linux-kernel - RE: [PATCH] kbuild: treat char as always signed

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Date:   Wed, 19 Oct 2022 20:58:18 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Linus Torvalds' <torvalds@...ux-foundation.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-kbuild@...r.kernel.org" <linux-kbuild@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-toolchains@...r.kernel.org" <linux-toolchains@...r.kernel.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: RE: [PATCH] kbuild: treat char as always signed

From: Linus Torvalds
> Sent: 19 October 2022 20:54
> 
> On Wed, Oct 19, 2022 at 9:27 AM Jason A. Donenfeld <Jason@...c4.com> wrote:
> >
> > So let's just eliminate this particular variety of heisensigned bugs
> > entirely. Set `-fsigned-char` globally, so that gcc makes the type
> > signed on all architectures.
> 
> Btw, I do wonder if we might actually be better off doing this - but
> doing it the other way around.
> 
> IOW, make 'char' always UNsigned. Unlike the signed char thing, it
> shouldn't generate any worse code on any common architecture.
> 
> And I do think that having odd architecture differences is generally a
> bad idea, and making the language rules stricter to avoid differences
> is a good thing.
> 
> Now, you did '-fsigned-char', because that's the "common default" in
> an x86-centric world.

I'm pretty sure char is signed because the pdp11 only had
sign-extending byte loads.

> You are also right that people might think that "char" works like
> "int", and that if you don't specify the sign, it's signed.

But even 'unsigned char' works like int.
The values are promoted to int (thanks to the brain-dead ANSI-C
committee) rather than unsigned int (which I think was in K&R C).
(There is an exception, int, short and char can all be the same size.
In which case unsigned char promotes to unsigned int.)

	David

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