Groups > comp.lang.c > #112054

Hexadecimal float constants

From	Steve Keller <keller@no.invalid>
Newsgroups	comp.lang.c
Subject	Hexadecimal float constants
Date	2017-06-12 18:34 +0200
Organization	Aioe.org NNTP Server
Message-ID	<ohmfqn$q4l$1@gioia.aioe.org> (permalink)

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Hm, until a few days ago, I thought I know the C standard quite well.
Now, there are a number of surprises, like the exact meaning of
tentative definitions, scope of variables, and now hexadecimal float
constants.

I read in the thread "High quality resources for learning modern C?"
that 0x1234e-1 is an illegal preprocessing token, and in fact, gcc and
clang refuse it:

    xf.c:3:9: error: invalid suffix "-1" on integer constant
     int x = 0x1234e-1;

Searching the standard, I found the reason for this surprising
behavior.  The preprocessor doesn't scan for the C constants, which
consist of integer-constant, floating-constant, enumeration-constant,
and character-constant, but instead it scans for pp-number, which
allows "0x1234e-1" as a preprossing token, which then becomes a token
that is not a legal C token.

But what is the reason for this definition?  Why does the preprocessor
not simply look for the C tokens (plus header-names for #include),
which would produce the (expected) token sequence "0x1234e", "-" , "1"?

Steve

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Hexadecimal float constants Steve Keller <keller@no.invalid> - 2017-06-12 18:34 +0200
  Re: Hexadecimal float constants "James R. Kuyper" <jameskuyper@verizon.net> - 2017-06-12 12:44 -0400
    Re: Hexadecimal float constants supercat@casperkitty.com - 2017-06-12 10:54 -0700
      Re: Hexadecimal float constants Keith Thompson <kst-u@mib.org> - 2017-06-12 11:26 -0700
        Re: Hexadecimal float constants bartc <bc@freeuk.com> - 2017-06-12 19:38 +0100
          Re: Hexadecimal float constants scott@slp53.sl.home (Scott Lurndal) - 2017-06-12 18:45 +0000
          Re: Hexadecimal float constants Robert Wessel <robertwessel2@yahoo.com> - 2017-06-12 13:54 -0500
            Re: Hexadecimal float constants bartc <bc@freeuk.com> - 2017-06-12 20:40 +0100
              Re: Hexadecimal float constants Keith Thompson <kst-u@mib.org> - 2017-06-12 13:06 -0700
        Re: Hexadecimal float constants supercat@casperkitty.com - 2017-06-12 13:37 -0700
          Re: Hexadecimal float constants bartc <bc@freeuk.com> - 2017-06-12 22:22 +0100
          Re: Hexadecimal float constants Keith Thompson <kst-u@mib.org> - 2017-06-12 16:01 -0700
            Re: Hexadecimal float constants supercat@casperkitty.com - 2017-06-12 16:36 -0700
      Re: Hexadecimal float constants jameskuyper@verizon.net - 2017-06-12 12:39 -0700
  Re: Hexadecimal float constants bartc <bc@freeuk.com> - 2017-06-12 18:42 +0100
  Re: Hexadecimal float constants Thomas David Rivers <rivers@dignus.com> - 2017-06-12 17:27 -0400
    Re: Hexadecimal float constants "James R. Kuyper" <jameskuyper@verizon.net> - 2017-06-12 18:03 -0400
      Re: Hexadecimal float constants bartc <bc@freeuk.com> - 2017-06-12 23:31 +0100
        Re: Hexadecimal float constants Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-06-13 00:10 +0100
          Re: Hexadecimal float constants jacobnavia <jacob@jacob.remcomp.fr> - 2017-06-13 01:33 +0200
            Re: Hexadecimal float constants Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-06-13 01:10 +0100
      Re: Hexadecimal float constants supercat@casperkitty.com - 2017-06-12 16:01 -0700
        Re: Hexadecimal float constants Keith Thompson <kst-u@mib.org> - 2017-06-12 17:00 -0700
          Re: Hexadecimal float constants supercat@casperkitty.com - 2017-06-13 07:51 -0700
            Re: Hexadecimal float constants Keith Thompson <kst-u@mib.org> - 2017-06-13 09:17 -0700
              Re: Hexadecimal float constants bartc <bc@freeuk.com> - 2017-06-13 17:45 +0100
                Re: Hexadecimal float constants Keith Thompson <kst-u@mib.org> - 2017-06-13 11:58 -0700
                Re: Hexadecimal float constants supercat@casperkitty.com - 2017-06-13 12:57 -0700
                Re: Hexadecimal float constants Keith Thompson <kst-u@mib.org> - 2017-06-13 13:01 -0700
                Re: Hexadecimal float constants bartc <bc@freeuk.com> - 2017-06-13 22:11 +0100
                Re: Hexadecimal float constants Keith Thompson <kst-u@mib.org> - 2017-06-13 14:22 -0700
                Re: Hexadecimal float constants "James R. Kuyper" <jameskuyper@verizon.net> - 2017-06-13 17:43 -0400
                Re: Hexadecimal float constants Keith Thompson <kst-u@mib.org> - 2017-06-13 14:48 -0700
                Re: Hexadecimal float constants bartc <bc@freeuk.com> - 2017-06-13 23:03 +0100
                Re: Hexadecimal float constants GOTHIER Nathan <nathan.gothier@gmail.com> - 2017-06-14 01:30 +0200
              Re: Hexadecimal float constants supercat@casperkitty.com - 2017-06-13 12:24 -0700
                Re: Hexadecimal float constants jacobnavia <jacob@jacob.remcomp.fr> - 2017-06-13 21:52 +0200
                Re: Hexadecimal float constants Keith Thompson <kst-u@mib.org> - 2017-06-13 13:12 -0700
                Re: Hexadecimal float constants supercat@casperkitty.com - 2017-06-13 16:18 -0700
                Re: Hexadecimal float constants Ike Naar <ike@iceland.freeshell.org> - 2017-06-14 21:28 +0000
    Re: Hexadecimal float constants supercat@casperkitty.com - 2017-06-12 15:21 -0700
    Re: Hexadecimal float constants Thomas David Rivers <rivers@dignus.com> - 2017-06-13 12:13 -0400

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