From torek@elf.ee.lbl.gov Sun Sep 29 00:12:51 1991
From: torek@elf.ee.lbl.gov (Chris Torek)
Newsgroups: comp.lang.c,comp.unix.questions
Subject: Re: cc error output
Date: 28 Sep 91 20:36:53 GMT
Reply-To: torek@elf.ee.lbl.gov (Chris Torek)
Organization: Lawrence Berkeley Laboratory, Berkeley
X-Local-Date: Sat, 28 Sep 91 13:36:53 PDT

In article <1991Sep27.150314.6422@hellgate.utah.edu>
jwindley@cs.utah.edu (Jay Windley) writes:

>If you mean "bad lhs," then it is indicating that the left side of an
>assignment statement is improper,

This is correct ...

>such as the following:
>
>int x[X_SIZE];
>int y;
>x = &y;

and this is indeed improper ...

>x indeed has the correct type -- "pointer to int," but its value is constant
>and cannot be reassigned.

but x is *not* constant, and x does *not* have type `pointer to int'.
x has type `array X_SIZE of int'.  Pick up almost any C book on the
shelf of your local bookstore, turn to the chapter on `pointers and
arrays', and you will probably find a flat statement---not just a hint
or a `temporary concept' to be cleared up later---claiming that the
name of an array is a pointer to the first element of the array.  This
statement is false; all of these books are wrong.  The name of the
array is not a pointer; the name of the array *becomes* a pointer *in
a value context*.  This is easily demonstrated:

	printf("sizeof array = %ld, sizeof pointer = %ld\n",
	    (long)sizeof(x), (long)sizeof(int *));

As X_SIZE varies, the first number also varies, while the second number
remains constant.  If x were `pointer to int', it would have the same
size as `int *'; it does not, so it is not.

Note that the left hand side of an assigment operator is not a value
context---it is an object context---so the array `x' remains an array.
It is illegal to have an array object on the left hand side of an
assignment operator.

The next time you are in a bookstore, look at the C books and count the
number that get this right.  There will be very few, or even none.
Chances are that the authors *believe* that an array is a pointer, and
that sizeof and multi-dimensional arrays are special cases.  This is
backwards: *values* are special cases, and arrays are otherwise
arrays.  (Function parameters are also special cases.  Both special
cases occur very often, but they are still special.)
-- 
In-Real-Life: Chris Torek, Lawrence Berkeley Lab CSE/EE (+1 510 486 5427)
Berkeley, CA		Domain:	torek@ee.lbl.gov