Re: man termios

[Peter Hurley <peter@hurleysoftware.com>]

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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