Edsger W. Dijkstra

Written b ecause p eople ask me for these data.

Professor Edsger W. Dijkstra 1930 worked for 10 years as programmer at the Math-

ematical Centre in Amsterdam, for 11 years as Full Professor of Mathematics at the

Eindhoven Institute of Technology, for 11 years as ResearchFellow of Burroughs Cor-

p oration, and now|i.e., 1997|for 13 years as Professor of Computer Sciences at the

UniversityofTexas at Austin.

Professor Dijkstra has done pioneering work in graph algorithms, programming lan-

guage implementation, op erating systems, semantic theories and programming metho d-

ology in the broadest sense of the word, and has invented a numb er of seminal prob-

lems mutual exclusion, dining philosophers, distributed termination detection, and

self-stabilization.

His currentinterests fo cus on the formal derivation of pro ofs and programs, and the

streamlining of the mathematical argument in general.

From my Life

I was the third of for children. My father was an excellent chemist, who

worked in the town of Rotterdam, rst as a teacher of chemistry at a secondary

scho ol, later as its sup erintendent. His knowledge was vast and he was p erfectly

able and willing to teach cosmographyor physics when needed. He made a

few inventions, one of which really sophisticated. Among chemists all over the

country he was well-known|he had, for instance, b een President of the Dutch

Chemical So ciety|but steadfastly refused all forms of scienti c promotion, ex-

plaining that he would rather b e the rst among the counts than the last among

the dukes, but the net e ect of adhering to this lofty principle was that he lost

the colleagues that were intellectually his equals to the university campus. In the

From my Life 87

vague idealism after WWI I, Dutch secondary education deteriorated to suchan

extent that he forbade me to b ecome a scho olteacher. My mother was a brilliant

mathematician who had no job|as was customary in those days, even for most

women with a college degree. Her mind was much less encyclopaedic than that

of my father but very quick; she had a great agility in manipulating formulae

andawonderful gift for nding very elegant solutions, and I learned a lot from

her though she had not the patience to b e a go o d teacher.

During the last class of the Gymnasium|that was 1947/48|I considered

going to University to study law, as I hop ed to represent my country in the

United Nations. Then we did our nal exams. Before we got our grades and

were told whether we had passed, I was very depressed, for I reviewed all the

mistakes and stupidities I had committed and I got convinced that I would fail.

My consolation was the conviction that I would get a second chance, always

having b een the b est pupil in the class. My grades turned out to b e b etter than

most teachers had ever seen at a nal examination, and older and wiser p eople

convinced me that it was safer to base mychoice on ability than on idealism,

and that it would b e a pity if I did not devote myself to science. So I wentto

Leyden University to study mathematics and physics during the rst years, and

theoretical physics during the latter part.

On the whole I enjoyed the years a the universityvery much. Wewere very

p o or, worked very hard, never slept enough and often did not eat enough, but

life was incredibly exciting. Now, more than 40 years later, I still remember

the rst \international" piano recital I attended: Clara Haskil playing Mozart!

Another thing I rememb er|b ecause it surprised me|was that a number of my

professors|Klo osterman, Haantjes, Gorter, Kramers|to ok me seriously.

My father, who was a subscrib er to \Nature", had seen the announcementof

a three-week course in Cambridge on programming for an electronic computer

for the EDSAC, to b e precise, and when I had passed the midway exam b efore

the end of my third year|whichwas relatively early|he o ered me bywayof

reward the opp ortunity to attend that course. I thought it a go o d idea, b ecause

I intended to b ecome an excellent theoretical physicist and I felt that in my

e ort to reach that goal, the ability to use an electronic computer might come

in handy. The course was in Septemb er 1951; by accident, A. van Wijngaarden,

then Director of the Computation Departmentof the Mathematical Centre in

Amsterdam, heard of my plans, invited me for a visit, and o ered me a job. Due

to obligations in Leyden I could accept that invitation only in March 1952, when

I b ecame the rst Dutchman with on the payroll the quali cation \programmer".

