Re: Untyped Variables

From	Keith Thompson <kst-u@mib.org>
Newsgroups	comp.lang.c
Subject	Re: Untyped Variables
Date	2015-08-10 08:29 -0700
Organization	None to speak of
Message-ID	<lnegjbduvo.fsf@kst-u.example.com> (permalink)
References	<83a58dd3-53a6-422a-8502-4fef870baf70@googlegroups.com> <ln37ztfte6.fsf@kst-u.example.com> <8eb33dc8-5c47-4b7b-a20e-584166a50d58@googlegroups.com> <55C69FFA.7050309@verizon.net> <mqaadk$2hpv$1@macpro.inf.ed.ac.uk>

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richard@cogsci.ed.ac.uk (Richard Tobin) writes:
> In article <55C69FFA.7050309@verizon.net>,
> James Kuyper  <jameskuyper@verizon.net> wrote:
>
>>On 08/08/2015 07:06 PM, David Kleinecke wrote:
>>...
>>> I haven't been thinking of constraints as part of syntax. ...
>
>>It would be more accurate to say that some constraints constrain the
>>syntax.
>
> It would make sense that a program must conform to both the grammar
> and the constraints.
>
> But in the case of implicit int in C90, the "constraint" in 6.5.2
> explicitly allows something that is not allowed by the grammar.

Interesting, so it does.

For those who don't have a copy of C90, 6.5.2 covers type
specifiers.  A type specifier is one of void, char, short,
int, etc.  A declaration starts with a sequence of one or more
declaration specifiers, each of which is a storage-class specifier,
a type-specifier, or a type-qualifier.  The constraint in 6.5.2
restricts the list of type specifiers to one of a list of sets
(changed to "multisets" in C99 with the addition of long long,
though the wording wasn't updated until C11); for example, you
can't have "int int" or "short long" in a declaration.

One of the permitted sets of type specifiers is:

    -- int, signed, signed int, or no type specifiers

So this:

    x;

does not satisfy the grammar, but it is permitted by the constraint.
("auto x;" or "static x;" is permitted by the grammar.)

(Note that "x;" is ambiguous at block scope, where it could be an
expression statement; it's unambiguous at file scope where statements
are not permitted.  I think C90 has wording to resolve the ambiguity,
but I don't know exactly where it does so.)

This is yet another reason to be glad that the implicit int rule
is no longer with us.

-- 
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"

Thread

Untyped Variables David Kleinecke <dkleinecke@gmail.com> - 2015-08-07 18:19 -0700
  Re: Untyped Variables Lőrinczy Zsigmond <nospam@for.me> - 2015-08-08 06:53 +0200
  Re: Untyped Variables James Kuyper <jameskuyper@verizon.net> - 2015-08-08 11:09 -0400
    Re: Untyped Variables Keith Thompson <kst-u@mib.org> - 2015-08-08 12:09 -0700
      Re: Untyped Variables James Kuyper <jameskuyper@verizon.net> - 2015-08-08 14:24 -0400
    Re: Untyped Variables Keith Thompson <kst-u@mib.org> - 2015-08-08 12:54 -0700
      Re: Untyped Variables David Kleinecke <dkleinecke@gmail.com> - 2015-08-08 16:06 -0700
        Re: Untyped Variables Keith Thompson <kst-u@mib.org> - 2015-08-08 17:29 -0700
          Re: Untyped Variables David Kleinecke <dkleinecke@gmail.com> - 2015-08-09 17:52 -0700
            Re: Untyped Variables Ian Collins <ian-news@hotmail.com> - 2015-08-10 12:57 +1200
            Re: Untyped Variables Keith Thompson <kst-u@mib.org> - 2015-08-10 08:41 -0700
              Re: Untyped Variables David Kleinecke <dkleinecke@gmail.com> - 2015-08-10 17:06 -0700
        Re: Untyped Variables Richard Damon <Richard@Damon-Family.org> - 2015-08-08 19:44 -0400
        Re: Untyped Variables James Kuyper <jameskuyper@verizon.net> - 2015-08-08 20:34 -0400
          Re: Untyped Variables glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-08-09 02:03 +0000
            Re: Untyped Variables Richard Damon <Richard@Damon-Family.org> - 2015-08-08 22:52 -0400
              Re: Untyped Variables glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-08-09 04:57 +0000
                Re: Untyped Variables Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2015-08-09 09:58 -0600
                Re: Untyped Variables Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-08-10 03:54 -0700
                Re: Untyped Variables Richard Damon <Richard@Damon-Family.org> - 2015-08-09 16:59 -0400
          Re: Untyped Variables David Kleinecke <dkleinecke@gmail.com> - 2015-08-09 18:02 -0700
            Re: Untyped Variables James Kuyper <jameskuyper@verizon.net> - 2015-08-10 11:01 -0400
              Re: Untyped Variables James Kuyper <jameskuyper@verizon.net> - 2015-08-10 13:20 -0400
              Re: Untyped Variables David Kleinecke <dkleinecke@gmail.com> - 2015-08-10 16:54 -0700
                Re: Untyped Variables James Kuyper <jameskuyper@verizon.net> - 2015-08-11 00:20 -0400
            Re: Untyped Variables Keith Thompson <kst-u@mib.org> - 2015-08-10 08:11 -0700
              Re: Untyped Variables David Kleinecke <dkleinecke@gmail.com> - 2015-08-10 17:15 -0700
                Re: Untyped Variables James Kuyper <jameskuyper@verizon.net> - 2015-08-11 00:30 -0400
                Re: Untyped Variables David Kleinecke <dkleinecke@gmail.com> - 2015-08-11 16:50 -0700
                Re: Untyped Variables Keith Thompson <kst-u@mib.org> - 2015-08-11 17:24 -0700
                Re: Untyped Variables Richard Heathfield <rjh@cpax.org.uk> - 2015-08-12 01:37 +0100
                Re: Untyped Variables glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-08-12 04:46 +0000
                Re: Untyped Variables David Kleinecke <dkleinecke@gmail.com> - 2015-08-12 18:12 -0700
                Re: Untyped Variables Keith Thompson <kst-u@mib.org> - 2015-08-13 12:08 -0700
                Re: Untyped Variables David Kleinecke <dkleinecke@gmail.com> - 2015-08-13 16:25 -0700
                Re: Untyped Variables Keith Thompson <kst-u@mib.org> - 2015-08-14 10:36 -0700
                Re: Untyped Variables Bartc <bc@freeuk.com> - 2015-08-14 19:13 +0100
                Re: Untyped Variables Keith Thompson <kst-u@mib.org> - 2015-08-11 07:58 -0700
          Re: Untyped Variables richard@cogsci.ed.ac.uk (Richard Tobin) - 2015-08-10 13:53 +0000
            Re: Untyped Variables Keith Thompson <kst-u@mib.org> - 2015-08-10 08:29 -0700
              Re: Untyped Variables David Kleinecke <dkleinecke@gmail.com> - 2015-08-10 17:01 -0700
                Re: Untyped Variables Keith Thompson <kst-u@mib.org> - 2015-08-10 17:42 -0700
                Re: Untyped Variables glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-08-11 02:56 +0000
            Re: Untyped Variables David Kleinecke <dkleinecke@gmail.com> - 2015-08-10 16:50 -0700
              Re: Untyped Variables Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-08-11 01:39 +0100
                Re: Untyped Variables Keith Thompson <kst-u@mib.org> - 2015-08-10 19:03 -0700
                Re: Untyped Variables Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-08-11 11:21 +0100
                Re: Untyped Variables David Kleinecke <dkleinecke@gmail.com> - 2015-08-11 16:37 -0700

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