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From: pds@lemming.webo.dg.com (Paul D. Smith)
Newsgroups: comp.lang.c
Subject: Re: &&**
Message-ID: <941@dg.dg.com>
Date: 12 Sep 90 14:19:18 GMT
References: <0926@sheol.UUCP> <20757:Sep1115:30:0390@kramden.acf.nyu.edu> <853.26ed0446@iccgcc.decnet.ab.com> <23314:Sep1122:28:1390@kramden.acf.nyu.edu>
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In some article, brnstnd@kramden.acf.nyu.edu (Dan Bernstein) writes:

[]> > Question for the standards people: Is && undefined, unspecified,
[]> > implementation-defined, or what have you? 
[]> See page 49 of K&R 1, or page 53 of K&R 2.
[]
[]K&R1 is obviously an incorrect answer, as I was wondering exactly what
[]level of error-ness &(&(...)) has under ANSI. I don't have K&R2 (does it
[]really indicate specific error levels?) or the standard, which is why I
[]was asking in the first place.
[]
[...]
[]
[]---Dan

Well, I have K&R II, and I didn't see anything on p. 53 which related
to this question ??

However, I did look up some stuff in the reference (Appendix A), which
is where all quotes from K&R II about the ANSI standard should come
from, anyhoo ...  (emphasis added by me)

  Sec. A7.4.2 "Address Operator" (p. 203)

    ... --> The operand must be an lvalue <-- referring neither to a
    bit-field nor to an object declared as register, or must be of
    function type.

  Sec. A7.4.3 "Indirection Operator" (p. 204)

    ... The unary * operator denotes indirection ... --> It is an lvalue
    if the operand is a pointer to an object of arithmetic, structure,
    union, or pointer type <--

So, what does this mean?  This means that

    struct foo *ptr, *ptr2;

    ptr2 = &*ptr;

is definitely legal.  Unfortunately, Sec A7.4.2 does not explicitly
state whether or not "&foo" is an lvalue, although all common sense
would indicate that it is not (can you write "&foo = barp"?  I think
not!)  However, "*" _can_ be applied to something which is not an
lvalue (eg. the name of an array!), and it generates an lvalue.

So,

    ptr = &&&&****ptr2;

is *not* legal (which is fairly intuitive), however it seems that

    ptr = &*&*&*&*ptr2;

_is_ legal.  Whatcha think?
                                                                paul
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| Paul D. Smith                          | pds@lemming.webo.dg.com |
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