Professor H.A. Kramers, who had accepted me as student of theoretical physics,

died, I did not like his successor, and for a while I considered continuing my

studies in Utrecht, but it turned out that the lab oratory work I would have

to do in Utrechtwas incompatible with my half time job at the Mathematical

Centre. That was the moment I had to cho ose b etween theoretical physics and

computing; I chose the latter and my study of physics in Leyden b ecame a

formality. I got my degree in the Spring of 1956, moved immediately from Leyden

88 Dijkstra

to Amsterdam, and from that momentonI worked full time at the Mathematical

Centre.

My closest collab orators there were Bram J. Lo opstra and Carel S. Scholten,

who had b een hired while they were still students of exp erimental physics at the

Municipal University of Amsterdam to build a computer for the mathematical

Centre. They were ab out ve years older than I, had b een close friends since

the age of 12, and had already worked on their machinery for quite a few years

when I joined them. When I came their machine did not work, but they had

learned a lot; among the things they had learned was to recognise that their rst

e ort contained to o many technological mistakes to be salvageable, and that

they should start afresh. So, early in the game I b ecame involved in the design

of \the next machine", something that would b e rep eated a numb er of times. In

retrosp ect, the most noteworthy asp ect of that co op eration is that we accepted

without any discussion a discipline that seems rare to day: the very rst thing

we did was to write a programmer's manual for the next machine, including its

complete functional description. For some reason, I was always the one that

wrote that do cument. This do cument then served as a contract b etween them

and me: they knew what they had to build and I knew that I could build up on.

And ab out a year later, wewould b e b oth ready: they had built their machine

and I had all the basic software nished. Not tested, of course, but that turned

always out to be sup er uous. At the time I did not realise how valuable this

training was of programming for a machine that did not exist yet; now I know.

I designed my rst non trivial algorithms. The algorithm for The Shortest

Path was designed for the purp ose of demonstrating the p ower of the ARMAC

at its ocial inauguration in 1956, the one for the Shortest Spanning Tree was

designed to minimise the amount of copp er in the backplane wiring of the X1. In

retrosp ect, it is revealing that I did not rush to publish these two algorithms: at

that time, discrete algorithms had not yet acquired mathematical resp ectability,

and there were no suitable journal. Eventually they were o ered in 1959 to

\Numerische Mathematik" in an e ort of helping that new journal to establish

itself. For manyyears, and in wide circles, The Shortest Path has b een the main

pillar for my name and fame, and then it is a strange thought that it was designed

without p encil and pap er, while I had a cup of co ee with my wife on a sunny

cafe terrace in Amsterdam, only designed for a demo...

Life b ecame serious when Bram and Carel embarked on the design of the X1,

which after a while they conceived with a real-time interrupt. I was frightened

out of my wits, only b eing to o muchaware that this would confront me with a

to all intents and purp oses non deterministic machine. For ab out three months

Ihave b een able to delay the decision to include the interrupt, until eventually,

they attered me out of my resistance. Their plea was along the lines \Yes, we

understand that this makes your task much harder, but surely someone likeyou

must b e up to it, etc.". The real-time interrupt handler b ecame my thesis topic;

I earned my Do ctorate in late 1959.

In the mean time, the ALGOL pro ject was underway. I had attended a few

conferences outside the Netherlands, but it was ALGOL that intro duced me to

From my Life 89

frequent foreign travel. Those trips were very tiring, for my English was un-

trained. I am very grateful to MikeWo o dger from the National Physical Lab-

oratory, who quickly started correcting my English, but in the b eginning only

the errors he had heard me make more than once. In the summer of 1959 I had

discovered how to implement subroutines that could call themselves, by the end

of the year I sawhow to use a sto ck for the evaluation of expression and how

to translate expressions from the usual in x notation into \reverse Polish" only

we did not call it that. For several months, Harry D. Huskey had b een working

at the Mathematical Centre, having b een invited by van Wijngaarden in the

hop e that he would teachushow to write what was then called \an algebraic

translator". Van Wijngaarden wanted us to base our ALGOL 60 compiler on

Huskey's work whichwas a mess Huskey b eing an exp erimental programmer; I

had seen that work and knew that we had to ignore it, but in order to convince

van Wijngaarden I had to bang my st on his table. I rememb er this, b ecause

this has b een the only time that I did so. Late Decemb er I started with J.A.

