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From: crash@jc3b21.UUCP (Frank (Crash) Edwards)
Newsgroups: comp.unix.wizards
Subject: Re: have I found a bug in K&R?
Message-ID: <190@jc3b21.UUCP>
Date: Sun, 11-Oct-87 13:18:25 EDT
Article-I.D.: jc3b21.190
Posted: Sun Oct 11 13:18:25 1987
Date-Received: Wed, 14-Oct-87 05:37:57 EDT
References: <517@hubcap.UUCP> <321@laticorp.UUCP>
Organization: Transportation Systems Consulting, Palm Harbor, FL
Lines: 53
Keywords: argv definition
Summary: not necessarily...

In article <321@laticorp.UUCP>, sarah@laticorp.UUCP (Sarah Groves Hobart) writes:
> What is actually being declared is a pointer to an array of pointers. 
> Remember that the following declarations are equivalent:
> 
> char s[];
> 
> char *s;
> 
> Most people would tend to call the first declaration a character string, 
> and the second a character pointer, but they are the same thing.  So K&R
> are just using slightly different terminology, probably in the interests
> of clarity.
> 
> Sarah Groves Hobart
> {ihnp4,amdahl,sun}!ptsfa!laticorp!sarah

They are the same but they are not the same!  Using the strategy above,
try the following:

----------

foo.c:

char	s[] = "This is the character array.";

bar.c:

extern	char	*s;

----------

There is a subtle but startling difference between the two; primarily in the
storage allocated for each.  The declaration in "foo.c" defines storage for
the string and sets the array variable s to that address.  The extern decl-
aration in file "bar.c" indicates that s is an n-byte pointer to the string.
This means that to print the string, say, the compiler finds the address of
s and the code generated loads a pointer value from it, whereas for a true
array the compiler simply uses the value of s at compile time.

Perhaps this is obvious to most ;-) but it wasn't to me.  My particular
run-in with this type of coding was more devious because I declared an array
of character strings (similar to argv...) and then referenced them as "ptr
to ptr to char".

I should note that there won't be a problem if these constructs are used
within function calls since the array reference generates an address (for
case #1), and the compiler code loads the ptr from address s (for case #2).
And so either way a parameter declared as "char *s" would work.  This problem
only crops up when global declarations are used.

Anyway, I hope the above is intelligible.  If not, oh well.

Frank (Crash) Edwards