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From: tim@amdcad.AMD.COM (Tim Olson)
Newsgroups: comp.sys.amiga
Subject: Re: Manx C
Message-ID: <17827@amdcad.AMD.COM>
Date: Wed, 5-Aug-87 23:44:15 EDT
Article-I.D.: amdcad.17827
Posted: Wed Aug  5 23:44:15 1987
Date-Received: Sat, 8-Aug-87 09:08:12 EDT
References: <4540@jade.BERKELEY.EDU> <1836@vax135.UUCP> <4595@jade.BERKELEY.EDU> <3199@zen.berkeley.edu>
Reply-To: tim@amdcad.UUCP (Tim Olson)
Distribution: comp
Organization: Advanced Micro Devices, Inc., Sunnyvale, Ca.
Lines: 60
Keywords: look at the assembly.
Summary: Watch out for the C police!

In article <3199@zen.berkeley.edu> waterman@cory.Berkeley.EDU.UUCP (T.S. Alan Waterman) writes:
+-----
| >Of course, "if (ptr == 0)" is as valid as "if (ptr == NULL)", and
| >probably isn't fixed by that change. That the identical "if (!ptr)"
| >doesn't produce an error message may indicate that "if (ptr == 0)"
| >won't, either.
| >
| >	<mike
| 
| Mike, the deal with this problem is this:
| 
|   if (ptr == 0)  compares the pointer with an INT, which is gonna be 16 bits.
|     That is, is hacks ptr in half and compares the bottom half with 0 (16 bits).
|     If you don't expect your pointer to end up on such a huge page-boundary,
|     this'll work, but I wouldn't trust it....
+-----
No.  Mike is right.  From K&R, section 7.7 (Equality Operators):

	"... A pointer to which 0 has been assigned is guaranteed not to
	point to any object, *and will appear to be equal to 0*; in
	conventional usage, such a pointer is considered to be null."
	[emphasis mine]

	From the X3J11 ANSI C Draft, section 3.2.2.3 (Pointers):

	"... An integral constant expression with the value 0, or such an
	expression cast to type void, is called a null pointer constant. 
	If a null pointer constant is assigned to or compared for
	equality to a pointer, the constant is converted to a pointer of
	that type.  Such a pointer, called a *null pointer*, is
	guaranteed not to point to any object or function."

Any C compiler which doesn't perform in this manner is broken.

+-----
|   if (!ptr) loads the thing into a register (all 32 bits of it) and checks
|     the zero flag. It doesn't run a cmp instruction.
+-----
No. !ptr is *exactly* equivalent to ptr==0.  K&R, section 7.2:

	"... The result of the logical negation operator ! is 1 if the
	value of its operand is 0, 0 if the value of its operand is
	non-zero.  ...  It is applicable to any arithmetic type *or to
	pointers*."
	[emphasis mine]

	From the X3J11 ANSI C Draft, section 3.3.3.3 (Unary Operators)

	"... The result of the logical negation operator ! is 0 if the
	value of its operand is nonzero, 1 if the value of its operand
	is 0.  ...  The expression !E is equivalent to (0==E)."

Sorry for the diatribe, but us C police want to eradicate any
disinformation as soon as possible so that it doesn't corrupt young
minds ;-) ;-) ;-)

	-- Tim Olson
	(tim@amdcad.amd.com)

	"C-Police 'R' Us"