Groups > comp.lang.c > #162013

Re: How to disambiguate macro?

From	John Forkosh <forkosh@panix.com>
Newsgroups	comp.lang.c
Subject	Re: How to disambiguate macro?
Date	2021-07-21 10:10 +0000
Organization	PANIX Public Access Internet and UNIX, NYC
Message-ID	<sd8rra$f4o$1@reader1.panix.com> (permalink)
References	(1 earlier) <878s21gney.fsf@bsb.me.uk> <sd6h5j$hng$2@reader1.panix.com> <87fsw9m4qd.fsf@bsb.me.uk> <sd7ud2$hkh$1@reader1.panix.com> <sd8r8p$fph$1@dont-email.me>

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James Kuyper <jameskuyper@alumni.caltech.edu> wrote:
> On 7/20/21 9:48 PM, John Forkosh wrote:
> ...
>> i.e., "What the heck is he doing that for?" But then again, I never
>> much liked relying that  (a&&b)  always evaluates a first, immediately
>> becoming 0 without evaluating b at all if a is itself 0. You never know
>> exactly what the next C standard might mess around with.
> 
> That is a pretty fundamental feature of C; it's one of the last features
> of C that I would ever expect to see changed. It would break far too
> much existing code.

I wouldn't be too complacent about that in the future.
And even today I wouldn't be so blithely sure.
For example, && is commutative, i.e., a&&b == b&&a.
So suppose you're compiling with -O3 optimization,
and your program contains something like...
   ( (very_very_complicated_expression) && (++n>3) )
So maybe the optimizer says to itself, "Hey, I'll
just execute that very simple right-hand-side first,
and if it's false then I don't have to bother evaluating
that very complicated left-hand-side at all. Could save lots
of time." Now maybe that's not supposed to happen,
but it's way too subtle for my liking.

I'd never consider a&&b to mean that a is a guard on b's execution.
It certainly doesn't look like a guard, and I wouldn't
trust interpreting it that way. Clearly very bad practice
and very bad advice, even if it happens to work.
-- 
John Forkosh  ( mailto:  j@f.com  where j=john and f=forkosh )

Thread

How to disambiguate macro? John Forkosh <forkosh@panix.com> - 2021-07-20 11:15 +0000
  Re: How to disambiguate macro? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-07-20 13:49 +0100
    Re: How to disambiguate macro? John Forkosh <forkosh@panix.com> - 2021-07-20 12:56 +0000
      Re: How to disambiguate macro? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-07-20 15:36 +0100
        Re: How to disambiguate macro? John Forkosh <forkosh@panix.com> - 2021-07-21 01:48 +0000
          Re: How to disambiguate macro? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2021-07-20 19:18 -0700
          Re: How to disambiguate macro? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-07-20 20:27 -0700
            Re: How to disambiguate macro? John Forkosh <forkosh@panix.com> - 2021-07-21 04:15 +0000
              Re: How to disambiguate macro? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-07-21 02:00 -0700
              Re: How to disambiguate macro? James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-07-21 06:02 -0400
          Re: How to disambiguate macro? James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-07-21 06:00 -0400
            Re: How to disambiguate macro? John Forkosh <forkosh@panix.com> - 2021-07-21 10:10 +0000
              Re: How to disambiguate macro? Bart <bc@freeuk.com> - 2021-07-21 11:30 +0100
              Re: How to disambiguate macro? David Brown <david.brown@hesbynett.no> - 2021-07-21 13:31 +0200
              Re: How to disambiguate macro? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-07-21 13:02 +0100
                Re: How to disambiguate macro? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-07-21 14:20 +0100
                Re: How to disambiguate macro? Bart <bc@freeuk.com> - 2021-07-21 15:15 +0100
                Re: How to disambiguate macro? David Brown <david.brown@hesbynett.no> - 2021-07-21 16:28 +0200
                Re: How to disambiguate macro? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-07-21 16:20 +0100
                Re: How to disambiguate macro? Bart <bc@freeuk.com> - 2021-07-21 17:34 +0100
                Re: How to disambiguate macro? David Brown <david.brown@hesbynett.no> - 2021-07-21 19:46 +0200
                Re: How to disambiguate macro? Manfred <noname@add.invalid> - 2021-07-21 20:42 +0200
                Re: How to disambiguate macro? David Brown <david.brown@hesbynett.no> - 2021-07-22 08:37 +0200
                Re: How to disambiguate macro? Bart <bc@freeuk.com> - 2021-07-21 20:47 +0100
                Re: How to disambiguate macro? David Brown <david.brown@hesbynett.no> - 2021-07-22 08:47 +0200
                Re: How to disambiguate macro? Bart <bc@freeuk.com> - 2021-07-22 11:31 +0100
                Re: How to disambiguate macro? antispam@math.uni.wroc.pl - 2021-07-22 14:37 +0000
                Re: How to disambiguate macro? David Brown <david.brown@hesbynett.no> - 2021-07-22 18:15 +0200
                Re: How to disambiguate macro? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-07-21 21:19 +0100
                Re: How to disambiguate macro? Manfred <noname@add.invalid> - 2021-07-21 15:42 +0200
                Re: How to disambiguate macro? scott@slp53.sl.home (Scott Lurndal) - 2021-07-21 15:39 +0000
              Re: How to disambiguate macro? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2021-07-21 09:22 -0700
              Re: How to disambiguate macro? James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-07-21 17:24 -0400
                Re: How to disambiguate macro? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-07-21 16:42 -0700
                Re: How to disambiguate macro? John Forkosh <forkosh@panix.com> - 2021-07-22 05:49 +0000
                Re: How to disambiguate macro? James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-07-22 19:11 -0400
                Re: How to disambiguate macro? John Forkosh <forkosh@panix.com> - 2021-07-22 05:30 +0000
                Re: How to disambiguate macro? scott@slp53.sl.home (Scott Lurndal) - 2021-07-22 14:14 +0000
                Re: How to disambiguate macro? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2021-07-22 17:06 +0100
                OT: possessive adjectives (Was: How to disambiguate macro?) Manfred <noname@add.invalid> - 2021-07-22 15:04 +0200
                Re: OT: possessive adjectives Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-08-14 13:37 -0700
          Re: How to disambiguate macro? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-08-14 13:58 -0700
  Re: How to disambiguate macro? David Brown <david.brown@hesbynett.no> - 2021-07-20 14:50 +0200
    Re: How to disambiguate macro? John Forkosh <forkosh@panix.com> - 2021-07-20 13:19 +0000
      Re: How to disambiguate macro? Bart <bc@freeuk.com> - 2021-07-20 14:40 +0100
        Re: How to disambiguate macro? David Brown <david.brown@hesbynett.no> - 2021-07-20 21:02 +0200
      Re: How to disambiguate macro? David Brown <david.brown@hesbynett.no> - 2021-07-20 21:00 +0200
      Re: How to disambiguate macro? David Brown <david.brown@hesbynett.no> - 2021-07-21 08:15 +0200
      Re: How to disambiguate macro? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-08-14 23:17 -0700
  Re: How to disambiguate macro? John Forkosh <forkosh@panix.com> - 2021-07-20 12:51 +0000
  Re: How to disambiguate macro? Andrey Tarasevich <andreytarasevich@hotmail.com> - 2021-07-20 08:47 -0700

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