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Date:	Fri, 21 Mar 2014 15:17:28 +0100
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	linux kernel <linux-kernel@...r.kernel.org>,
	linux-serial <linux-serial@...r.kernel.org>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Ivan <athlon_@...l.ru>
Subject: Re: man termios

On Fri, Mar 21, 2014 at 3:03 PM, Peter Hurley <peter@...leysoftware.com> wrote:
> On 03/21/2014 09:15 AM, Michael Kerrisk (man-pages) wrote:
>>
>> On 03/21/2014 12:21 PM, Peter Hurley wrote:
>>>
>>> On 03/21/2014 06:45 AM, Michael Kerrisk (man-pages) wrote:
>
>
>>>>> Finally, if the 'count' parameter is less than MIN, read() may return
>>>>> before
>>>>> MIN bytes have been received, if 'count' bytes have been received.
>>>>
>>>>
>>>> Yes. But it's not clear to me here: do you mean that something in the
>>>> man page (or in TLPI) needs fixing?
>>>
>>>
>>> Well, what I mean here is that read() may also _not_ return until MIN
>>> bytes have
>>> been received, even if 'count' bytes have been received.
>>
>>
>> Ahh -- I see what you mean. And, it looks like there is a point here where
>> Linux
>> differs from POSIX and (at least) Solaris. See the current man-page text
>> below,
>> in particular the MIN>0, TIME>0 case. I've also attached a simple test
>> program
>> that I used, below.
>>
>>         In noncanonical mode input is available immediately (without  the
>>         user  having  to  type a line-delimiter character), no input pro‐
>>         cessing is performed, and line editing is disabled.  The settings
>>         of  MIN (c_cc[VMIN]) and TIME (c_cc[VTIME]) determine the circum‐
>>         stances in which a read(2) completes;  there  are  four  distinct
>>         cases:
>>
>>         MIN == 0; TIME == 0:
>>                If  data  is  available, read(2) returns immediately, with
>>                the lesser of the number of bytes available, or the number
>>                of  bytes  requested.   If  no  data is available, read(2)
>>                returns 0.
>>
>>         MIN > 0; TIME == 0:
>>                read(2) blocks until MIN bytes are available, and  returns
>>                up to the number of bytes requested.
>>
>>         MIN == 0; TIME > 0:
>>                TIME  specifies  the limit for a timer in tenths of a sec‐
>>                ond.   The  timer  is  started  when  read(2)  is  called.
>>                read(2)  returns  either when at least one byte of data is
>>                available, or  when  the  timer  expires.   If  the  timer
>>                expires  without  any  input  becoming  available, read(2)
>>                returns 0.  If data is already available at  the  time  of
>>                the call to read() the call behaves as though the data was
>>                received immediately after the call.
>>
>>         MIN > 0; TIME > 0:
>>                TIME specifies the limit for a timer in tenths of  a  sec‐
>>                ond.  Once an initial byte of input becomes available, the
>>                timer is restarted after each further  byte  is  received.
>>                read(2)  returns  when  any of the following conditions is
>>                met:
>>
>>                *  MIN bytes have been received.
>>
>>                *  The interbyte timer expires.
>>
>>                *  The number of  bytes  requested  by  read(2)  has  been
>>                   received.   (POSIX  does  not  specify this termination
>>                   condition, and on  some  other  implementations  read()
>>                   does not return in this case.)
>>
>>                Because  the  timer is started only after the initial byte
>>                becomes available, at least one byte  will  be  read.   If
>>                data  is  already  available  at  the  time of the call to
>>                read() the call behaves as though the  data  was  received
>>                immediately after the call.
>>
>>         POSIX does not specify whether the setting of the O_NONBLOCK file
>>         status flag takes precedence over the MIN and TIME settings.   If
>>         O_NONBLOCK is set, a read() in noncanonical mode may return imme‐
>>         diately, regardless of the setting of MIN or TIME.   Furthermore,
>>         if  no  data is available, POSIX permits a read() in noncanonical
>>         mode to return either 0, or -1 with errno set to EAGAIN.
>
>
> All looks good.

Peter, do you agree that Linux appears to differ from POSIX here? (Not
sure if you tried my test program to verify...)

Cheers,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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