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From: Jeff King <peff@peff.net>
To: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Cc: Alexandr Miloslavskiy <alexandr.miloslavskiy@syntevo.com>,
	Johannes Schindelin via GitGitGadget <gitgitgadget@gmail.com>,
	git@vger.kernel.org, Junio C Hamano <gitster@pobox.com>
Subject: Re: [PATCH v2 1/1] vreportf(): avoid relying on stdio buffering
Date: Tue, 29 Oct 2019 16:09:36 -0400	[thread overview]
Message-ID: <20191029200936.GA12337@sigill.intra.peff.net> (raw)
In-Reply-To: <nycvar.QRO.7.76.6.1910292049530.46@tvgsbejvaqbjf.bet>

On Tue, Oct 29, 2019 at 08:57:33PM +0100, Johannes Schindelin wrote:

> > On 29.10.2019 14:37, Johannes Schindelin via GitGitGadget wrote:
> >
> > > -	vsnprintf(msg, sizeof(msg), err, params);
> > > -	for (p = msg; *p; p++) {
> > > +	p = msg + off < pend ? msg + off : pend - 1;
> >
> > According to my understanding, this is undefined behavior:
> 
> It is not entirely obvious to me what exactly you mean by "this",
> assuming that you refer to comparing two pointers via `<`, I agree that
> this is not the best idea, I changed it to `off < pend - msg`. If my
> assumption is correct, however, we are still talking about C, so I
> wonder how this C++ document you linked could bear any relevance:

I think the issue is not the comparison, but rather that forming the
pointer "msg + off" is undefined, since its point outside of any object
(including the "one past" rule). And this is illegal in both C and C++,
though of course it works fine in practice most of the time.

Doing "off < pend - msg" is legal. Or note that "pend" is just
"msg + sizeof(msg)", subtract out the common term and end up with
"off < sizeof(msg)". :)

> > Can you please preserve the research text about fprintf() behavior on
> > different platforms that I provided before? Change formatting to what you
> > think fits best.
> 
> Quite honestly, I purposefully refrained from copying that information.
> While even the following paragraphs are incomplete by necessity (there
> are many more standard C libraries we try to work against, e.g. musl and
> newlibc, and it would be insanity to try to cover them all in an
> analysis of stdio buffering), the only finding that is relevant to the
> patch under discussion is that MSVC's runtime outputs to `stderr` byte
> by byte (or more correctly, character by character, I guess), and I did
> mention this in the commit message (in the part you quoted).

One other case is impacted, which is:

> > 1) If `stderr` is fully buffered:
> >    the ordering of `stdout` and `stderr` messages could be wrong,
> >    because `stderr` output waits for the buffer to become full.

We'll now output the stderr message closer to its time-of-printing,
which is a good thing (it may still be in a weird place with respect to
a buffered stdout, but getting the error message out ASAP is the best we
can do there).

I agree that the rest of the research is not especially relevant to the
code change (though I'm glad it is available in the list archive).

-Peff

  reply	other threads:[~2019-10-29 20:09 UTC|newest]

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-28 15:00 [PATCH 0/1] Fix t5516 flakiness in Visual Studio builds Johannes Schindelin via GitGitGadget
2019-10-28 15:00 ` [PATCH 1/1] vreportf(): avoid buffered write in favor of unbuffered one Johannes Schindelin via GitGitGadget
2019-10-29  3:18   ` Junio C Hamano
2019-10-29 12:30     ` Johannes Schindelin
2019-10-29 13:49       ` Jeff King
2019-10-29 14:13         ` Johannes Schindelin
2019-10-29 14:32           ` Jeff King
2019-10-29 20:09             ` Johannes Schindelin
2019-10-30  1:43               ` Junio C Hamano
2019-10-29 16:44         ` Junio C Hamano
2019-10-29 10:38   ` SZEDER Gábor
2019-10-29 12:38     ` Johannes Schindelin
2019-10-29 13:52       ` Jeff King
2019-10-29 14:18         ` Johannes Schindelin
2019-10-29 13:37 ` [PATCH v2 0/1] Fix t5516 flakiness in Visual Studio builds Johannes Schindelin via GitGitGadget
2019-10-29 13:37   ` [PATCH v2 1/1] vreportf(): avoid relying on stdio buffering Johannes Schindelin via GitGitGadget
2019-10-29 14:21     ` Alexandr Miloslavskiy
2019-10-29 19:57       ` Johannes Schindelin
2019-10-29 20:09         ` Jeff King [this message]
2019-10-29 20:24           ` Alexandr Miloslavskiy
2019-10-29 20:11         ` Alexandr Miloslavskiy
2019-10-29 20:01   ` [PATCH v3 0/1] Fix t5516 flakiness in Visual Studio builds Johannes Schindelin via GitGitGadget
2019-10-29 20:01     ` [PATCH v3 1/1] vreportf(): avoid relying on stdio buffering Johannes Schindelin via GitGitGadget
2019-10-29 20:32       ` Jeff King
2019-10-30  8:54         ` Johannes Schindelin
2019-10-31  6:24           ` Jeff King
2019-10-31 10:26             ` Johannes Schindelin
2019-10-31 15:48               ` Jeff King
2019-11-01 18:41                 ` Johannes Schindelin
2019-10-30  2:01       ` Junio C Hamano
2019-10-30  9:13         ` Johannes Schindelin
2019-10-30 10:44     ` [PATCH v4 0/1] Fix t5516 flakiness in Visual Studio builds Johannes Schindelin via GitGitGadget
2019-10-30 10:44       ` [PATCH v4 1/1] vreportf(): avoid relying on stdio buffering Johannes Schindelin via GitGitGadget
2019-11-02  4:05         ` Junio C Hamano

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