The RISKS Digest
Volume 4 Issue 96

Saturday, 6th June 1987

Forum on Risks to the Public in Computers and Related Systems

ACM Committee on Computers and Public Policy, Peter G. Neumann, moderator

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o Lightning Strikes Twice At NASA
Matthew P Wiener
o Iraqi cockpit navigation system placed Stark in exclusion zone?
Jon Jacky
o Run-time checks
Howard Sturgis
Henry Spencer
James M. Bodwin
Alan Wexelblat
o Error Checking and Norton's Assembly Language Book
James H. Coombs
o Re: Risks of Compulsive Computer Use
Douglas Jones
o A reference on Information Overload; a Paradox of Software
Eugene Miya
o Computerholics
James H. Coombs
o Naval Warfare — on possible non-detonation of missiles
Mike McLaughlin
o Info on RISKS (comp.risks)

Lightning Strikes Twice At NASA

Matthew P Wiener <weemba@brahms.Berkeley.EDU>
Sat, 6 Jun 87 04:22:40 PDT
The 22 May 87 issue of _Science_, p903, has an article about the rocket
that was hit by lightning two months ago:

    Jon Busse, chairman of NASA's inquiry into the accident, disclosed ...
  that the Atlas-Centaur ... was launched in a heavily charged electrical
  atmosphere on 26 March.  The rocket itself triggered a lightning bolt.  A
  single bolt punched through the fiberglass nose cone, spread fingers of
  electricity around the computerized brain [sic!] that commands the motors,
  and changed one word of program language.  As a result, the motors sent the
  rocket veering off course 51 seconds after lift-off, at the moment of peak
  strain.  The $160-million package began to break up, and flight controllers
  had no choice but to deliver the coup de grace.
    ... If [shuttle launch criteria] had been applied, they would have
  prevented the launch of the unmanned Atlas-Centaur this spring.  According
  to one researcher, NASA has installed the ``most sophisticated lightning
  monitoring system in the world''....  But its data were not used.
    ... There was a failure of communication, Busse said, and a failure of
  NASA to ``exercise awareness, judgment, and leadership.''

I find it interesting how this example is doubly appropriate for RISKS.
We have both an accidental computer failure, and of the larger human
failure to match existing data with actual procedures.

ucbvax!brahms!weemba        Matthew P Wiener/Brahms Gang/Berkeley CA 94720

Iraqi cockpit navigation system placed Stark in exclusion zone?

Jon Jacky <>
Thu, 04 Jun 87 21:45:29 PDT
The SEATTLE TIMES, Thurs June 4, 1987, p. A4 includes a story titled
"Seconds to react, Pentagon report on Stark says," attributed to The 
Washington Post and Newhouse News Service.  It includes the statements:

"Iraq has claimed that the Stark was inside the "exclusion zone" of the 
(Persian) Gulf where Iraq had warned ships would be subject to attack, the
Pentagon said.  The U.S. government disputes the claim.

The Iraqis base their claim on the navigation system in the fighter-plane
cockpit, according to the Pentagon, which added "we are convinced Stark
was 10 to 15 nautical miles outside" that zone.  The Pentagon said it had
received a "wealth of position data" on the Stark from that ship, the AWACS
plane that monitored the attack and two other ships in the area."

- Jon Jacky, University of Washington

       [It does little good after an autombile wreck to argue that your late
       spouse had the right of way.  Unfortunately, this case is similar.  
       Whether or not the Iraqi navigation system was in error, there were
       quite a few human lapses that rendered technology useless.  PGN]

Run-time checks

Thu, 4 Jun 87 11:13:00 PDT
Most of us in the Computer Science Laboratory at the Xerox Palo Alto
Research Center have been using the programming language "Cedar" for several
years.  Cedar has a "safe" subset which guarantees the "safe" behavior of
compiled programs.  One has to use keywords like "LOOPHOLE" or "TRUSTED" in
order to program outside the safe subset.  Unsafe behavior includes
references outside the bounds of an array.  Programming in the safe subset
causes the automatic insertion of run-time bounds checks, among other run
time checks.  (It also involves extensive compile time checking as well.)

