linux-kernel - Re: [PATCH] [DISCUSS] Make the variable NULL after freeing it.

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Date:	Mon, 01 Jan 2007 17:25:03 +0100
From:	Andreas Schwab <schwab@...e.de>
To:	Ingo Oeser <ioe-lkml@...eria.de>
Cc:	Amit Choudhary <amit2030@...oo.com>,
	Bernd Petrovitsch <bernd@...mix.at>,
	Jan Engelhardt <jengelh@...ux01.gwdg.de>,
	Pavel Machek <pavel@....cz>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] [DISCUSS] Make the variable NULL after freeing it.

Ingo Oeser <ioe-lkml@...eria.de> writes:

> Hi,
>
> On Monday, 1. January 2007 07:37, Amit Choudhary wrote:
>> --- Ingo Oeser <ioe-lkml@...eria.de> wrote:
>> > #define kfree_nullify(x) do { \
>> > 	if (__builtin_constant_p(x)) { \
>> > 		kfree(x); \
>> > 	} else { \
>> > 		typeof(x) *__addr_x = &x; \
>
> Ok, I should change that line to 
> 		typeof(x) *__addr_x = &(x); \
>
>> > 		kfree(*__addr_x); \
>> > 		*__addr_x = NULL; \
>> > 	} \
>> > } while (0)
>> > 
>> > Regards
>> > 
>> > Ingo Oeser
>> > 
>> 
>> This is a nice approach but what if someone does kfree_nullify(x+20).
>
> Then this works, because the side effect (+20) is evaluated only once. 

It's not a side effect, it's a non-lvalue, and you can't take the address
of a non-lvalue.

Andreas.

-- 
Andreas Schwab, SuSE Labs, schwab@...e.de
SuSE Linux Products GmbH, Maxfeldstraße 5, 90409 Nürnberg, Germany
PGP key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
"And now for something completely different."
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