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Re: Macro Expansion

From BartC <bc@freeuk.com>
Newsgroups comp.lang.c
Subject Re: Macro Expansion
Date 2017-01-08 23:53 +0000
Organization A noiseless patient Spider
Message-ID <o4ujb6$d72$1@dont-email.me> (permalink)
References <o4rcbl$4cp$1@dont-email.me> <o4ufvd$geq$1@z-news.wcss.wroc.pl>

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On 08/01/2017 22:54, antispam@math.uni.wroc.pl wrote:
> BartC <bc@freeuk.com> wrote:

>> * The result is now scanned again looking for new macros to expand.
>>   However, once any name (eg. M and W) has been expanded once, it is
>>   never expanded again (in a subsequent pass). So only XX is changed
>>   to result in:
>>
>>    10 20 "10" 99 M(a,b) W+W+20+x
>
> AFAICS this is your interetation, which is mostly correct but
> in corner cases may produce wrong result and seem to be much
> less efficient then what standard says:
>
> : After all parameters in the replacement list have been substituted
> : and # and ## processing has taken place, all placemarker preprocessing
> : tokens are removed. The resulting preprocessing token sequence is
> : then rescanned, along with all subsequent preprocessing tokens of
> : the source file, for more macro names to replace.
>
> So, after macro expanded parameters are substituted and # and ##
> handling is done you simply prepend the result to source text
> following macro call and start reading from the start of resulting
> token list.  In other words there are _no_ repeated rescanning,
> rescanning is done just once and during rescanning you may have
> macro call which begins withing replacement list but ends in the
> source text.

I don't understand. If you insert the expanded macro result back into 
the source, and re-start normal tokenising from the start of that new 
sequence, isn't that effectively doing a re-scan anyway? And if a new 
macro is identified, then the process repeats.

But an advantage I can see if that if a new macro is encountered near 
the end of the replacement sequence, you don't have to re-scan all of 
the sequence up to that point after that macro is expanded, only the 
last bit.

Also, it takes care of the f(2)(9) example from elsewhere, as this is 
awkward when the f(2) expands to macro g, but the g and the () are in 
different placed.

> Note: most of the time repeated rescanning just wastes time.
> But sometimes it gives wrong result (see examples at the end).

> Executive summary: rules in the standard are carefully written
> to allow fast one pass implementation, but give freedom to
> the implementer to choose other, less efficient way.
>
> Few examples:
>
> #define f a d d (x)
> #define g a b (c)
> #define h(x, y) x y
> #define hash #
> #define i j j
> #define j i
>
> // This form used to check for modern comment handling
>
> // No rescanning when reading from source
> a b (c)
> a d (c)
> // Expanded macro argument is rescanned
> d(a b (c))
> d(a d (c))

I don't get these. Have you left out definitions for a and d?

