linux-kernel - Re: epoll_ctl and const correctness

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Date:	Fri, 27 Mar 2009 23:17:05 +0100
From:	nicolas sitbon <nicolas.sitbon@...il.com>
To:	Michael Tokarev <mjt@....msk.ru>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	linux-kernel@...r.kernel.org,
	Davide Libenzi <davidel@...ilserver.org>
Subject: Re: epoll_ctl and const correctness

Well, first, thanks for your answer, then, there is a difference
between saying the kernel modify or not the structure and the kernel
keep track of it. Consider this user :
http://www.csplayer.org/2009/02/a-tutorial-example-of-epoll-usage/, he
thinks the kernel doesn't keep track of the event, and I'm sure he is
not the first, so please, at least, be more explicit in the
documentation and again thanks for your answer. And what about size
parameter in epoll_create()? why is it an int and not a size_t?

2009/3/27 Michael Tokarev <mjt@....msk.ru>:
> nicolas sitbon wrote:
>>
>> Please, can anyone answer me, I need a response.
>
>> 2009/3/25 nicolas sitbon <nicolas.sitbon@...il.com>:
>>>
>>> You don't teach me anything, I know that, the fact is the
>>> documentation is incomplete, so rather saying that, please answer my
>>> questions. For the moment, only the documenation and the prototype of
>>> epoll are buggy.
>
> So which response do you want -- the one saying that the documentation
> is buggy or or epoll prototype?  Or something else?
>
> []
>>>>>
>>>>>  or the good prototype is
>>>>>
>>>>> int epoll_ctl(int epfd, int op, int fd, struct epoll_event const
>>>>> *event);
>
> Why should it be const?  There is no guarantee the argument will not be
> modified by the kernel.  Documentation does not say that.  Current prototype
> does not say that.  If you need such a guarantee, you're free to add another
> system call into your kernel, and fix both your documentation and your
> prototype to match.  What's the deal?
>
> Back from useless rants and to the technical points.
>
> Again: there's no guarantee the `event' argument will not be modified.
> Even if kernel CURRENTLY indeed does not modify it, but the interface
> does not PROMISE it to be that way for ever.
>
> Why does it not promise that is another question.  Just one example:
> what, some day, stops us from adding some EPOLL_CTL_GET operation
> to RETRIEVE information associated with that filedescriptor in kernel
> currently and STORE that info in the structure pointed to by `event'
> argument?  That way it will not be const anymore.
>
> So.. what's your problem?
>
> /mjt
>
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