From: Carl Worth <cworth@cworth.org>
To: Jed Brown <jed@59A2.org>, Karl Wiberg <kha@treskal.com>,
	Dirk-Jan Binnema <djcb.bulk@gmail.com>
In-Reply-To: <87ws1h48up.fsf@59A2.org>
References: <87skc5yd6v.wl%djcb@djcbsoftware.nl>
	<b8197bcb0911222334v26e96890t3632369ff32ff80f@mail.gmail.com>
	<1873022c0911230311o77e55fdqa5464a2dd16ec265@mail.gmail.com>
	<b8197bcb0911230519l4edf1b72w26d993834f25a417@mail.gmail.com>
	<87ws1h48up.fsf@59A2.org>
Date: Tue, 01 Dec 2009 07:57:30 -0800
Message-ID: <87ljhmadqt.fsf@yoom.home.cworth.org>
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Cc: "notmuch@notmuchmail org" <notmuch@notmuchmail.org>, djcb@djcbsoftware.nl
Subject: Re: [notmuch] [PATCH 1/2] * avoid gcc 4.4.1 compiler warnings due
 to ignored write return values
Precedence: list

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On Mon, 23 Nov 2009 15:24:46 +0100, Jed Brown <jed@59A2.org> wrote:
> >From the gcc man page:
>=20
>        -Wunused-value
>            Warn whenever a statement computes a result that is explicitly
>            not used. To suppress this warning cast the unused expression
>            to void. This includes an expression-statement or the left-
>            hand side of a comma expression that contains no side effects.
>            For example, an expression such as x[i,j] will cause a
>            warning, while x[(void)i,j] will not.
>=20
>            This warning is enabled by -Wall.
>=20
> But I'm confused here because I don't currently see any warnings with
> gcc-4.4.2.  Actually this must be a bug because I get no warnings for
> the blatantly unused
>=20
>   malloc(5);

I'm guessing that the -Wunused-value warning doesn't consider values
computed by function calls.

> with -Wall -Wextra -pedantic.  Anyway, if your system headers specify
> __attribute__((warn_unused_result)) for write, then you could be running
> into this bug/feature
>=20
>   http://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D35579

Yes, this is the attribute that's triggering the warnings. I poked
around in the glibc headers to see how to get this warning myself, and I
finally found:

	make CFLAGS=3D"-O -D_FORTIFY_SOURCE"

That makes the warning appear even with gcc 4.3.4, (and the definitions
in the headers suggest it will work with any gcc >=3D 4.1).

So I've pushed the patch now, (with an updated commit message to reflect
the above analysis).

=2DCarl

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