Path: utzoo!attcan!uunet!auspex!guy
From: guy@auspex.UUCP (Guy Harris)
Newsgroups: comp.unix.wizards
Subject: Re: printf, data presentation
Message-ID: <845@auspex.UUCP>
Date: 14 Jan 89 05:52:00 GMT
References: <19@xenlink.UUCP> <8800006@gistdev> <13327@ncoast.UUCP>
Reply-To: guy@auspex.UUCP (Guy Harris)
Organization: Auspex Systems, Santa Clara
Lines: 34

>At least *some* System V's will treat VMIN == 0, VTIME == 0 as the inkey()
>function originally requested.

In the code as distributed by AT&T, reads with VMIN == 0, VTIME == 0 are
non-blocking reads, and reads with VMIN == 0, VTIME == 10 wait for up to
1 second, or until a character arrives.  I would hope *all* System V's
work that way, but it's conceivable some are broken.... 

From the SunOS 4.0 TERMIO(4) (it implements an S5 superset - this part
is not a superset, though, S5 does the same stuff):

  Case C: MIN = 0, TIME > 0
     In this case, since MIN = 0, TIME no  longer  represents  an
     intercharacter timer.  It now serves as a read timer that is
     activated as soon as a read() is done.  A read is  satisfied
     as  soon as a single character is received or the read timer
     expires.  Note: in this case if the timer expires, no  char-
     acter  will  be returned.  If the timer does not expire, the
     only way the read can be satisfied  is  if  a  character  is
     received.  In this case the read will not block indefinitely
     waiting for a character - if no character is received within
     TIME*.10  seconds after the read is initiated, the read will
     return with zero characters.

  Case D: MIN = 0, TIME = 0
     In this case return is immediate.  The minimum of either the
     number  of  characters requested or the number of characters
     currently available will be  returned  without  waiting  for
     more characters to be input.

>I *hope* that POSIX is dealing with this.

POSIX specifies an S5-like tty driver, with S5-style behavior for VMIN
and VTIME.