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Date:   Fri, 11 Aug 2023 15:28:17 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc:     Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Petr Mladek <pmladek@suse.com>, Marco Elver <elver@google.com>,
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        linux-mm@kvack.org, Sergey Senozhatsky <senozhatsky@chromium.org>,
        Alexander Potapenko <glider@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH v2 2/3] lib/vsprintf: Split out sprintf() and friends
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On Thu, 10 Aug 2023 16:17:57 +0200
Rasmus Villemoes <linux@rasmusvillemoes.dk> wrote:

> > Btw, it's easy to enforce IIUC, i.e. by dropping
> > 
> >   #ifndef _FOO_H
> >   #define _FOO_H
> >   #endif
> > 
> > mantra from the headers.
> >   
> 
> No, you can't do that, because some headers legitimately include other
> headers, often for type definitions. Say some struct definition where
> one of the members is another struct (struct list_head being an obvious
> example). Or a static inline function.
> 
> We _also_ don't want to force everybody who includes a.h to ensure that
> they first include b.h because something in a.h needs stuff from b.h.
> 
> So include guards must be used. They are a so well-known idiom that gcc
> even has special code for handling them: If everything in a foo.h file
> except comments is inside an ifndef/define/endif, gcc remembers that
> that foo.h file has such an include guard, so when gcc then encounters
> some #include directive that would again resolve to that same foo.h, and
> the include guard hasn't been #undef'ed, it doesn't even do the syscalls
> to open/read/close the file again.

I hope Andy was just joking with that recommendation.

-- Steve