Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!wuarchive!mit-eddie!bloom-beacon!primerd!ENI!RELAY!ARIEL
From: ARIEL@RELAY.Prime.COM
Newsgroups: comp.lang.c
Subject: Re: What C compilers have non-zero null
Message-ID: <108300010@RELAY.Prime.COM>
Date: 12 Jul 90 20:17:00 GMT
References: <422@minya.UUCP>
Lines: 21
Nf-ID: #R:minya.UUCP:-42200:RELAY:108300010:000:590
Nf-From: RELAY.Prime.COM!ARIEL    Jul 12 20:17:00 1990


A small clarification re "certain Prime computers":

The Prime 50 Series does use a non-zero representation
for NULL in some languages, particularily PL/I.

The C NULL is all 0-bits. Sometimes (in IX mode) 32 of
them, other times (V mode, on older machines) 48 bits.

If i is an int, and p is a pointer, then (in IX mode):

i = (int)p;
p = (void *)i;

changes the bit pattern. (i.e after these two statements
*(int *)&i != *(int *)&p is true if p != NULL)

The change is invertable (p ends up with the same value)
IF p is actually a valid pointer (including NULL of course)

Robert Ullmann