Zonneveld on an implementation of ALGOL 60 for the X1. We co ded the trans-

lator in duplo, fully aware that we were making history: eachevening each of

us to ok his own copy of the manuscript home so that in case of a re at the

Mathematical Centre our translator would not b e lost. There was no re. In

August 1960 our implementation was working, more than a year b efore the one

of our nearest comp etitor, the group of in Cop enhagen. In May 1960

I submitted the short article \Recursive Programming" to \Numerische Math-

ematik", which published it quickly; here I intro duced the term \stack", while

describing the quintessence of our implementation. For many years I thought

that that pap er had b een ignored in the USA, but later I learned that it had

established my name among America's scholarly computing scientists.

The ALGOL compiler had b een a ma jor e ort on which I had worked day

and night for 8 months, and I needed to recover; more than anything I needed

to b e free from the resp onsibility for a large pro ject. For a few years I did not

build something sophisticated and ambitious, but thought, mainly ab out the

logical intricacies of multiprogramming. This was when I formulated the Mu-

tual Exclusion Problem, thought ab out synchronizing primitives and designed

\semaphores" for that purp ose. I did not publish that last invention b ecause the

company \Electrologica", for whichI was a consultant, was going to incorp orate

them in their next machine, but I freely lectured ab out the topic. In 1961, at

Brighton, Peter Naur and I lectured on ALGOL 60 and its implementation; at

that o ccasion, and I met for the rst time.

At the end of summer of 1962 I attended the IFIP Congress in Munich,

where thanks to van Wijngaarden I was given the opp ortunity to deliver an

invited sp eech. My sp eechwas well received, and the congress was the o ccasion

at which and I met each other for the rst time. When I came

back from Munich, it was Septemb er, and I was Professor of Mathematics at the

Eindhoven UniversityofTechnology.

Late I learned that I had b een the the Department's third choice, after two

numerical analysts had turned the invitation down; the decision to invite me

90 Dijkstra

had not b een an easy one, on the one had b ecause I had not really studied

mathematics, and on the other hand b ecause of my sandals, my b eard and my

\arrogance" whatever that may b e. I have also b een told that I should b e very

grateful to the Department b ecause it had taken a great risk by app ointed me.

For two years I gave the courses in Numerical Analysis b ecause they had no

one else to do so, at the same time I b egan building a little group of computing

scientists. In the years that followed, I built with Cor Ligtmans, Piet Vo orho eve,

Nico Hab ermann and Frits Hendriks what b ecame known as the \THE Multi-

programming System" for the X8 the next machine of Electrologica, of which

the university had ordered a copy; machine and op erating system served the

computation centre of the universityvery well for ab out half a decade, until the

X8 had to b e replaced by a more p owerful engine.

With the THE Multiprogramming System we also knew wewere making his-

tory, b ecause it was the world's rst op erating system conceived or partitioned

asanumb er of lo osely coupled, explicitly synchronized, co-op erating sequential

pro cesses, a structure that made pro ofs of absence of the danger of deadlo ck and

pro ofs of other correctness prop erties feasible. I have presented the quintessence

of this design in Teddinton, in Edinburgh, and in Gatlinburg, Tennessee at an

ACM Conference on Op erating Systems Principles, and it got all the attention

and recognition it deserved. The Departmentsaw it di erently and disbanded

the little group, in reaction to which most of them left the University.It was

obviously to o dicult for the Department to honour what had b een achieved: as

classical mathematicians they were not used to group e orts at all, and further

more it had no connection to the kind of mathematics they were used to. In

addition, some b egan to fear that, instead of a strong comp onent of the Depart-

ment, Computing Science could b ecome to o p owerful a comp etitor for the true

mathematics that were the Department's real concern. For me, life b ecame a

little bit dicult.

For more than half a year I su ered from a rather severe depression, but

things b ecame b etter when I realised that I was frightened by the next challenge,

viz. the study of diculty as such, and of how to overcome it. How do we

do dicult things? My previous more ambitious pro jects acquired the status

of nger exercises, and for therap eutic reasons I wrote \Notes on Structured

Programming"; as far as my depression was concerned, the therapyworked.

