C Coding Question

[Chris Torek]

In article <138 at darth.UUCP> gary at darth.UUCP (Gary Wisniewski) writes:
>As far as your question about "char *help[]" and "char **help": the two
>forms are IDENTICAL to virtually every C compiler (that's worth its
>salt).  Arrays in C are merely special cases of pointers.  In other
>words, both forms are correct.

	NO!

Ai!  This has been asserted far too often.  Arrays and pointers are
not at all the same thing in C!

>Section 5.3 of K&R explain this more fully.

Indeed it does, and I suggest you read it rather more carefully.

  The correspondence between indexing and pointer arithmetic is
  evidently very close. ... The effect is that an array name *is*
  a pointer expression.  (p. 94)

This does not say that arrays and pointers are *the same*.

  There is one difference between an array name and a pointer
  that must be kept in mind.

Aha!  See p. 94 for that difference.

  As formal parameters in a function defintion,

	char s[];

  and

	char *s;

  are exactly equivalent.... (p. 95)

Here they *are* the same---but note the qualifier: `As formal
parameters'.  In the (unquoted) original example, the array was a
global variable.

There is one other thing which, I guess, adds to this confusion.
Both of the following are legal global declarations in C:

	char	msg0[] = "Hello, world";
	char	*msg1 = "Hello, world";

Given both declarations,

	printf("%s\n", msg0);

and

	printf("%s\n", msg1);

produce the same output.  Yet msg0 and msg1 are not the same:

	printf("%d %d\n", sizeof (msg0), sizeof (msg1));

prints

	13 4

on a Vax; for msg0 is an array, and msg1 is a pointer.  The code
generated for the two declarations is different:

	/* edited assembly output from ccom */
		.data			# Switch to data segment.
		.globl	_msg0		# The array ...
	_msg0:	.asciz	"Hello, world"	# and here it is.

		.data	2		# Switch to alternate data segment.
	L12:	.asciz	"Hello, world"	# The object to which msg1 will point.
		.data			# Back to regular data segment.
		.globl	_msg1		# The pointer ...
	_msg1:	.long	L12		# which points to the object.

String constants comprise two special cases in the compiler.  The
first case is when the constant appears anywhere *except* as an
initialiser for a `char' array.  Here the compiler uses the alternate
data segment to suddenly `create' a new array, initialised to the
string text; it then generates a pointer to that array.  In the
second case the string constant is generated in the primary data
segment, and `is' the array being initialised: the constant is
`unwrapped' into an aggregate initialisation.

The second case is actually the more `conventional' of the two;
other aggregates cannot be created at run time:

	int a[] = { 0, 1, 2, 3 };

is legal only outside functions.  What seems surprising to some is
that the same is true of

	char s[] = "foo";

because, unwrapped, this is equivalent to

	char s[] = { 'f', 'o', 'o', '\0' };
	
---even though

	char *s = "foo";

is legal anywhere a declaration is legal.

Ah, if only C had aggregate initialisers!
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 1516)
UUCP:	seismo!umcp-cs!chris
CSNet:	chris at umcp-cs		ARPA:	chris at mimsy.umd.edu