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

From: Joshua Juran <jjuran@gmail.com>
Date: Feb 10, 2011, 13:23 UTC
Message-ID: <E84FF9D8-92D7-4CA0-9CF4-77FF33893977@gmail.com>
In-Reply-To: <AANLkTinqkSxj0C+GyMVR4a7d=yy_mh+oDuar0moZjZ8_@mail.gmail.com>

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

Show 14 quoted lines

> 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