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Date: Fri, 21 Mar 2014 14:15:27 +0100
From: "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To: Peter Hurley <peter@...leysoftware.com>
CC: mtk.manpages@...il.com,
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 03/21/2014 12:21 PM, Peter Hurley wrote:
> On 03/21/2014 06:45 AM, Michael Kerrisk (man-pages) wrote:
>>> This is also true of the other non-canonical
>>> read()'s with timeout (TIME > 0).
>>
>> Here, if I understand you correctly, you mean this case:
>> * TIME > 0
>> * MIN == 0
>> * O_NONBLOCK set on the FD
>> * No input available
>>
>> You are saying that read() returns 0 in this case? This doesn't appear
>> to me to be correct (on Linux). How did you verify this?
>
> Apologies, my mistake here.
Okay -- thanks for confirming.
> (aside: O_NONBLOCK can be changed while the reader is blocked, but that's
> not what I meant originally, and I wouldn't document it there)
>
>>> 'man termios' is silent here, but 62.6.2 in LPI implies that O_NONBLOCK will
>>> return -1 with errno==EAGAIN; it does not.
>>
>> I guess you are referring to this text in TLPI:
>
> Yes.
>
>> [[
>> This mode is somewhat similar to setting the O_NONBLOCK flag for the
>> terminal (Section 5.9). However, with O_NONBLOCK, if no bytes are
>> available for reading, then read() returns –1 with the error
>> EAGAIN.
>> ]]
>>
>> Oops. The text was not meant to imply that. Rather the comparison was
>> intended to be with O_NONBLOCK *in canonical mode*. However, I agree
>> that I could have made that more explicit. (I'll add an erratum to
>> mention canonical mode.)
>>
>>> This is unspecified by POSIX (11.1.7).
>>
>> Yep, I see. XBD 11.1.7 says:
>>
>> [[
>> Therefore, if O_NONBLOCK is set, read( ) may return immediately,
>> regardless of the setting of MIN or TIME. Also, if no data is
>> available, read( ) may either return 0, or return -1 with errno set to
>> [EAGAIN].
>> ]]
>>
>> So, it seems to be saying that either behavior is allowed, right?
>
> Exactly.
>
>> And as far as I can see, for the TIME>0 case on Linux, read() returns
>> -1 + EGAIN.
>
> You're right about the TIME>0 case.
>
>> In any case, I've added this text to termios(3):
>>
>> POSIX does not specify whether the setting of the O_NONBLOCK
>> file status flag takes precedence over the MIN and TIME set‐
>> tings. If O_NONBLOCK is set, a read() in noncanonical mode may
>> return immediately, 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.
>
>
> Great; I think that will really help clarify the usage wrt O_NONBLOCK.
[...]
>>> 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.
Cheers,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
View attachment "noncanonical.c" of type "text/x-csrc" (1444 bytes)
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