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Re: // comments and \

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From Keith Thompson <kst-u@mib.org>
Newsgroups comp.lang.c
Subject Re: // comments and \
Date Tue, 01 Dec 2015 11:07:58 -0800
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supercat@casperkitty.com writes:
> On Tuesday, December 1, 2015 at 11:15:51 AM UTC-6, Ben Bacarisse wrote:
>> By the way, handling them before anything else sounds unwise to me since
>> you'd interfere with trigraphs and the implementation-defined
>> translation to the source character set.  I think you mean they would be
>> done in a new translation phase between 1 and 2.  Even so, some existing
>> conforming code would be broken.
>
> Actually, I meant before phase 1.  A stand-alone utility which feeds each
> line through a simple state machine and echoes everything as it goes along
> unless it reaches a "strip remainder" state.
[...]

To be clear, you're suggesting handling // comments before phase 1.

Phase 1 is:
    Physical source file multibyte characters are mapped, in an
    implementation-defined manner, to the source character set
    (introducing new-line characters for end-of-line indicators)
    if necessary. Trigraph sequences are replaced by corresponding
    single-character internal representations.

Before phase 1, the input isn't even necessarily in the source character
set.  String literals and character constants aren't identified until
phase 3, so to handle:

    puts("// This is not a comment");

you'd have to recognize string literals and character constants in
phases 0 *and* 3.  Phase 0 would also have to recognize trigraphs.

Would your phase 0 handle both "//" and "/*...*/" comments?  It would at
least have to *recognize* "/*...*/" comments:

    /* This // is a valid comment */
    puts("This is not a comment");

Here's an alternative suggestion to solve the (IMHO minor) problem
of line-spliced "//" comments:

Currently, in phase 2 a backslash immediately followed by a newline
is deleted.

Proposal: Do the same thing for a backslash followed by whitespace
followed by a newline.

Proposal: Each backslash-whitespace-newline sequence, rather than
simply being deleted, is replaced by a special marker (which could be
implemented by the compiler remembering where the deletion occurred).

In phase 3, any occurrence of that marker within a // comment is
a syntax error.  The marker may appear in the middle of a token,
and is otherwise ignored in this and all following phases.

The changes in behavior from the current standard would be:

