rule specifies that the string is a constant.

julia
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