C declarations (with examples)

[Ray Lubinsky]

>    I have a question about C declarations.  The [] notation is equivalent
> to the * notation, right?

   Well, not exactly.  To declare something as ptr[] is to say that you want
an array of objects of the type that you specify and that the identifier 'ptr'
is to point to the zeroth element.  Declaring *ptr only reserved 'ptr' to mean
a pointer to that type.  For example here are the errors I got on two test
programs:

% cat > test1.c <<EOF			|	% cat > test2.c <<EOF
main() {				|	char	x[];
	char	y[];			|	main()
	y = "abc";			|	{
}					|	}
EOF					|	EOF
% cc test1.c				|	% cc test2.c
"test1.c", line 3: illegal lhs of	|	Undefined:
assignment operator			|	_x

   In test1, the compiler tells us that you can't change the value of the
identifier which indicates the start of an array.  No matter that they array
has no elements -- it just won't permit it.  Otherwise, a programmer could
lose track of his array.  In test2, the compiler assumes that the (evidently)
null array 'x' must be declared in some other load module; when it's not found,
the loader complains.

   When you declare an array, you must define how much storage you want to
allocate for it.  There are three possiblities:

int	x[2];			/* x points to a block of 2 elements */
int	y[] = { 0 , 1 };	/* y has implicitly 2 elements */
extern	int	z[];		/* z has dimensions declared elsewhere */

   If you have a pointer to integer call 'ptr', it can be assigned to point to
an element of any of these arrays.  The statement  ptr = z;  just points 'ptr'
to the zeroth element of array z.

>   Is this a desirable characteristic of C?

   What can I say?  C will let you do all sorts of crazy things that you had no
intention of doing (like accessing the 11th element of a ten-element array) but
it won't let you risk losing all references to a block of allocated memory.
Seems like a good idea to me.

------------------------------------------------------------------------------

Ray Lubinsky		     University of Virginia, Dept. of Computer Science
			     uucp: decvax!mcnc!ncsu!uvacs!rwl