-- 
Bartc

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Thread

Macro Expansion BartC <bc@freeuk.com> - 2017-01-07 18:35 +0000
  Re: Macro Expansion Thiago Adams <thiago.adams@gmail.com> - 2017-01-07 11:23 -0800
    Re: Macro Expansion Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-01-07 21:28 +0000
  Re: Macro Expansion Siri Cruise <chine.bleu@yahoo.com> - 2017-01-07 11:45 -0800
    Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-07 20:16 +0000
      Re: Macro Expansion Siri Cruise <chine.bleu@yahoo.com> - 2017-01-07 12:23 -0800
        Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-07 20:31 +0000
          Re: Macro Expansion Siri Cruise <chine.bleu@yahoo.com> - 2017-01-07 12:42 -0800
          Re: Macro Expansion Ian Collins <ian-news@hotmail.com> - 2017-01-08 14:42 +1300
            Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-08 13:18 +0000
  Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-07 20:15 +0000
    Re: Macro Expansion Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-01-07 20:42 +0000
      Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-07 22:50 +0000
        Re: Macro Expansion Ian Collins <ian-news@hotmail.com> - 2017-01-08 11:59 +1300
          Re: Macro Expansion Malcolm McLean <malcolm.mclean5@btinternet.com> - 2017-01-07 15:10 -0800
        Re: Macro Expansion supercat@casperkitty.com - 2017-01-07 17:24 -0800
          Re: Macro Expansion Tim Rentsch <txr@alumni.caltech.edu> - 2017-01-15 21:55 -0800
            Re: Macro Expansion supercat@casperkitty.com - 2017-01-16 09:06 -0800
              Re: Macro Expansion Tim Rentsch <txr@alumni.caltech.edu> - 2017-01-31 08:51 -0800
                Re: Macro Expansion supercat@casperkitty.com - 2017-01-31 13:06 -0800
                Re: Macro Expansion Tim Rentsch <txr@alumni.caltech.edu> - 2017-02-03 07:49 -0800
                Re: Macro Expansion supercat@casperkitty.com - 2017-02-03 12:03 -0800
                Re: Macro Expansion Tim Rentsch <txr@alumni.caltech.edu> - 2017-02-09 09:00 -0800
        Re: Macro Expansion Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-01-08 01:37 +0000
          Re: Macro Expansion Jean-Marc Bourguet <jm@bourguet.org> - 2017-01-08 10:22 +0100
  Re: Macro Expansion Thiago Adams <thiago.adams@gmail.com> - 2017-01-07 12:18 -0800
    Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-07 20:28 +0000
      Re: Macro Expansion Thiago Adams <thiago.adams@gmail.com> - 2017-01-07 12:36 -0800
        Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-07 22:59 +0000
  Re: Macro Expansion Thiago Adams <thiago.adams@gmail.com> - 2017-01-08 00:20 -0800
    Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-08 13:30 +0000
      Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-10 14:00 +0000
        Re: Macro Expansion Thiago Adams <thiago.adams@gmail.com> - 2017-01-10 07:43 -0800
          Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-10 17:47 +0000
            Re: Macro Expansion Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-01-10 22:49 +0000
              Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-10 23:28 +0000
              Re: Macro Expansion "James R. Kuyper" <jameskuyper@verizon.net> - 2017-01-10 18:51 -0500
                Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-11 00:06 +0000
          Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-10 20:30 +0000
            Re: Macro Expansion Thiago Adams <thiago.adams@gmail.com> - 2017-04-26 10:04 -0700
              Re: Macro Expansion Keith Thompson <kst-u@mib.org> - 2017-04-26 12:00 -0700
                Re: Macro Expansion Thiago Adams <thiago.adams@gmail.com> - 2017-04-26 12:24 -0700
              Re: Macro Expansion bartc <bc@freeuk.com> - 2017-04-26 20:12 +0100
                Re: Macro Expansion Thiago Adams <thiago.adams@gmail.com> - 2017-04-26 12:34 -0700
                Re: Macro Expansion jameskuyper@verizon.net - 2017-04-26 13:46 -0700
  Re: Macro Expansion Jean-Marc Bourguet <jm@bourguet.org> - 2017-01-08 10:13 +0100
    Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-08 13:10 +0000
      Re: Macro Expansion Jean-Marc Bourguet <jm@bourguet.org> - 2017-01-08 15:51 +0100
        Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-08 17:17 +0000
          Re: Macro Expansion supercat@casperkitty.com - 2017-01-08 11:04 -0800
            Re: Macro Expansion Malcolm McLean <malcolm.mclean5@btinternet.com> - 2017-01-08 11:10 -0800
              Re: Macro Expansion supercat@casperkitty.com - 2017-01-08 14:08 -0800
            Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-08 19:49 +0000
              Re: Macro Expansion supercat@casperkitty.com - 2017-01-08 14:17 -0800
          Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-08 19:19 +0000
          Re: Macro Expansion Jean-Marc Bourguet <jm@bourguet.org> - 2017-01-08 21:43 +0100
      Re: Macro Expansion supercat@casperkitty.com - 2017-01-08 10:43 -0800
      Re: Macro Expansion Keith Thompson <kst-u@mib.org> - 2017-01-08 12:39 -0800
  Re: Macro Expansion antispam@math.uni.wroc.pl - 2017-01-08 22:54 +0000
    Re: Macro Expansion "James R. Kuyper" <jameskuyper@verizon.net> - 2017-01-08 18:36 -0500
      Re: Macro Expansion antispam@math.uni.wroc.pl - 2017-01-09 01:15 +0000
        Re: Macro Expansion "James R. Kuyper" <jameskuyper@verizon.net> - 2017-01-08 20:49 -0500
    Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-08 23:53 +0000
      Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-09 01:11 +0000
      Re: Macro Expansion antispam@math.uni.wroc.pl - 2017-01-09 01:31 +0000
        Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-10 11:28 +0000

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