Re: [PATCH v2 1/1] vreportf(): avoid relying on stdio buffering

From: Jeff King <peff@peff.net>
Date: Oct 29, 2019, 20:09 UTC
Message-ID: <20191029200936.GA12337@sigill.intra.peff.net>
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:

Show 13 quoted lines

> > 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)". :)

Show 12 quoted lines

> > 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