Path: utzoo!attcan!uunet!tut.cis.ohio-state.edu!snorkelwacker!bloom-beacon!ora!minya!jc
From: jc@minya.UUCP (John Chambers)
Newsgroups: comp.unix.wizards
Subject: Re: What machines core dump on deref of NULL?
Summary: Dereferencing a NULL pointer is *not* an error.
Message-ID: <412@minya.UUCP>
Date: 29 Jun 90 11:13:27 GMT
References: <444@mtndew.UUCP> <31079@cup.portal.com> <13226@smoke.BRL.MIL>
Lines: 57

In article <13226@smoke.BRL.MIL>, gwyn@smoke.BRL.MIL (Doug Gwyn) writes:
> In article <2389@aix.aix.kingston.ibm.com> rlc@aix.aix.kingston.ibm.com (Roger Collins) writes:
> >In article <31079@cup.portal.com> thad@cup.portal.com (Thad P Floryan) writes:
> >> Point being: there "may" be some method (perhaps under software control) to
> >> disable the MMU on YOUR system to catch NULL dereferencing...
> >First, the way that the systems causes null pointer dereferencing
> >*not* to fail is so simple that all systems should do it: just make
> >sure address 0 (page 0) is allocated and set to 0.
> 
> To the contrary, this merely prevents genuine bugs from being caught
> as soon as they would be were dereference of a null pointer to trap.
> Dereferencing a null pointer is a serious BUG in any application and
> can indicate an algorithmic error that should be tracked down before
> it is too late.

Hey, wait just a minute here.  I can't let such an erroneous error go
unchallenged!  Dereferencing a null pointer is quite definitely *not*
an error, bug, mistake, or any other pejorative, in a great many sorts
of applications.

The trouble with generalizing to all C code is that C outgrew Unix
about a decade ago.

I've written many programs that are to be downloaded and run all by
their lonesome in an "embedded" processor.  Lots of people use C for
such applications, and there are a *lot* of processors (in fact, the
overwhelming majority of them) that run only one program (without an
operating system) for their entire life.  Such embedded, standalone
programs not only can, but are required to access all of physical
memory, including address zero.  The hardware says that something 
particular resides at the bottom end of memory, and such a program 
must access that byte/word/whatever, or it can't do its job.  If a 
C compiler won't let me access that address, then I look for another 
that was written by someone more competent.

Now, if you were to suggest an *optional* feature that would allow
me to turn null-pointer checking off and on (preferably at run time,
but a compile-time flag would be useful), I'd grab it and use it.
After all, it is pretty much true that when a process running on
a Unix system dereferences a null pointer, it is generally a bug.

Of course, all C compilers already have a perfectly good tool for
doing a run-time check for a null pointer:
	if (p == 0 ) ...
I use it a lot.  In particular, I introduce it all over the place
in code that I'm porting from VAXen to other machines.  I suggest 
that others try it.  It's a whole lot nicer than having your program
bomb out on a SEGV.

(Now if there were only an equivalent piece of code that would tell
me whether a non-null pointer is valid.)

-- 
Uucp: ...!{harvard.edu,ima.com,eddie.mit.edu,ora.com}!minya!jc (John Chambers)
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