From: Corinna Vinschen <corinna-cygwin@cygwin.com>
To: cygwin@cygwin.com
Subject: Re: Problem with line buffering and getc function on 1.7.33.
Date: Sun, 13 Mar 2016 10:47:00 -0000 [thread overview]
Message-ID: <20160313104712.GC25791@calimero.vinschen.de> (raw)
In-Reply-To: <ebae78105462dee607c99bc191f797fc@mail.kylheku.com>
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On Mar 12 16:37, Kaz Kylheku wrote:
> On 12.03.2016 14:29, Corinna Vinschen wrote:
> >On Mar 12 20:39, Corinna Vinschen wrote:
> >>On Mar 11 16:05, Kaz Kylheku wrote:
> >>> We can reproduce the problem with just file streams using
> >>> a much simpler program:
> >>> [...]
> >>> The contents of file end up being "\n": one empty
> >>> line, instead of "abcde\n":
> >>>
> >>> $ cat file
> >>>
> >>> $
> >>
> >>Thanks for the testcase. I can reproduce the issue and I see where the
> >>problem occurs, but I'm still puzzled. Comparing the code in our newlib
> >>C library with its BSD counterparts, I could swear the same behaviour
> >>happens on OpenBSD as well. If not (which needs testing), I wonder why
> >>and where newlib's actually different. Right now I don't see the
> >>difference.
> >
> >I do now. Basically it's setvbuf screwing up the internal flags in the
> >FILE structure. I took the liberty to update newlib's setvbuf to the
> >OpenBSD version locally and I'm going to apply my patches to newlib
> >soon. I'll provide a new 2.5.0 test release of Cygwin with this patch
> >tomorrow or early next week.
>
>
> Indeed, I thought it would have something to do with the
> stream->_flags, because when I single-stepped over the getc(stream)
> line (necessary for the problem to repro, IIRC) the only apparent state
> change in the stream was to the _flags member:
The actual problem was that setvbuf set the _lbfsize member to -bufsize
in the _IOLBF case indiscriminately. This in turn leads to __sputc_r
doing the wrong thing:
if (--_p->_w >= 0 || (_p->_w >= _p->_lbfsize && (char)_c != '\n'))
return (*_p->_p++ = _c);
else
return (__swbuf_r(_ptr, _c, _p));
Note the check for _p->_w >= _p->_lbfsize. By setting _lbfsize in
setvbuf, what happens is that the first branch is called for all
chars up to the first \n. However, the *first* call to __sputc_r
is supposed to call __swbuf_r anyway, otherwise bookkeeping is broken.
And that's the major difference to the OpenBSD setvbuf; it only sets
fp->_lbfsize to -bufsize if we're already writing (__SWR flag is set).
Corinna
--
Corinna Vinschen Please, send mails regarding Cygwin to
Cygwin Maintainer cygwin AT cygwin DOT com
Red Hat
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next prev parent reply other threads:[~2016-03-13 10:47 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-03-11 21:57 Kaz Kylheku
2016-03-11 22:14 ` Marco Atzeri
2016-03-12 0:05 ` Kaz Kylheku
2016-03-12 19:39 ` Corinna Vinschen
2016-03-12 22:29 ` Corinna Vinschen
2016-03-13 0:37 ` Kaz Kylheku
2016-03-13 10:47 ` Corinna Vinschen [this message]
2016-03-13 0:41 ` Kaz Kylheku
2016-03-13 11:06 ` Corinna Vinschen
2016-03-13 11:37 ` Corinna Vinschen
-- strict thread matches above, loose matches on Subject: below --
2016-03-11 19:17 Kaz Kylheku
2016-03-11 20:09 ` Yaakov Selkowitz
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