Re: [PATCH] compat: helper for detecting unsigned overflow

From: Joshua Juran <jjuran@gmail.com>
To: Sverre Rabbelier <srabbelier@gmail.com>
Cc: Jonathan Nieder <jrnieder@gmail.com>,
	git@vger.kernel.org, Erik Faye-Lund <kusmabite@gmail.com>,
	Jeff King <peff@peff.net>, Nicolas Pitre <nico@fluxnic.net>,
	Pierre Habouzit <madcoder@debian.org>,
	Ilari Liusvaara <ilari.liusvaara@elisanet.fi>
Subject: Re: [PATCH] compat: helper for detecting unsigned overflow
Date: Thu, 10 Feb 2011 05:23:25 -0800	[thread overview]
Message-ID: <E84FF9D8-92D7-4CA0-9CF4-77FF33893977@gmail.com> (raw)
In-Reply-To: <AANLkTinqkSxj0C+GyMVR4a7d=yy_mh+oDuar0moZjZ8_@mail.gmail.com>

On Feb 10, 2011, at 4:11 AM, Sverre Rabbelier wrote:

> On Thu, Feb 10, 2011 at 10:35, Jonathan Nieder <jrnieder@gmail.com>  
> wrote:
>>       unsigned_add_overflows(a, b)
>
>> Define such a macro, expanding roughly to ((a) < UINT_MAX - (b)).
>> Because the expansion uses each argument only once outside of  
>> sizeof()
>> expressions, it is safe to use with arguments that have side effects.
>
>> +#define unsigned_add_overflows(a, b) \
>> +    ((b) > maximum_unsigned_value_of_type(a) - (a))
>
> I'm confused, you say you won't use it twice, and then you do use it  
> twice?

The author is asserting that maximum_unsigned_value_of_type() is a  
function of sizeof a, which would have no runtime effect.

Josh