1. Invisible trailing whitespace doesn't affect line splicing
   (gcc and clang already behave this way).

2. Line splicing within a // comment is an unambiguous error.

It's likely such a change would affect some existing code -- but most
such code is likely incorrect anyway.  The most common case is probably
Windows-style file paths at the end of // comments.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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// comments and \ BartC <bc@freeuk.com> - 2015-12-01 12:52 +0000
  Re: // comments and \ me <self@example.org> - 2015-12-01 13:08 +0000
    Re: // comments and \ Eric Sosman <esosman@comcast-dot-net.invalid> - 2015-12-01 08:20 -0500
      Re: // comments and \ me <self@example.org> - 2015-12-01 13:36 +0000
        Re: // comments and \ Eric Sosman <esosman@comcast-dot-net.invalid> - 2015-12-01 09:03 -0500
        Re: // comments and \ Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-12-01 14:14 +0000
          Re: // comments and \ supercat@casperkitty.com - 2015-12-01 07:40 -0800
            Re: // comments and \ Keith Thompson <kst-u@mib.org> - 2015-12-01 08:55 -0800
            Re: // comments and \ Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-12-01 17:15 +0000
              Re: // comments and \ supercat@casperkitty.com - 2015-12-01 09:58 -0800
                Re: // comments and \ Richard Heathfield <rjh@cpax.org.uk> - 2015-12-01 18:11 +0000
                Re: // comments and \ supercat@casperkitty.com - 2015-12-01 10:18 -0800
                Re: // comments and \ Richard Heathfield <rjh@cpax.org.uk> - 2015-12-01 18:20 +0000
                Re: // comments and \ Keith Thompson <kst-u@mib.org> - 2015-12-01 11:07 -0800
                Re: // comments and \ supercat@casperkitty.com - 2015-12-01 11:39 -0800
                Re: // comments and \ Keith Thompson <kst-u@mib.org> - 2015-12-01 12:33 -0800
                Re: // comments and \ glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-12-02 01:05 +0000
                Re: // comments and \ Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-12-01 19:30 +0000
            Re: // comments and \ David Brown <david.brown@hesbynett.no> - 2015-12-01 20:59 +0100
              Re: // comments and \ James Kuyper <jameskuyper@verizon.net> - 2015-12-01 15:22 -0500
              Re: // comments and \ Richard Heathfield <rjh@cpax.org.uk> - 2015-12-01 20:37 +0000
                Re: // comments and \ Robert Wessel <robertwessel2@yahoo.com> - 2015-12-01 14:56 -0600
                Re: // comments and \ David Brown <david.brown@hesbynett.no> - 2015-12-01 22:11 +0100
                Re: // comments and \ Philip Lantz <prl@canterey.us> - 2015-12-02 03:01 -0800
            Re: // comments and \ glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-12-02 00:45 +0000
        Re: // comments and \ Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2015-12-01 07:52 -0700
          Re: // comments and \ Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-12-01 15:19 +0000
            Re: // comments and \ Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2015-12-01 08:29 -0700
              Re: // comments and \ Richard Heathfield <rjh@cpax.org.uk> - 2015-12-01 15:35 +0000
                Re: // comments and \ Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2015-12-01 09:21 -0700
                Re: // comments and \ Richard Heathfield <rjh@cpax.org.uk> - 2015-12-01 16:41 +0000
                Re: // comments and \ Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2015-12-01 10:15 -0700
                Re: // comments and \ Richard Heathfield <rjh@cpax.org.uk> - 2015-12-01 17:24 +0000
                Re: // comments and \ Richard Heathfield <rjh@cpax.org.uk> - 2015-12-01 17:43 +0000
                Re: // comments and \ Philip Lantz <prl@canterey.us> - 2015-12-02 03:05 -0800
                Re: // comments and \ Richard Heathfield <rjh@cpax.org.uk> - 2015-12-02 11:40 +0000
                Re: // comments and \ Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-12-02 04:29 -0800
                Re: // comments and \ BartC <bc@freeuk.com> - 2015-12-01 19:11 +0000
                Re: // comments and \ Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-12-01 20:43 +0000
                Re: // comments and \ BartC <bc@freeuk.com> - 2015-12-01 21:04 +0000
                Re: // comments and \ Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-12-01 21:16 +0000
                Re: // comments and \ Philip Lantz <prl@canterey.us> - 2015-12-02 03:12 -0800
                Re: // comments and \ BartC <bc@freeuk.com> - 2015-12-02 11:52 +0000
                Re: // comments and \ Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-12-02 04:40 -0800
                Re: // comments and \ David Brown <david.brown@hesbynett.no> - 2015-12-02 13:55 +0100
                Re: // comments and \ BartC <bc@freeuk.com> - 2015-12-02 13:56 +0000
                Re: // comments and \ David Brown <david.brown@hesbynett.no> - 2015-12-02 15:24 +0100
