Re: man termios

[Peter Hurley]

On 03/21/2014 10:17 AM, Michael Kerrisk (man-pages) wrote:
> On Fri, Mar 21, 2014 at 3:03 PM, Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> 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 proâ
> > >          cessing is performed, and line editing is disabled.  The settings
> > >          of  MIN (c_cc[VMIN]) and TIME (c_cc[VTIME]) determine the circumâ
> > >          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 secâ
> > >                 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  secâ
> > >                 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 immeâ
> > >          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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