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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sun, 28 Feb 2021 12:00:22 -0800
Message-ID: <CAHk-=wi54DEScexxpMrO+Q2Nag_Tup+Y5YBHc_9_xGLeRfP8pA@mail.gmail.com>
Subject: Re: [PATCH 00/11] pragma once: treewide conversion
To:     Alexey Dobriyan <adobriyan@gmail.com>
Cc:     Luc Van Oostenryck <luc.vanoostenryck@gmail.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Sparse Mailing-list <linux-sparse@vger.kernel.org>
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On Sun, Feb 28, 2021 at 11:34 AM Alexey Dobriyan <adobriyan@gmail.com> wrote:
>
> >
> > End result: #pragma is fundamentally less reliable than the
> > traditional #ifdef guard. The #ifdef guard works fine even if you
> > re-read the file for whatever reason, while #pragma relies on some
> > kind of magical behavior.

You continue to not even answer this very fundamental question.

"#pragma once" doesn't seem to have a _single_ actual real advantage.

Everybody already does the optimization of not even opening - much
less reading and re-parsing - headers that have the traditional #ifdef
guard.

And even if you _don't_ do that optimization, the #ifdef guard
fundmentally semantically guarantyees the right behavior.

So the #ifdef guard is
 (a) standard
 (b) simple
 (c) reliable
 (d) traditional

and you have yet to explain a _single_ advantage of "#pragma once".

Why add this incredible churn that has no upside?

So no. We're not using #pragma once unless y9ou can come up with some
very strong argument for it

And no, having to come up with a name for the #ifdef guard is not a
strong argument. It's simply not that complicated.

               Linus