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SubjectRe: man termios
On 03/21/2014 10:17 AM, Michael Kerrisk (man-pages) wrote:
> On Fri, Mar 21, 2014 at 3:03 PM, Peter Hurley <peter@hurleysoftware.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...)

I did run the test program to validate that it's observed behavior is that
implemented by Linux, with which I'm very familiar.
I don't have a test setup for other *nixes.

I would be interested to know the results of

./noncanonical 0 5 3 0
hello

and

./noncanonical 0 5 3 2
hel

on other platforms.

With respect to POSIX compliance, it's hard to say. I'm not sure the
spec contemplates the degenerate case where max bytes < MIN. And specifically
with regard to terminal i/o behavior, POSIX is essentially ex post facto,
and is really documenting existing behavior.

Other than the degenerate case of max bytes < MIN, is there any other
variation between Solaris and Linux in non-canonical mode?

Regards,
Peter Hurley

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