In our workstation operating system, also called "Cedar", all programs run
in a common multi-process address space.  This includes programs that are
being debugged, as well as editors and the file system.  The operating
system itself is written in Cedar, and a large part of that system is
written in the safe subset.  (Some old parts of the system have not been
converted to safe Cedar.  It is a matter of controversy how much could be
converted.)  The screen managers, editors, compilers, and mail program are
all mostly in the safe subset.  Thus, my day to day work is conducted using
programs that involve extensive run-time checking.

I rarely have a "crash" on my workstation that can be traced to a failure
that could have been caught by a run-time check, such as a bounds check.
That is, crashes in which the language abstraction is violated.  "Rarely"
means on the order of significant fractions of a year between occurrences,
probably more than a year.  I do have more frequent crashes than this.
There are a few resources in our system that are consumed in a monotonic
fashion, and under certain conditions one can run out of these resources
during a day of work; this event requires a re-boot.  In addition, there are
some lingering deadlocks in the window manager that strike infrequently.

I have recently written a large compiler-compiler system.  Except for a few
lines of code in the midst of a parser generator subsytem, it is written
entirely in the safe subset.  This system generates Cedar source code, and
generates this source code faster than the Cedar compiler can compile it.
Thus, the cost of the run-time checks in this program is inconsequential.  I
debugged this system on my workstation, running it concurrently with may
day-to-day support tools, including the window manager, text editor,
compiler, and mail-program.  This undebugged code never interfered with the
execution of these support tools.

All of the above is written to support the notion that one can work on a
day-to-day basis in a system with extensive built in run-time checks.  I
find it very comforting to work in this system.  I personally would find
it very painful to go back to a world that was not "safe".

Howard Sturgis

Run-time checks

Sat, 6 Jun 87 02:54:42 edt
I should clarify a couple of points in my previous contribution.  Several
people have pointed out IBM's internal PL.8 compiler, which does generally
resemble what I was thinking of:  it works quite hard to determine whether
run-time checks are really needed, and succeeds in eliminating most of them.
It typically manages to bring the run-time-check overhead down to one or
two percent, sometimes to zero.  (PL.8 has not been publicized much, but
there are a couple of papers on it in the Sigplan 82 compiler symposium.)

My notion was a little different, though:  the compiler should *never*
generate a run-time check; if a run-time check is necessary anywhere, and
the programmer has not explicitly overridden that particular check in that
particular place, this should be considered a fatal error and code generation
should be abandoned.

The reasons behind this are two.  First, the occurrence of a run-time error
is almost certainly a symptom of a bug; programs should be more careful.
It seems better to flag such bugs at compile time, when it is convenient
to do something about them.  Second, since the actual error will generally
show up at some remove from the underlying bug that caused it, it is really
very difficult to do anything intelligent about it after the fact, barring
global recovery methods like recovery blocks.  (This is a major reason why
programs should avoid generating such errors.)  Dying with a core dump is
not acceptable for e.g. an air-traffic-control program.  It seems wise to
head off such errors in advance.

Clearly, it is not realistic to expect a compiler to eliminate all run-time
checks from arbitrarily complex programs.  That's more a job for a theorem
prover, which isn't practical as a routine programming tool for normal
applications just yet.  But:  programmers do not write arbitrary programs.
My conjecture — which PL.8 strengthens but does not prove — is that modest
intelligence in the compiler, plus some willingness to clean up code to make
it clearer, would reduce the "compiler can't be sure about this one" cases
to an occasional annoyance that could be handled by manual overrides.

