C question

[chris at umcp-cs.UUCP]

In article <306 at cord.UUCP> fjh at cord.UUCP (FJ Hirsch) writes:
>What is the difference between:
>
>extern	char	*A;
>
>and
>
>extern	char	A[];

Quite a bit.  The former tells the compiler that there is a variable
named `A' defined externally, and that its type is `pointer to
char'.  The latter tells it that there is a variable named `A'
defined externally, and that its type is `array of char'.  These
are most certainly *not* equivalent.  A Vax compiler, for example,
treats the former as a four-byte variable, and the latter as a
`link-time constant'.

>If you do: printf("A=%s\n",A);
>the first causes a core dump, the second works.

In that case, the actual defintion must have been

	char A[<constant>];

>I thought pointers and arrays were equivalent?

No.  This confusion seems to arise from C's convention of treating
the name of an array (in many places, but not all) as a pointer to
the first element of that array.  It is heightened by C's convention
of allowing an `array' declaration as a formal parameter:

	f(s)
		char s[];
	{
		...

It might have been better if compilers refused to accept such a
declaration, rather than silently altering it to read

		char *s;

So when *is* an object of type `array of <stuff>' treated as an
object of type `pointer to <stuff>'?  If I have not forgotten
anything, the answer is `everywhere except in sizeof and in actual
declarations'.  `extern char *A' and `extern char A[]' are actual
(not formal) declarations.
-- 
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