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From: cameron@elecvax.eecs.unsw.oz (Cameron Simpson)
Newsgroups: comp.unix.wizards
Subject: Re: have I found a bug in K&R?
Message-ID: <3802@elecvax.eecs.unsw.oz>
Date: Fri, 16-Oct-87 05:12:51 EDT
Article-I.D.: elecvax.3802
Posted: Fri Oct 16 05:12:51 1987
Date-Received: Sun, 18-Oct-87 09:51:34 EDT
References: <18668@amdcad.AMD.COM> <517@hubcap.UUCP> <321@laticorp.UUCP> <7608@steinmetz.steinmetz.UUCP>
Organization: EE and CS, Uni of NSW, Sydney, Australia
Lines: 35
Summary: char s[], *t; /* t is an lvalue, s is not */

In article <18668@amdcad.AMD.COM> tim@amdcad.AMD.COM (Tim Olson) writes:
>In article <7608@steinmetz.steinmetz.UUCP> dawn!stpeters@steinmetz.UUCP (Dick St.Peters) writes:
>| In article <321@laticorp.UUCP> sarah@laticorp.UUCP (Sarah Groves Hobart) writes:
>| >Remember that the following declarations are equivalent:
>| >char s[];
>| >char *s;
>| Actually, they're not. [ explaination deleted ]
>Actually, you are both right.  There are two places where a declaration
>of an array with no size (and no initializer) are legal: 1) as a formal
>parameter, and 2) as an external array declaration, where the array is
>defined elsewhere.
>As a formal parameter, char s[] and char *s are equivalent -- the
>compiler coerces the former into the later.  In that case you *can*
>change s.  As an external array declartion, the two are not equivalent.
>	-- Tim Olson
>	Advanced Micro Devices
>	(tim@amdcad.amd.com)

I've been following this for a while. In all this discussion I have not
once seen a word which crops up several times in K&R, namely `lvalue'.
An lvalue is something which may occur on the left-hand side of an
assignment statement. If I say
	char s[];	/* a character array of unspecified size */
	char *t;	/* a pointer to a character */
then I may say `t++;' or `t = s;' or `t = &s[2];' and so on.
I may not say `s++;' or `s = t;'.
This applies whether they are declared as formal parameters or as
external array declarations. I often declare functions like
	fn(a,b)
	char *a;
	char b[];
in order to *prevent* myself from changing b during the function.
It eliminates some coding errors by making the compiler do some
extra checking for me.
	- Cameron Simpson, Uni of NSW, Australia