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From: nextuid@xyzzy.UUCP
Newsgroups: comp.lang.c
Subject: Re: offsetof (was Arithmetic on NULL)
Message-ID: <355@xyzzy.UUCP>
Date: Fri, 30-Oct-87 10:20:09 EST
Article-I.D.: xyzzy.355
Posted: Fri Oct 30 10:20:09 1987
Date-Received: Sun, 1-Nov-87 06:41:12 EST
References: <1459@cullvax.UUCP> <48400001@tub.UUCP> <1151@haddock.ISC.COM> <2500@mmintl.UUCP> <907@sugar.UUCP>
Reply-To: meissner@xyzzy.UUCP (Michael Meissner)
Organization: Data General (Languages @ Research Triangle Park, NC.)
Lines: 39

In article <907@sugar.UUCP> peter@sugar.UUCP (Peter da Silva) writes:
| In article <2500@mmintl.UUCP>, franka@mmintl.UUCP (Frank Adams) writes:
| > In article <1076@jenny.cl.cam.ac.uk> am@cl.cam.ac.uk (Alan Mycroft) writes:
| > |I have a concern about this.  ANSI (Oct 86) specifically disallows
| > |unary-& from appearing in constant expressions (no doubt
| > |except in sizeof() context)....
| 
| Does this mean the following use of & in &STD_Gadget[n], which is being
| used in a context that requires a constant expression, is not ANSI standard?

To try and quell the followups, here is the wording from the August 3 draft
(page 49) pertaining to constant expressions.

      ....

          The operands in an integral constant expression shall consist
      only of integer, enumeration, and character constants, sizeof
      expressions, and casts of aritmetic types to integral types.  The
      array-subscript [] and member-acess . and -> operators, the
      address & and indirection * unary operators, and arbitrary casts
      may appear only in the operand of a sizeof operator[36].  The
      value of an integral constant expression shall be representable in
      the storage appropriate for its type in the execution environment.

          More latitude is permitted for constant expressions in
      initializers.  Floating constants and arbitrary casts may also be
      used.  Lvalues designating objects that have static storage
      duration or function designators may also be used to specify
      addresses, explicitly with the unary & operator, or implicitly,
      for expressions of array or function type.

      ....

      [36] The operand of a sizeof operator is not evaluated.


Seems clear enough to me.  At the last meeting, it was voted to restore
the constraint that accidently got deleted stating that when you use an
address in an initializer, you can only add or subtract and integral
constant to it  (ie, you can't do:  int i = ((int) &j) * 50;).