                Re: // comments and \ supercat@casperkitty.com - 2015-12-02 06:54 -0800
                Re: // comments and \ David Brown <david.brown@hesbynett.no> - 2015-12-02 16:40 +0100
                Re: // comments and \ Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-12-02 18:24 +0000
                Re: // comments and \ Öö Tiib <ootiib@hot.ee> - 2015-12-02 20:27 -0800
                Re: // comments and \ Keith Thompson <kst-u@mib.org> - 2015-12-02 23:22 -0800
                Re: // comments and \ Öö Tiib <ootiib@hot.ee> - 2015-12-03 00:57 -0800
                Re: // comments and \ Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-12-03 23:44 +0000
                Re: // comments and \ Öö Tiib <ootiib@hot.ee> - 2015-12-03 21:17 -0800
                Re: // comments and \ Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-12-05 00:43 +0000
                Re: // comments and \ Les Cargill <lcargill99@comcast.com> - 2015-12-04 20:32 -0600
                Re: // comments and \ Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-12-05 16:06 +0000
                Re: // comments and \ Les Cargill <lcargill99@comcast.com> - 2015-12-05 13:10 -0600
                Re: // comments and \ Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-12-06 10:18 +0000
                Re: // comments and \ Les Cargill <lcargill99@comcast.com> - 2015-12-06 14:26 -0600
                Re: // comments and \ David Brown <david.brown@hesbynett.no> - 2015-12-03 12:45 +0100
                Re: // comments and \ raltbos@xs4all.nl (Richard Bos) - 2015-12-04 11:45 +0000
                Re: // comments and \ Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-12-04 04:20 -0800
                Re: // comments and \ David Brown <david.brown@hesbynett.no> - 2015-12-04 14:20 +0100
                Re: // comments and \ James Kuyper <jameskuyper@verizon.net> - 2015-12-04 10:05 -0500
                Re: // comments and \ David Brown <david.brown@hesbynett.no> - 2015-12-04 17:31 +0100
                Re: // comments and \ Les Cargill <lcargill99@comcast.com> - 2015-12-04 17:24 -0600
                Re: // comments and \ James Kuyper <jameskuyper@verizon.net> - 2015-12-04 18:50 -0500
                Re: // comments and \ Les Cargill <lcargill99@comcast.com> - 2015-12-04 20:34 -0600
                Re: // comments and \ raltbos@xs4all.nl (Richard Bos) - 2015-12-10 20:40 +0000
                Re: // comments and \ Keith Thompson <kst-u@mib.org> - 2015-12-04 09:36 -0800
                Re: // comments and \ Richard Damon <Richard@Damon-Family.org> - 2015-12-05 09:05 -0500
                Re: // comments and \ Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-12-05 17:42 +0000
                Re: // comments and \ David Brown <david.brown@hesbynett.no> - 2015-12-06 22:23 +0100
                Re: // comments and \ Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-12-07 21:59 +0000
                Re: // comments and \ Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-12-05 15:50 +0000
                Re: // comments and \ David Brown <david.brown@hesbynett.no> - 2015-12-05 18:17 +0100
                Re: // comments and \ Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-12-06 10:01 +0000
                Re: // comments and \ Öö Tiib <ootiib@hot.ee> - 2015-12-06 03:05 -0800
                Re: // comments and \ Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-12-06 04:07 -0800
                Re: // comments and \ Öö Tiib <ootiib@hot.ee> - 2015-12-06 04:37 -0800
                Re: // comments and \ Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-12-06 05:05 -0800
                Re: // comments and \ Keith Thompson <kst-u@mib.org> - 2015-12-06 12:19 -0800
                Re: // comments and \ Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-12-06 13:59 +0000
                Re: // comments and \ Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-12-06 06:09 -0800
                Re: // comments and \ Öö Tiib <ootiib@hot.ee> - 2015-12-06 07:30 -0800
                Re: // comments and \ Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-12-06 18:55 +0000
                Re: // comments and \ Keith Thompson <kst-u@mib.org> - 2015-12-06 12:23 -0800
                Re: // comments and \ Keith Thompson <kst-u@mib.org> - 2015-12-06 12:22 -0800
                Re: // comments and \ Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-12-06 21:06 +0000
                Re: // comments and \ Les Cargill <lcargill99@comcast.com> - 2015-12-05 13:14 -0600
                Re: // comments and \ Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-12-06 10:09 +0000
                Re: // comments and \ mark.bluemel@gmail.com - 2015-12-03 06:15 -0800
                Re: // comments and \ Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2015-12-02 08:28 -0700
                Re: // comments and \ Keith Thompson <kst-u@mib.org> - 2015-12-02 09:01 -0800
                Re: // comments and \ Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-12-02 21:52 +0000