                Henry Spencer @ U of Toronto Zoology

Some experience with run-time checking

Thu, 4 Jun 87 15:49:24 EDT
I'm the author of a Pascal compiler that is compatible with the IBM
Pascal/VS compiler.  Among other things, the compiler does a MUCH more
complete job of detecting run-time errors than the IBM product does.  For
instance, my compiler detects run-time uses of variables before they have
been assigned to or otherwise initialized.  Since the compiler is completely
compatible with the IBM product, one of the first things that people did
when the compiler was released was to dust off existing Pascal/VS programs
and run them through the new compiler.  The number of bugs that the new
compiler found in these supposedly "fully debugged" programs was absolutely
frightening.  I now regret that I did not have the foresight to keep
statistics on this since it would have given some kind of indication of the
effectiveness of the particular debugging techniques used.  Even more
frightening was the number of programmers who refused to believe that this
new compiler had actually found errors in production programs.  I've often
thought that it would be an interesting experiment to have a group of
programmers debug some programs with all compiler run-time checking off and
then to turn the run-time checking on in order to see how many errors they

One risk of putting strong run-time checking into a compiler is that the
programmers may assume that the compiler is doing more than it is.  For
instance, the IBM Pascal/VS compiler does not detect integer overflows in
EVERY circumstance (it isn't clear if this is a "bug" or a "feature").
Thus, a few programmers were really surprised when the new compiler detected
integer overflows in their programs.  They had not bothered to test for that
themselves since they were convinced that the Pascal/VS compiler was doing
the checking for them.
                                          Jim Bodwin

Bounds checking

Alan Wexelblat <wex@MCC.COM>
Thu, 4 Jun 87 12:37:04 CDT
There's a risk associated with hardware bounds checking which continues to nab
me.  After working for months on Burroughs hardware (in Modula2 and ALGOL)
I became accustomed to having my occasional bounds errors caught by the
compiler or the hardware.  Now I work in C on a UNIX box and occasionally am
astonished when the first element of an array "miraculously" takes on a new
value.  Makes debugging a lot harder, too.

Alan Wexelblat 
UUCP: {seismo, harvard, gatech, pyramid, &c.}!sally!im4u!milano!wex

Error Checking and Norton's Assembly Language Book

"James H. Coombs" <>
Thu, 04 Jun 87 13:07:40 EDT
While working through *Peter Norton's Assembly Language Book for the IBM 
PC*, I was struck again and again by it's "risky" approach to programming.  
A few quotations:

On error checking:

         In this first version of READ_SECTOR we'll deliberately ignore
         errors, such as having no disk in the disk drive.  This is not
         good practice, but this isn't the final version of READ_SECTOR.
         We won't be able to cover error-handling in this book, but you
         will find error-handling procedures in the version of Dskpatch
         on the disk that is available for this book (169).

At another point that I could not locate just now, they (Norton and Socha)
say that they will not check for X, since it is a programming error.

One could interpret the ignoring of error-checking in a number of
ways---perhaps it is so important that the reader should spend $24.95
(for the diskette) to get some good examples; perhaps the authors are
bored by it; perhaps it would make the book too large; etc.  In any case,
the book does not make an effort to impress upon readers the importance of
extensive and careful error checking.  Since they attempt to teach
modular programming at basic levels, it would be appropriate for them to
teach error checking at basic levels as well.  They do stress the importance
of TESTING, especially boundary conditions, but they clearly believe that
testing can replace most error checking.

On completeness of programs:

         Dskpatch won't be finished then, as we said, programs never
         are; but the scope of our coverage in this book will be
         complete (280).

         "a program is never done . . . but there comes a time when it
         has to be shipped to users" (295; quotation marks are in the
         book, apparently to indicate that this is conventional wisdom).

         Remember:  Programs are never complete, but we have to stop
         somewhere (276; and then they kludge a routine that they admit
         should be completely rewritten).

