Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sun-barr!apple!heksterb
From: heksterb@Apple.COM (Ben Hekster)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Using Malloc to create variables.  help?
Message-ID: <48214@apple.Apple.COM>
Date: 17 Jan 91 18:14:08 GMT
References: <6996@crash.cts.com>
Organization: Apple Computer Inc., Cupertino, CA
Lines: 37

You write:

extern int	**map;
 
if ( (map = (int **)( malloc( (100 * 100) * sizeof(int) + 1 ) )) == NULL)
	ExitToShell();

	map[x][y] = -1;

Well, for one thing, malloc returns pointers to nonrelocatable blocks, not
handles.  Since arrays and pointer types are equivalent in C, the compiler
interprets `map' as an array of pointers to arrays of int.  Although the
variables are declared incorrectly, the access `map[x][y] = -1' is correct.
My guess is that the compiler you use employs 2-byte ints, and since pointers
are 4 bytes in the Macintosh world, the array accesses are therefore writing
twice as much stuff into memory as you expect (0xffffffff rather than 0xffff)
so you end up stepping all over the Memory Manager data structures.

	You can go two ways--either stick with C's malloc-type allocation, or
use the Memory Manager NewHandle calls.  Obviously the Memory Manager stuff
is a little friendlier since it gives the system a chance to juggle memory
when it gets tight.

	By the way, what is the `+ 1' for in your malloc?  Paranoia??  If so,
I'd suggest taking it out because otherwise it's a great way of hiding bugs
in your algorithms.  You *do* want to find bugs, don't you?

	If you haven't got it yet, I strongly recommend buying Kernighan &
Ritchie's "The C Programming Language".  An excellent book that will teach
you everything you need to know and renders everything else obsolete.

_______________________________________________________________________________
Ben Hekster                           | "I've had my fun
Student intern                        |  but now it's time
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