Hi,
On Thu, Aug 22, 2013 at 5:38 PM, G.raud <graud@xxxxxxx> wrote:
>> Date: Thu, 22 Aug 2013 09:22:32 +0200
>> From: "Michael Kerrisk (man-pages)" <mtk.manpages@xxxxxxxxx>
>> Subject: Re: select.2: s/minimum/maximum/
>>
>> On 08/20/13 01:00, G.raud wrote:
>> > In this part of the manpage:
>> >
>> > " The timeout argument specifies the minimum interval that select()
>> > should block waiting for a file descriptor to become ready. "
>>
>> This isn't correct (though maybe there is room for a better wording).
>>
>> The 'timeout' places a ceiling on how long the call will block,
>> but that ceiling has the *minimum* value provided by the argument.
>> See what I mean?
>
> The text that you cite and that is not correct is extracted from the
> present manpage.
Your original report was rather briefly worded, and my "This" referred
to your subject line, not the text you cited.
> The corrected text should be:
>
> " The timeout argument specifies the maximum interval that select()
> should block waiting for a file descriptor to become ready. "
Again, this is NOT correct. According to that statement, an
implementation that did not block at all would also be conformant,
since 0 is always less than or equal to whatever value is specified in
'timeout'. This point was the subject of changes in several man pages
(poll(2), epoll_pwait(2), select(2), sigtimedwait(2)) back in May
2012, after someone (I can't remember who) reported the point about
one of those pages.
> which is close to the POSIX description and is correct in that "specify"
> does not mean "is equal to" but can be understood as "is close to",
> which is all most of the programmers need to know.
Either of these interpretations would be a surprising way of defining
"specify", IMO.
By the way, I see what you mean about the POSIX description of
select(). It speaks of "maximum". But there, I would say the problem
is an imprecision in the standard. They get things worded better in
the poll() spec, which says: "poll() shall wait at least timeout
milliseconds".
> I suggest prepending the following before the complete description bewtween
> parentheses if a better (i.e. more explicit) wording is desired:
>
> " timeout is a lower bound on the actual maximum waiting time."
Now, I do (I think) understand why the current text troubles you: it's
because the description is counter-intuitive. But, when one thinks
about it, one realizes that it is accurate (and s/minimum/maximum/ is
not). And the very next sentences are there to help the reader
understand this point:
(This interval will be rounded up to the system clock granular‐
ity, and kernel scheduling delays mean that the blocking interval
may overrun by a small amount.)
One could I suppose reword something like this:
The timeout argument specifies the interval that select() should
block waiting for a file descriptor to become ready. In effect,
timeout specifies a minimum interval: the timeout will be rounded
up to the system clock granularity, and kernel scheduling delays
mean that the blocking interval may overrun by a small amount.
But I'm not sure that that is really much of an improvement.
Cheers,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Author of "The Linux Programming Interface"; http://man7.org/tlpi/
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