This attitude explains why the book gradually gets sloppier and sloppier.  The
first few chapters impressed me as award-winning material, wonderful tutorial,
careful, thoughtful.... Then I found that care replaced more and more by an
effort to sell the diskette and by haste to finish the book and get more
dollars in their pockets.  In some ways, I appreciate the errors that I found
in their programs, since that gave me a chance to try out my knowledge of
assembly language, but that does not excuse their attitude and certainly does
not excuse their teaching others to meet deadlines by handing programs over as
is instead of completed.

I would still recommend this book for experienced programmers who want a good
tutorial on assembly language, but it would be a little dangerous for people
who have not already absorbed some of the principles of risk reduction.

In addition to this example of risky programming, I would like to offer some
thoughts on the discussion of such built-in error checking as checking for
the overflowing of array bounds.

First, aren't the gains fairly limited?  If the error is captured in the
hardware, it's still going to cause an abort, right?  It may be a little
cleaner and may help the programmer locate the problem, but the program is
still going to crash.  I certainly wouldn't want the hardware or even any
compiler-generated code to attempt to correct such an error in my code.  I
suppose that there may be some disagreement about the gains of terminating
gracefully over just falling apart, but the point is that one still has to
terminate, which is not much consolation to the user.  If the error condition
could cause damage to the system, then the gains might be considerable.  Such
damage is usually prevented at the systems level though.  On an IBM mainframe,
one has such things as "operation exceptions."  What about other machines?
In three years of program development, I have yet to damage an IBM PC.  In
"critical" software---well, maybe there is something here---would built-in
boundary checking prevent a robot from smashing someone's head?  From the
user's point of view, that certainly would be consolation!

Second, what is the overhead for such checking?  In an inner loop, the
overhead might be substantial, especially if the loop consists of only a
few lines, as in searching for a character in a string.  The programmer
has to code some termination anyway, so error checking should be redundant
here.  E.g.,

   for (i = 0; i < BUFSIZE && buf[i]!=CHAR_X; i++);

I suppose the programmer could slip and use BUFSIZE when it should be BUFSIZE2
or something like that.  (For a real example of such an error in distributed
code, see the public domain version of SED, "a freeware component of the GNU
operating system"---in the incarnation in which I received it from a BBS, same
version on Genie, I believe.)

Finally, checking for array bounds is of little value in languages/programs
that use pointers extensively.  The loop above is much more likely to be coded
as follows:

   for (s = buffer; *s && *s!=CHAR_X; s++);

A superb compiler might be able to determine that this is an appropriate point
to check for going past the bounds of 'buffer', which may not be properly
terminated.  By such standards, however, today's compilers are Neanderthals,
and most of our machines don't have the power to drive more intelligent
compilers at reasonable speeds anyway.

In conclusion, I don't believe that built-in error checking is worth the
trouble for most environments.  First, the gains are limited.  Second, the
overhead may be substantial and cannot be controlled by the programmer.
Finally, compilers are not capable of interpreting the code well enough to
determine just what sort of error checking should be performed (perhaps I
should say compilers that I am familiar with on equipment that I am
familiar with: PL/I on IBM mainframes; C86 and MSC on IBM PCs).

James H. Coombs, Mellon Postdoctoral Fellow in English, Brown University

Re: Risks of Compulsive Computer Use

Douglas Jones <>
Thu, 4 Jun 87 10:29:25 CDT
Steve Thompson wrote (Risks 4.94) "Need there be a Hackers-Anonymous."

In 1972 or 73, while I was an undergraduate at Carnegie-Mellon, I made up
a supply of flyers for 'Computer Nurds Anonymous' and posted them around
campus.  (Note:  Nurd was the accepted spelling at CMU back then; we had
not picked up on the MIT spelling, nor was the word 'hacker' used much at
CMU at the time.)  Computer Nurds Anonymous offered a withdrawal plan that
moved the nurd from interactive computing to a batch environment, and from
there, in graduated steps, to hand cyphering.  I understand that some
copies of the flyer ended up at other universities; I no longer have a

More seriously, while I was an undergraduate, I and another student did
a term project for a personality theory course which involved what may well
be one of the first serious psychological studies of hackers.  We came up
with the following characterization:

    This particular group of students uses the computer as a
    social surrogate and as an object of their creative energies
    because, being passive individuals, they prefer to deal with
    the stable environment of the machine world.  As these people
    are basically intelligent but introverted, they employ the
    computer because it is intellectually stimulating without
    allowing the trauma of social contact.

