linux-kernel - Re: [PATCH 00/11] pragma once: treewide conversion

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Date:   Sat, 6 Mar 2021 14:07:27 +0100
From:   Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     David Laight <David.Laight@...lab.com>,
        Tom Tromey <tom@...mey.com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Sparse Mailing-list <linux-sparse@...r.kernel.org>
Subject: Re: [PATCH 00/11] pragma once: treewide conversion

On Fri, Mar 5, 2021 at 10:25 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> And regardless of even _those_ issues, you still should do all the
> other syntactic tokenization stuff (ie all the quoting, the the
> character handling: 'a' is a valid C token, but if you see the string
> "it's" outside of a comment, that's a syntax error even if it's inside
> a disabled region. IOW, this is an incorrect file:
>
>    #if 0
>    it's a bug to do this, and the compiler should scream
>    #endif
>
> because it's simply not a valid token sequence. The fact that it's
> inside a "#if 0" region doesn't change that fact at all.  So you did
> need to do all the tokenization logic.

Compilers don't scream that much, only GCC seems to give a warning. I
assume it is because it is just undefined rather than a required
error/diagnostic, i.e. the "If a ’ or a " character matches the last
category, the behavior is undefined." in 6.4.

Concerning #pragma once: I actually would like to have a standard
#once directive if what is a "seen file" could be defined a bit more
precisely. Even if just says it creates a guard with something similar
to the result of `__FILE__` would be good enough for many projects out
there, and one can still use guards when flexibility is needed and/or
corner cases are expected (which, if detected, the compiler could also
warn about).

Cheers,
Miguel