Xref: utzoo comp.std.c:1007 comp.lang.c:17409
Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!decwrl!megatest!djones
From: djones@megatest.UUCP (Dave Jones)
Newsgroups: comp.std.c,comp.lang.c
Subject: Re: realloc
Message-ID: <3255@goofy.megatest.UUCP>
Date: 4 Apr 89 05:58:47 GMT
References: <9132@alice.UUCP>
Organization: Megatest Corporation, San Jose, Ca
Lines: 35

From article <9132@alice.UUCP>, by andrew@alice.UUCP (Andrew Hume):
> what is
> all the whining about? can't malloc(0) return a pointer to 1 (or 2 or 4)
> bytes? of course, everyone would be happy now, wouldn't they?

Hear! Hear!

Indeed, most implementations could actually return a valid pointer to
ZERO bytes, if they wanted to!

Yep.  Typically a pointer to a heap-packet is the machine-address of the
memory just after a heap-packet HEADER, which the heap routines use
to keep books.  Since no two heap-packets have the same headers, no
two heap-packets have the same heap-packet pointer.  This remains
true even if no memory beyond the header is actually reserved.

In practice, a heap-packet-pointer-to-nothing would typically
be the machine address of another heap-packet-header.  We dare not 
dereference it, but so what?  It is always an error to use a
heap-packet-pointer to index as much as heap-packet-size bytes into the 
heap-packet. (Doing so usually gets you into another heap-packet-header,
or another heap-packet, and can keep you awake debugging all night
and most of the next day, so don't DO that!)

Ego, with a pointer to zero bytes, it is an error to index even zero-bytes
beyond it.  That is to say, the pointer-to-nothing is good only for
comparing for equality with similarly typed pointers.

No problem.

Maybe on a Prevert-2000, that keeps different kinds of pointers in
different shapes of mayonaise jars, you would have to allocate a byte
or four of memory.  Okay. Fine.

Ain' no deal, Home!