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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 27 Oct 2022 11:28:02 -0700
Message-ID: <CAHk-=wgadqCK7xAOzCdDHa7CQWC1z3df6a-pHQaF73ZjSZ58wg@mail.gmail.com>
Subject: Re: make ctype ascii only? (was [PATCH] kbuild: treat char as always signed)
To:     Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc:     "Jason A. Donenfeld" <Jason@zx2c4.com>,
        linux-kernel@vger.kernel.org, linux-kbuild@vger.kernel.org,
        linux-arch@vger.kernel.org, linux-toolchains@vger.kernel.org,
        Masahiro Yamada <masahiroy@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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On Thu, Oct 27, 2022 at 12:59 AM Rasmus Villemoes
<linux@rasmusvillemoes.dk> wrote:
>
> AFAICT, the differences are:
>
> - 0xaa (FEMININE ORDINAL INDICATOR), 0xb5 (MICRO SIGN), 0xba (FEMININE
> ORDINAL INDICATOR) should be lower (hence alpha and alnum), not punct.
>
> - depending a little on just exactly what one wants latin1 to mean, but
> if it does mean "first 256 codepoints of Unicode", 0x80-0x9f should be cntrl
>
> - for some reason at least glibc seems to classify 0xa0 as punctuation
> and not space (hence also as isgraph)
>
> - 0xdf and 0xff are correctly classified as lower, but since they don't
> have upper-case versions (at least not any that are representable in
> latin1), correct toupper() behaviour is to return them unchanged, but we
> just subtract 0x20, so 0xff becomes 0xdf which isn't isupper() and 0xdf
> becomes something that isn't even isalpha().

Heh.

Honestly, I don't think we should care at all.

For the byte range 128-255, anybody who uses ctype on them gets what
they get. In the kernel, the most likely use of it is for 'isprint()',
and if those care, they can (and some do) use 'isascii()' in addition.

I don't know if you realize, but the kernel already says "screw libc",
and makes all the isxyz() things just cast the argument to 'unsigned
char', and doesn't care about EOF.

And for the rest, let's just call it the "kernel locale", and just
admit that the kernel locale is entirely historical.

Boom - problem solved, and it's entirely standards conformant (apart
possibly from the EOF case, I think that is marked as a "lower case
character" right now ;)

Looking through

    https://pubs.opengroup.org/onlinepubs/9699919799/

I'm not actually seeing anything that says that we don't do *exactly*
what the standard requires.

You thinking that the kernel locale is US-ASCII is just wrong.

              Linus