linux-kernel - Re: [PATCH] Fix strim() semantics for strings that have only blanks

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Message-Id: <20111013165431.b84ffab4.akpm@linux-foundation.org>
Date:	Thu, 13 Oct 2011 16:54:31 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	holzheu@...ux.vnet.ibm.com
Cc:	andre.goddard@...il.com, schwidefsky@...ibm.com,
	heiko.carstens@...ibm.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Fix strim() semantics for strings that have only blanks

On Wed, 12 Oct 2011 10:31:57 +0200
Michael Holzheu <holzheu@...ux.vnet.ibm.com> wrote:

> Hello Andre,
> 
> With git commit 84c95c9acf088c99d8793d78036b67faa5d0b851 a patch from
> you went upstream where you wanted to improve the performance of the
> strim() function.
> 
> Unfortunately this changed the semantics of strim() and broke my code. Before
> the patch it was possible to use strim() without using the return value for
> removing trailing spaces from strings that had either only blanks or only
> trailing blanks.
> 
> Now this does not work any longer for strings that *only* have blanks.
> 
> Before patch: "   " -> ""    (empty string)
> After patch:  "   " -> "   " (no change)
> 
> I think we should remove your patch to restore the old behavior.
> 
> >From the description (lib/string.c):
> 
>  * Note that the first trailing whitespace is replaced with a %NUL-terminator
> 
> => The first trailing whitespace of a string that only has whitespace
>    characters is the first whitespace

Yes, that change makes sense.

> Also strim() explicitly does not have "__must_check", in order to use it
> without using the return value.
> 

This observation seems to have nothing to do with the patch?

I think that strim() _should_ have __must_check annotation (which means
that strim() simply disappears, which is good).  It is only safe to
ignore the strim() return value if the caller knows that the string
started with a non-space.  The number of situations where this is
guaranteed are faily small, I suspect.  Whereas the chances of someone
screwing this up are pretty high :(


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