linux-kernel - Re: A simple question of sys

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Message-ID: <20100921075334.GA5427@cr0.nay.redhat.com>
Date:	Tue, 21 Sep 2010 15:53:34 +0800
From:	Américo Wang <xiyou.wangcong@...il.com>
To:	Rofail Qu <rofail@...il.com>
Cc:	Américo Wang <xiyou.wangcong@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: A simple question of sys_

On Mon, Sep 20, 2010 at 04:28:36PM +0800, Rofail Qu wrote:
>2010/9/20 Américo Wang <xiyou.wangcong@...il.com>:
>> On Fri, Sep 17, 2010 at 02:58:30PM +0800, Rofail Qu wrote:
>>>How to use macro IS_ERR() ?
>>>
>>>It defines as,
>>>...
>>>#define IS_ERR_VALUE(x) unlikely((x) >= (unsigned long)-MAX_ERRNO)
>>>static inline long __must_check IS_ERR(const void *ptr)
>>>{
>>>       return IS_ERR_VALUE((unsigned long)ptr);
>>>}
>>>...
>>>so when pass x as a pointer and x>=-MAX_ERRNO (including NULL or any
>>>valid address),
>>>IS_ERR() will return true!
>>
>> NULL is not an error pointer, you missed the cast to unsigned long.
>Ahh, yeah. A stupid mistake i have.
>Thanks.
>>
>>
>>>IS_ERR(x) seems to use on judge if "x" is a valid error number, right?
>>>
>>
>> Strictly speaking, it checks if 'x' is an error pointer.
>Then what is the range of a valid pointer?
>At least less than (unsigned long)-MAX_ERRNO from this case.

Yeah, the last page in the virtual memory address is reserved,
so all but the last page are non-error addresses.
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