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From: minow@mountn.dec.com (Martin Minow)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Seemingly equivalent code fragments in C
Keywords: C pointers Lightspeed int
Message-ID: <559@mountn.dec.com>
Date: 7 Aug 89 20:53:36 GMT
References: <4882@tank.uchicago.edu>
Reply-To: minow%thundr.dec@decwrl.dec.com (Martin Minow)
Organization: Digital Equipment Corporation
Lines: 49

In article <4882@tank.uchicago.edu> s170@tank.uchicago.edu
(harmon g washington) tries to create a Pascal string:
>	int *bittenApple;
>	
>	*bittenApple = 0x0114;  	/* create Pascal string 1 byte long */
>	AppleMenu = NewMenu(ApplID, bittenApple);
>THIS CODE DID NOT WORK!  Instead of the Apple logo I got a random string of junkIt seemed it should have work: defining a pointer to an int; and then defining the item it points to.  Now the following fragment of code DID work:

This code sequence has the following meaning:
	int *bittenApple;
		This is a pointer to a default-sized integer.  (It will
		point to a 16-bit integer on Think-C, but you cannot
		count on that.)

	*bittenApple = 0x0114;
		The current value of bittenApple (whatever it is) is the
		address of an integer.  Store 0x0114 in that location.
		This writes 0x0114 in a random location in memory.
	... NewMenu(ApplID, bittenApple);
		Pass the random location in bittenApple to NewMenu.

Note that bittenApple was never initialized and a garbage value was
passed to NewMenu (This could be the address of ROM, or a location
on the stack that is overwritten).  The second example corrects this:
>	int *bittenApple, xx;
>	xx = 0x0114;
		Store 0x0114 in xx.
>	bittenApple = &xx;
		Store the address of xx in bittenApple
>	AppleMenu = NewMenu(ApplID, bittenApple);

There is one further, in some ways more serious, problem: an integer
containing the value 0x0114 is *not* necessarily equivalent to a string.
(There are a number of subtle C-language points here that would be
better explained in a C textbook.)  A better way to perform the function
in Think C would be:
	AppleMenu = NewMenu(ApplID, "\p\024");
Here "\p" introduces a Pascal format string.  (This is not compatible
with other C's) and \024 is the octal equivalent of \x14.  A more
compatible method would be:
	char bittenApple[] = { 0x01, 0x14 };
	AppleMenu = NewMenu(ApplID, &bittenApple[0]);

The last example should work anywhere.

Hope this clarifies the problem.

Martin Minow
minow%thundr.dec@decwrl.dec.com