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From: chris@mimsy.UUCP (Chris Torek)
Newsgroups: comp.lang.c
Subject: Re: Allocation for pointers
Message-ID: <8407@mimsy.UUCP>
Date: Fri, 4-Sep-87 10:56:35 EDT
Article-I.D.: mimsy.8407
Posted: Fri Sep  4 10:56:35 1987
Date-Received: Sat, 5-Sep-87 20:36:48 EDT
References: <1357@faline.bellcore.com>
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 96

In article <1357@faline.bellcore.com> gtchen@faline.bellcore.com
(George T. Chen) writes:
>main()
>{	char *a;
>	a = "text";
>	recurs(a);
>}
>
>void recurs(a);
>char *a;
>{	strcat(a,"more text");
>	if (some conditional test) recurs(a);
>}

This will not work (aside from the syntax error and the type clash
at the definition of `recurs').

>As I understand it, I'm passing the address of the variable a into
>the routine recurs.

No, to do that, you must say `recurs(&a)'.  You are passing the
value of a to the routine recurs; you have claimed that this
value points to zero or more characters (`char *a'), which it
does indeed.  We just covered this one:  The value passed from
main is the address of the first element of an anonymous array
of five characters which is initialised to {'t', 'e', 'x', 't', '\0'}.

>At no point do I specify how large of a buffer a will eventually
>point to.

At no point do you specify a buffer!  (For one thing, there is
no `buffer' construct *per se* in C, although `array N of char'
where N is `large' is conventionally called a buffer.)  You made
`a' point to this anonymous array of five characters, then told
strcat to write over a[4] with 'm', a[5] with 'o', a[6] with 'r',
and so forth.

>Is there a limit

Certainly:  The array to which `a' points contains only five
slots.

>and is it compiler dependent?

The result of writing nonexistent array elements is certainly
compiler (and runtime system) dependent!

You can, instead, pick your own limit:

	void	recurs();

	main(...)
	{
		char a[LIMIT];

		(void) strcpy(a, "text");
		recurs(a);
		...
	}

	recurs(a)
		char *a;
	{

		(void) strcat(a, "more text");
		if (...)
			recurs(a);
	}

or (to save a tiny bit of space and time)

	main(...) {
		static char a[LIMIT] = "text";
		...

or you can come up with fancy dynamic allocation routines:

	struct string {
		char	*s_addr;	/* address of text */
		int	s_len;		/* current length */
		int	s_space;	/* and space remaining */
	};

	struct string *string_append();

	#define	APPEND(result, app) \
	((result)->s_space < (app)->s_len ? string_append(result, app) : \
		((result)->s_len += (app)->s_len, \
			(result)->s_space -= (app)->s_len, \
			bcopy((app)->s_addr, (result)->s_addr, (app)->s_len), \
			result))

or whatnot.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7690)
Domain:	chris@mimsy.umd.edu	Path:	uunet!mimsy!chris