Xref: utzoo comp.arch:4822 comp.lang.c:10127
Newsgroups: comp.arch,comp.lang.c
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: negative addresses
Message-ID: <1988May15.222335.13174@utzoo.uucp>
Organization: U of Toronto Zoology
References: <2393@uvacs.CS.VIRGINIA.EDU> <21541@amdcad.AMD.COM>
Date: Sun, 15 May 88 22:23:35 GMT

> >Unfortunately, it is a real problem, because there are zillions of
> >programs that implicitly assume that pointers are all-zeros.
> 
> I don't think this is true.  How about an example?

Any program written by a programmer who believes the 4.3BSD manuals, or
any of their ancestors, all of which claim that the arg-list terminator
for execl is 0 rather than (char *)0.  A pox on the Berkloids for not
having fixed this long ago!

I'm not intimately acquainted with the problem myself, but I do know that
at least one computer project that wanted to use a non-zero null pointer
studied the situation and decided to change the hardware instead.

>Not that much of a problem, really.  The compiler knows about static
>data at compile time, and if not explicitly initialized it can output
>something like
>	ptrname: .word 0xF0F0F0F0 ; null pointer pattern
>in the data section of the code it generates.

True, which is why I described it as "a bit of a pain" rather than as
a significant problem.  The biggest nuisance, actually, is the loss in
effectiveness of the "BSS" optimization for object-module size.
-- 
NASA is to spaceflight as            |  Henry Spencer @ U of Toronto Zoology
the Post Office is to mail.          | {ihnp4,decvax,uunet!mnetor}!utzoo!henry