                Re: // comments and \ Keith Thompson <kst-u@mib.org> - 2015-12-02 16:55 -0800
                Re: // comments and \ David Brown <david.brown@hesbynett.no> - 2015-12-03 13:11 +0100
                Re: // comments and \ Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-12-03 23:21 +0000
                Re: // comments and \ David Brown <david.brown@hesbynett.no> - 2015-12-04 11:22 +0100
                Re: // comments and \ Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-12-05 01:12 +0000
                Re: // comments and \ Les Cargill <lcargill99@comcast.com> - 2015-12-04 20:39 -0600
                Re: // comments and \ raltbos@xs4all.nl (Richard Bos) - 2015-12-04 11:14 +0000
                Re: // comments and \ Richard Heathfield <rjh@cpax.org.uk> - 2015-12-04 12:25 +0000
                Re: // comments and \ David Brown <david.brown@hesbynett.no> - 2015-12-04 14:23 +0100
                Re: // comments and \ Keith Thompson <kst-u@mib.org> - 2015-12-01 09:00 -0800
                Re: // comments and \ Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-12-01 19:46 +0000
                Re: // comments and \ Richard Heathfield <rjh@cpax.org.uk> - 2015-12-01 20:36 +0000
                Re: // comments and \ Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2015-12-01 20:37 -0700
  Re: // comments and \ BartC <bc@freeuk.com> - 2015-12-01 14:26 +0000
    Re: // comments and \ Richard Heathfield <rjh@cpax.org.uk> - 2015-12-01 14:32 +0000
    Re: // comments and \ BartC <bc@freeuk.com> - 2015-12-01 14:52 +0000
      Re: // comments and \ David Brown <david.brown@hesbynett.no> - 2015-12-01 16:15 +0100
      Re: // comments and \ Richard Heathfield <rjh@cpax.org.uk> - 2015-12-01 15:28 +0000
        Re: // comments and \ glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-12-02 01:26 +0000
      Re: // comments and \ Keith Thompson <kst-u@mib.org> - 2015-12-01 09:12 -0800
        Re: // comments and \ Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-12-01 19:17 +0000
      Re: // comments and \ Robert Wessel <robertwessel2@yahoo.com> - 2015-12-01 12:32 -0600
    Re: // comments and \ Nobody <nobody@nowhere.invalid> - 2015-12-02 00:01 +0000
      Re: // comments and \ supercat@casperkitty.com - 2015-12-02 07:08 -0800
        Re: // comments and \ Nobody <nobody@nowhere.invalid> - 2015-12-02 21:05 +0000
          Re: // comments and \ Keith Thompson <kst-u@mib.org> - 2015-12-02 14:22 -0800
          Re: // comments and \ supercat@casperkitty.com - 2015-12-02 14:48 -0800
            Re: // comments and \ glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-12-02 23:37 +0000
              Re: // comments and \ supercat@casperkitty.com - 2015-12-02 17:10 -0800
                Re: // comments and \ Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-12-02 19:24 -0800
                Re: // comments and \ Richard Heathfield <rjh@cpax.org.uk> - 2015-12-03 04:10 +0000
                Re: // comments and \ Philip Lantz <prl@canterey.us> - 2015-12-02 21:49 -0800
                Re: // comments and \ Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2015-12-03 07:28 -0700
                Re: // comments and \ supercat@casperkitty.com - 2015-12-03 09:12 -0800
                Re: // comments and \ supercat@casperkitty.com - 2015-12-03 06:53 -0800
                Re: // comments and \ supercat@casperkitty.com - 2015-12-03 15:41 -0800
                Re: // comments and \ Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-12-03 15:56 -0800
                Re: // comments and \ supercat@casperkitty.com - 2015-12-03 16:44 -0800
                Re: // comments and \ Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-12-03 17:29 -0800
                Re: // comments and \ supercat@casperkitty.com - 2015-12-03 19:10 -0800
        Re: // comments and \ Philip Lantz <prl@canterey.us> - 2015-12-02 21:59 -0800
  Re: // comments and \ Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2015-12-01 07:52 -0700
  Re: // comments and \ Keith Thompson <kst-u@mib.org> - 2015-12-01 09:21 -0800
    Re: // comments and \ Richard Heathfield <rjh@cpax.org.uk> - 2015-12-01 17:26 +0000
      Re: // comments and \ Keith Thompson <kst-u@mib.org> - 2015-12-01 10:50 -0800
    Re: // comments and \ Keith Thompson <kst-u@mib.org> - 2015-12-01 10:49 -0800
      Re: // comments and \ Geoff <geoff@invalid.invalid> - 2015-12-01 11:34 -0800
    Re: // comments and \ David Thompson <dave.thompson2@verizon.net> - 2015-12-22 06:44 -0500
      Re: // comments and \ "Charles Richmond" <numerist@aquaporin4.com> - 2015-12-23 14:35 -0600
        Re: // comments and \ Keith Thompson <kst-u@mib.org> - 2015-12-23 13:00 -0800
          Re: // comments and \ James Kuyper <jameskuyper@verizon.net> - 2015-12-23 16:58 -0500
            Re: // comments and \ Keith Thompson <kst-u@mib.org> - 2015-12-23 14:21 -0800
        Re: // comments and \ James Kuyper <jameskuyper@verizon.net> - 2015-12-23 16:55 -0500
  Re: // comments and \ Öö Tiib <ootiib@hot.ee> - 2015-12-01 12:24 -0800

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