We then identified a group of 28 students who had completed some course
work in computer science, 6 of whom were identified by their peers as
'computer nurds'.  We administered the Edwards Personal Preference Scales
for change (Do they like familiar environments, or do they seek new
environments?) and order (Do they like an orderly life?), and the Minnesota
Multiphasic Personality Inventory social introversion scale.  We found
no correlation between being a 'computer nurd' and desire for change,
we found that, as expected, 'computer nurds' were more likely to be
introverted than extroverted, but that this was true of the entire population
tested also.  We found, contrary to our expectations, that 'computer nurds'
were significantly more disorderly than others.  

Charles Hedrick (who was then a PhD student at CMU) commented on our
unexpected result on disorder that, while an orderly disposition was
logically a prerequisite for a successful programmer, so-called 'computer
nurds' are not terribly successful.  Nurds spend a great deal of time
at the computer, but they operate it more as a toy than a learning
tool; they are adept at performing tricks with a teletype but lack
the orderliness required for true success.

                Douglas W. Jones

A reference on Information Overload; a Paradox of Software

04 Jun 87 11:13:28 PDT (Thu)
%A Spectrum Staff
%T Too Much, Too Soon: Information Overload
%J IEEE Spectrum
%V 24
%N 6
%D June 1987
%P 51-55
%K IFF, human factors

Jerry Saltzer's comment:  Ah, that's my "Paradox of Software" argument.  We
want it fast (who would use a computer slower than a person?), yet we want
it flexible, friendly and maintainable when it breaks down.  I wish I were
doing more research in software, but I am surrounded by people who only want
the computation done faster, almost at all costs.  E.g., would we prefer a
single button on ALL keyboards which reads "HELP" (single button, very
fast), or a more general, flexible keyboard which has 'H' 'E' 'L' 'P'?
--eugene miya


"James H. Coombs" <>
Thu, 04 Jun 87 14:01:26 EDT
Steve Thompson asks about people addicted to computing.  So far as I know, we
don't have a definition of addiction; we have, at best, some criteria for
determining when a person is addicted.  In fact, I believe that we have levels
of addiction.  For example, we have "problem drinkers," and we have
"alcoholics."  Do we also have "problem computors" and "compuholics"?

First, I think we need a careful characterization of addiction, with respect
to computing.  Until we have that characterization, I don't think we can
profitably address Steve's questions.

Just to start things off, I suggest that a person P is addicted to an activity
A (which may include the taking of substances) iff (if and only if):

1) A is being engaged in to an extent that is physically and/or mentally

2) A is such that the benefits of not engaging in it outweigh the benefits of
engaging in it.

3) P cannot cease A.

Clause (1) is necessary to exclude such necessary activites as eating.  Clause
(2) is necessary to account for such things as living, which is progressively
debilitating; presumably, we don't want to say that people are addicted to
living (and don't want to get into discussions of when people would be better
off dead?).

I'm not sure, but it seems in some ways right to add a clause specifying that P
knows (1 and 2) or even (1, 2, and 3).  I hesitate to add this though, because 
it also seems right to say that a person can be addicted to something without 
having any idea that there is a problem.  This consideration threatens to bring
up issues of social definitions, relativism, and all of that; but let's not.

