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From: chris@mimsy.umd.edu (Chris Torek)
Newsgroups: comp.lang.c
Subject: Re: A pointer to a 3-D array, or an array of ptrs to 2-D arrays?
Message-ID: <20773@mimsy.umd.edu>
Date: 15 Nov 89 13:48:32 GMT
References: <2765@levels.sait.edu.au>
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 44

In article <2765@levels.sait.edu.au> MARWK@levels.sait.edu.au writes:
>Why does (1) work but (2) does not?
>(1) p = (int (*) [13][2][64]) malloc (sizeof(int) * 13 * 2 * 64)
>    *p[i][j][k] = 6
>(2) p = (int (*) [2][64]) malloc (sizeof(int) * 13 * 2 * 64)
>    p[i][j][k] = 6

(I think you have this backwards: (1) fails and (2) works)

>These are both supposed to be ways of dynamically creating a 3-dimensional
>array for accessing as such, but (1) causes a segmentation error when run
>on the ENCORE.

(Aha, you think you have it backwards too :-) )

You need to read my recent treatises on `pointers and arrays'.

The short explanation for the failure of (1):

	*p[i][j][k]

is the same as

	*(((p[i])[j])[k])

which is the same as

	p[i][j][k][0]

which means to add i*sizeof(*p) to p, which will add i*13*2*64*sizeof(int)
bytes to whatever value is in p, which, if i is more than 0, will be
outside of the region allocated by malloc.

The code in (2) is much more `C-like': p is a pointer to an array 2
of array 64 of int, which points to the first of 13 consecutive such
things.  In (1), p is a pointer to an array 13 of array 2 of array 64
of int, and it points to the first of one such thing, so you must always
write (*p) or (p[0]) in order to get to that thing before you can
do anything useful with it.

(All of the above assumes p is properly declared.)
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
Domain:	chris@cs.umd.edu	Path:	uunet!mimsy!chris