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From: waterman@cory.Berkeley.EDU (T.S. Alan Waterman)
Newsgroups: comp.sys.amiga
Subject: Re: Manx C
Message-ID: <3199@zen.berkeley.edu>
Date: Wed, 5-Aug-87 18:33:09 EDT
Article-I.D.: zen.3199
Posted: Wed Aug  5 18:33:09 1987
Date-Received: Sat, 8-Aug-87 05:50:02 EDT
References: <4540@jade.BERKELEY.EDU> <1836@vax135.UUCP> <4595@jade.BERKELEY.EDU>
Sender: news@zen.berkeley.edu
Reply-To: waterman@cory.Berkeley.EDU.UUCP (T.S. Alan Waterman)
Distribution: comp
Organization: University of California, Berkeley
Lines: 28
Keywords: look at the assembly.
Summary: the problem is...

>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....

  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.


In short, the error message is correct. You are being rightly warned the the
compiler is about to do exactly what you told it to, and not really what
you wanted to do. Oh, BTW, defining anything as a ((void *) x) makes a long-
word (32 bit) value, whether x is long or not.


TS Waterman

'psionic code, the only way to fly'	waterman@cory.Berkeley.Edu
"I am my employer, and these are not his opinions"