So, a preliminary result given this definition.  The benefits of being addicted
to computing instead of to other activities are trivial, since the addiction
leads to physical and/or mental disability.  Note that under certain
circumstances, the disability may be justified, but this is covered by clause
(2).  In a state of emergency, it may be appropriate to exploit the addict's
computing abilities, just as it may be appropriate to ask the police to stop a
killer in a shopping mall.  ON THE OTHER HAND, if a person is so addictive
that he/she must be addicted to SOMETHING, then it would certainly be better
to be addicted to computing than to some more damaging activity (such as
drinking alcohol), and this is covered by clause (2).

Theoretically, I think this definition takes us a long way.  Practically, it
doesn't seem to offer us much.  How are we to determine whether or not, for
example, a person is so addictive that he/she must be addicted to something?
Are we to beat on that person until we teach him/her to live with no
addictions?  Or is it better to let up at some point and say "this is good
enough."  Also, we have the problem of levels of addiction, and this is not
addressed in the definition.  If one is computing at a level that is only
problematic, then is that better than being fully addicted to something that
is less dangerous than the computing?

Also, I see that I said in (3) that P CANNOT cease A. But, don't addicts often
(with extensive help) cease their addictive activities?  Perhaps we should say
in (3) that P cannot cease A on his/her own.  Then we might define a person
who has a problem with A as someone who can cease A on his/her own but is
having difficulty doing so.

Then, it seems that we have to say that being problematic with respect to A is
better than being addicted with respect to A' iff the costs of the full span of
engaging in A are less than the costs of the full span of engaging in A' (add 
after effects to each).  If being a problematic drinker ruins your liver, why 
not be a computing addict instead?  But if addictive computing ruins your eyes 
and causes skin cancer, but your liver problem is limited to a few spots, why 
not damage the liver a little?  If addictive computing ruins your health over a
twenty-year period but being more of a participant in normal social activities 
would have infected you with AIDS, why not compute yourself into the grave?

Finally, I would like to encourage people to quit worrying about other people
being isolated.  Whenever someone doesn't want to join the circle, people have
to figure out what his/her problem is.  To say that the person is addicted to
computing (or to work) seems like an au courant easy answer.  Some people
haven't an inkling of what happens in isolation, and many people seem to know
little more than herd instinct and the word "normal" (not the concept, just
the word).  To these people, isolation seems physically and/or mentally
debilitating.  It would be much better if people would accept others a little
more and give them credit for knowing their best interests instead of
addressing their own doubts by stigmatizing what they don't understand.

Ceteris paribus, then, I think we should drop "isolation" from our concerns
about the risks of computing.  There is nothing absolutely bad about isolation,
and some people can gain a lot by a week or so of solitude.  Of course, 
prophets tend to go off for much longer than a week; forty days and forty 
nights seems like a good healthy period for a prophet.  I think we should 
allow at least that to computer gurus, even if their activities are less grand.

James H. Coombs Mellon Postdoctoral Fellow in English Brown University

Naval Warfare — on possible non-detonation of missiles

Mike McLaughlin <>
Thu, 4 Jun 87 11:34:50 edt
In WWII, a Japanese battleship with HUGE (17" or 18") guns hit a U.S.
destroyer with very nearly a full salvo... of armor piercing projectiles.
The range was close, and the trajectory of the projectiles was close to the
horizontal.  The projectiles passed completely through the target, without
detonating - the destroyer was simply not there, as far as the fuzes were
concerned.  The U.S. ship took a fair amount of injuries and damage, but was
still able to sail on its own power back to a friendly port.  Just a couple
of the projectiles exited below the waterline, and the crew was able to
control the flooding and fire.

PT boats, on the other hand, were sometimes destroyed by the water spouts of
major caliber projectiles.  

Not much relation to today's Risks in Computing - except for a little 
perspective on the history of projectiles against ships.  Projectiles are
just ballistic missiles without on-board guidance.  The larger projectiles
have fuzes that used to be set mechanically just before firing.  If you 
don't have time to set them right, they do not do what is desired.  Sounds
like human error again... or perhaps, before.  

    - Mike McLaughlin   <>

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