linux-kernel - Re: [PATCH 2.6.19] drivers/media/video/cpia2/cpia2_usb.c: Free previously allocated memory (in array elements) if kmalloc() returns NULL.

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Message-ID: <Pine.LNX.4.64N.0612071619260.12627@attu4.cs.washington.edu>
Date:	Thu, 7 Dec 2006 16:36:31 -0800 (PST)
From:	David Rientjes <rientjes@...washington.edu>
To:	Mariusz Kozlowski <m.kozlowski@...land.pl>
cc:	Amit Choudhary <amit2030@...il.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2.6.19] drivers/media/video/cpia2/cpia2_usb.c: Free
 previously allocated memory (in array elements) if kmalloc() returns NULL.

On Fri, 8 Dec 2006, Mariusz Kozlowski wrote:

> > > Just for future. Shorter and more readable version of your for(...) thing:
> > > 
> > > 	while (i--) {
> > > 		...
> > > 	}
> > > 
> > 
> > No, that is not equivalent.
> > 
> > You want
> > 	while (i-- >= 0) {
> > 		...
> > 	}
> > 
> 
> Not really. That will stop at -1 not 0.
> 

It depends on your intent.  Generally speaking,

	while (i--) {
		...
	}

is never what you want.  Adding the check for being greater than 0 stops 
potential bugs from signed int i being negative.  The only drawback on x86 
is that it sets a byte based on the greater condition with setg and tests 
it later with testb for every iteration.  This use of testb will _always_ 
use the same addressable byte registers for both its operands.

Based on this particular patch, I agree that

	while (i-- > 0) {
		...
	}

will do the job.  This is equivalent to the original for loop and ensures 
that a negative value of i is never iterated on (even though it admittedly 
would never be negative in this instance to begin with).

		David
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