From owner-freebsd-chat  Sun Jan 28 16:39: 7 2001
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Date: Mon, 29 Jan 2001 01:31:45 +0100
To: freebsd-chat@freebsd.org
Subject: Re: C Puzzle
Message-ID: <20010129013145.A4834@crow.dom2ip.de>
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References: <20010128161353.C98223@trollkarl.skafte.org>
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In-Reply-To: <20010128161353.C98223@trollkarl.skafte.org>; from skafte+freebsd-chat@trollkarl.net on Sun, Jan 28, 2001 at 04:13:53PM -0700
From: Thomas Moestl <tmoestl@gmx.net>
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On Sun, Jan 28, 2001 at 04:13:53PM -0700, Greg Skafte wrote:
> doesn't work :
> 
> char* fn_mtrim __P((char*));
> char* fn_mtrim (s)
> char* s ;{
> 	short i=0;
>         char *buffer= (char *) calloc (256,sizeof(char));
> 	while (*s){
> 		( *s == ' ') ? s*++ : buffer[i++] = *s++ ;
> 	};
> 	return buffer; 
> }
> 
> works :
> 
> char* fn_mtrim __P((char*));
> char* fn_mtrim (s)
> char* s ;{
> 	short i=0;
>         char *buffer= (char *) calloc (256,sizeof(char));
> 	while (*s){
> 		( *s != ' ') ? buffer[i++] = s*++ : *s++ ;
> 	};
> 	return buffer; 
> }
> 
> Why does the first one not work and the second one work .... 
Precedence tells us:
	( *s == ' ') ? *s++ : buffer[i++] = *s++ ;
is equivalent to
	(( *s == ' ') ? *s++ : buffer[i++]) = *s++ ;
(notice the outer brackets). I guess you mean *s++ when you write s*++.
If you try this with string literal as argument (fn_mtrim("foo bar")), 
the string will get written into, and this may even segfault using gcc.
Otherwise, the result will be wrong anyway, because s gets incremented 
before it is tested against ' ' (this is not really well-defined), and 
then incremented again when *s == ' ' and the value is assigned to *s++.

	- thomas

P.S.: this is really ugly ;-)


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