Functions using malloc (style)

[John Hascall]

rh at smds.UUCP (Richard Harter) writes:
}Steven Lee writes:
}>   When you write a C function and it returns a pointer to some
}>   structure, how should the function do this?  Many of the system
}>   functions I know create a static area that gets rewritten when
}>   the user makes multiple calls to it.  This avoids the problem
}>   of allocating memory each time the function is called.

}>   The possible solutions I have come up with are:
}>   1. Force the user to malloc his own space and then call the function.

}>   2. The function mallocs space each time it is called.  It is up to
}>      the user to free (release) the memory.

}Either view may be appropriate, depending on usage.  The problem is that
}the function writer does not necessarily know what the users needs are.
}For a bit of extra work you can accomodate both sets of needs.  Make a
}package out of it.  [...big huge scheme...]

Or, a more modest solution is something like this:

#define ALLOC(type) ((type *)malloc(sizeof(type)))

struct woozy * get_woozy(wp)
	struct woozy *	wp;
{
	if (wp == NULL) {
		/* no woozy from user, get one for them */
		if ((wp == ALLOC(struct woozy)) == NULL) {
			/* can't get woozy:
			    fprintf, exit, return NULL,... as appropriate */
		}
	}
	/* ... etc ... */
	return (wp);
}

Or, possibly:

struct woozy * get_woozy(wp)
	struct woozy *	wp;
{
	static struct woozy	our_woozy;

	if (wp == NULL) {
		/* no woozy from user, use ours */
		wp = &our_woozy;
	}
	/* ... etc ... */
	return (wp);
}

John
-------------------------------------------------------------------------------
John Hascall                        An ill-chosen word is the fool's messenger.
Project Vincent
Iowa State University Computation Center                       john at iastate.edu
Ames, IA  50011                                                  (515) 294-9551