From: Warner Losh <imp@bsdimp.com>
To: Douglas McIlroy <douglas.mcilroy@dartmouth.edu>
Cc: TUHS main list <tuhs@minnie.tuhs.org>
Subject: Re: [TUHS] run commands at login in v6 and stty
Date: Mon, 28 Feb 2022 11:18:30 -0700 [thread overview]
Message-ID: <CANCZdfotYer6n6iuEc+Vocy+PyXXcETNgVoc4g6tQmS9b4JJZw@mail.gmail.com> (raw)
In-Reply-To: <CAKH6PiVCk6gSv-WVztRUiJrOt3QHVi1pCVEKzw1RcEi+m+G=bw@mail.gmail.com>
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On Sun, Feb 27, 2022 at 6:07 PM Douglas McIlroy <
douglas.mcilroy@dartmouth.edu> wrote:
> > The X11 tree was a heavily ifdef-ed. And it needed to be, I don't have
> > an answer as to how you would reuse all that code on different hardware
> > in a better way.
>
> Plan 9 did it with #include. The name of the included file was the same for
> every architecture. Only the search path for include files changed. Done
> with
> care, this eliminates the typical upfront #ifdefs.that define constants
> and set
> flags.
>
The qemu project does this, for the most part. It makes things a lot
easier, but
does take some getting used to to find the proper file to be included. It
helps
a lot to keep the twisty maze of #ifdefs down to a dull roar.
FreeBSD (and all the BSDs) has a similar philosophy. You define the page
size,
for example, of the target, and the rest of the system is written to that
abstraction
rather than having #ifdefs in a lot of places. NetBSD is much better about
keeping
the total number of #ifdefs out of MI code than FreeBSD. In both cases, most
of the #ifdefs are basically #ifdef __HAS_FUNKY_THING <do funky thing here>
#endif that are needed on a couple of architectures, though usually that
idiom
is expressed by a macro (that's empty for most platforms) or an inline
function.
> Other preprocessor conditionals can usually be replaced by a regular if,
> letting
> the compiler optimize away the unwanted alternative. This makes
> conditionals
> obey the scope rules of C.
>
As long as they are always defined... :) Though older gcc/clang compilers
get cranky
when they view the if expression as a tautology (warning about the idiom
rather than
encouraging it: stupid clang tricks).
Warner
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next prev parent reply other threads:[~2022-02-28 18:21 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-02-28 1:04 Douglas McIlroy
2022-02-28 7:22 ` Rob Pike
2022-02-28 14:07 ` Larry McVoy
2022-02-28 18:47 ` Dan Cross
2022-02-28 19:25 ` Dan Cross
2022-02-28 21:25 ` markus schnalke
2022-02-28 19:26 ` Clem Cole
2022-02-28 20:03 ` Adam Thornton
2022-02-28 18:18 ` Warner Losh [this message]
-- strict thread matches above, loose matches on Subject: below --
2022-02-27 7:48 Noel Chiappa
2022-02-26 21:45 Brian Walden
2022-02-26 22:16 ` Rob Pike
2022-02-27 20:32 ` Sven Mascheck
2022-02-26 20:39 Will Senn
2022-02-26 21:03 ` Michael Kjörling
2022-02-26 22:49 ` Clem Cole
2022-02-26 23:12 ` Rob Pike
2022-02-27 0:46 ` Clem Cole
2022-02-27 15:01 ` Larry McVoy
2022-02-27 17:10 ` Clem Cole
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