netdev - Re: [PATCH 2/6] treewide: remove using list iterator after loop body as a ptr

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Message-ID: <CAHk-=wgLYqYcw0xv65xrLSR7KDpS_6M+S9737m6NQorHGWsXYQ@mail.gmail.com>
Date:   Mon, 28 Feb 2022 16:54:45 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Segher Boessenkool <segher@...nel.crashing.org>
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Subject: Re: [PATCH 2/6] treewide: remove using list iterator after loop body
 as a ptr

On Mon, Feb 28, 2022 at 4:38 PM Segher Boessenkool
<segher@...nel.crashing.org> wrote:
>
> In C its scope is the rest of the declaration and the entire loop, not
> anything after it.  This was the same in C++98 already, btw (but in
> pre-standard versions of C++ things were like you remember, yes, and it
> was painful).

Yeah, the original C++ model was just unadulterated garbage, with no
excuse for it, and the scope was not the loop, but the block the loop
existed in.

That would never have been acceptable for the kernel - it's basically
just an even uglier version of "put variable declarations in the
middle of code" (and we use "-Wdeclaration-after-statement" to
disallow that for kernel code, although apparently some of our user
space tooling code doesn't enforce or follow that rule).

The actual C99 version is the sane one which actually makes it easier
and clearer to have loop iterators that are clearly just in loop
scope.

That's a good idea in general, and I have wanted to start using that
in the kernel even aside from some of the loop construct macros.
Because putting variables in natural minimal scope is a GoodThing(tm).

Of course, we shouldn't go crazy with it. Even after we do that
-std=gnu11 thing, we'll have backports to worry about. And it's not
clear that we necessarily want to backport that gnu11 thing - since
people who run old stable kernels also may be still using those really
old compilers...

            Linus