Re: [PATCH] trace2: refactor to avoid gcc warning under -O3

From: Johannes Schindelin <Johannes.Schindelin@gmx.de>
To: Junio C Hamano <gitster@pobox.com>
Cc: "Ævar Arnfjörð Bjarmason" <avarab@gmail.com>,
	git@vger.kernel.org, "Jeff King" <peff@peff.net>,
	"Denton Liu" <liu.denton@gmail.com>,
	"Jeff Hostetler" <jeffhost@microsoft.com>
Subject: Re: [PATCH] trace2: refactor to avoid gcc warning under -O3
Date: Tue, 11 May 2021 16:34:49 +0200 (CEST)	[thread overview]
Message-ID: <nycvar.QRO.7.76.6.2105111632380.57@tvgsbejvaqbjf.bet> (raw)
In-Reply-To: <xmqqh7jfy3du.fsf@gitster.g>

Hi Junio,

On Fri, 7 May 2021, Junio C Hamano wrote:

> Johannes Schindelin <Johannes.Schindelin@gmx.de> writes:
>
> >> Otherwise I'd strongly prefer to see a word that hints that this is
> >> an otherwise unneeded workaround for comiplers.  Your suggested
> >> title instead hints that it is wrong to assume that errno will be
> >> set to non-zero after a syscall.  I do not think that is the message
> >> we want to send to our readers.
> >
> > Right, the oneline I suggested was only for the original patch, with which
> > I disagreed.
>
> I actually do not know how your rewrite could be better, though.
>
> 		/* GCC thinks socket()/connect() might fail to set errno */
> 		return errno ? errno : EIO;
>
> If a compiler thinks errno may *not* be set, can 'errno' be reliably
> used to decide if it can be returned as-is or a fallback value EIO
> should be used, without triggering the same (incorrect) working in
> the first place?

Oh, I guess I mistook the problem for something else, then.

If the problem is that `errno` is not set in case of failure, the
resolution is easy (and even recommended in the manual page of `errno`):
simply set it to 0 before the syscall (or in the function that relies on
`errno == 0` means success).

There is really no need to introduce a `saved_errno` variable (which to me
would suggest that we need to save the current value of `errno` and
reinstate it later, as we do sometimes e.g. when we call `close()` after
noticing an error whose `errno` we need to preserve for the caller).

Ciao,
Dscho