Of the \Notes on Structured Programming" I sent only ab out 20 copies to

friends abroad, but by the time the copier had b ecome ubiquitous, and without

my knowledge the text spread like wild re: later I met p eople in far-away places

who treasured a fth or sixth generation copy! It was not a great text and

its sp ontaneous wide distribution surprised me when I heard of it; the only

explanation I have b een able to think of is that it was the rst text that op enly

to ok the programming challenge very seriously. It has certainly played a role in

the founding the IFIP Working Group 2.3 on \Programming Metho dology", a

topic that would only in the 70s b ecome a sub ject of academic concern.

Late one afterno on in 1972, the telephone in my oce at the University rang;

I picked up the receiver, and this was Franz L. Alt of the ACM, calling from

From my Life 91

the USA to tell me that I had won the ACM Turing Award. It to ok me by

complete surprise for I had never considered myself as a p otential recipientof

that most prestigious award, and I could hardly b elieve it, but Alt convinced

me that it was really true, and then we disconnected. When I realised that

my six predecessors had all b een native English sp eakers working at famous

institutes in the USA or the UK, I got even a bit overwhelmed. When I had

calmed downIwent to the oce of the Department's Chairman to inform him

if he was still in. He was, and I told him. His rst reaction was \In the world of

computing, they are rather lavish with prizes, aren't they?". When in the fall of

that year, another colleague, N.G. de Bruijn, launched a plan in which a massive

undergraduate course in programming would earn the money for the graduate

program in \genuine mathematics", I knew it was time to go. After having given

Philips the rst right of refusal, I b ecame Burroughs ResearchFellow on the 1st

of August 1973.

That same summer I designed the rst so-called \self-stabilizing systems". It

was a new problem of which no one knew whether it could b e solved at all, and

the solutions were at the time a tour de force. I submitted a short article to the

Communications of the ACM, which published it in 1974; it was ignored until

Leslie Lamp ort drew attention to it in 1984, and since then it has added a new

dimension to ways of dealing with error recovery.

In the fall of 1973 I designed \predicate transformers" as a to ol for de ning

program semantics in a way that would provide a suitable basis for a calculus

for the derivation of programs. In the late 60s, Tony Hoare had provided a basis

for this work: he taught us the relevance of the predicate calculus for reasoning

ab out programs and showed us how inference rules could b e tied to the formal

syntax of the , but it was at the time not clear to which

extent his \axiomatic basis" de ned the programming language semantics. I

tightened that up and intro duced in passing nondeterminacy. This formed the

starting p oint for the b o ok \A Discipline of Programming", whichkept me busy

for the next few years. Prentice-Hall published it in 1976, it is still for sale, it

has b een quite in uential; the organisation that publishes the Science Citation

Index has identi ed it as a \Citation Classic".

I did the \On-the- y garbage collection", which at the time was quite a feat of

ne-grained interleaving, and the distributed termination detection of di using

computation, which emerged as a fundamental problem. In 1981 I designed a

surprising sorting algorithm.

On the whole, myyears as Burroughs ResearchFellowhavebeenawonderful

time. I would visit the company in the USA a few times a year and my further

assignment was to do my own thing, which I did in the smallest Burroughs

research out t in the world, viz. my study on the second o or of our house in

Nuenen. I maintained my contracts by mail, by going to the University once

aweek, and by travelling all over the world. I lectured all over the world, but

foreign travel lost what little attraction it had I never wasavery go o d tourist:

I managed to visit Moscow without b eing dragged to the Kremlin. Whenever I

92 Dijkstra

was in the country,Iwould work on Friday with Carel S. Scholten; others I saw

as regularly were W.H.J. Feijen, A.J.M. van Gasteren and Alain J. Martin.

It was to o go o d to last, and it didn't. When Michael W. Blumenthal to ok

over, Burroughs Corp oration changed from an enlightened computer manufac-

turer into a nancial organisation, and precisely the kind of p eople I had es-

tablished relations with were the rst to leave the company: I lost my audience

there. Whereas Burroughs's intellectual horizon was shrinking, myown one was

widening, as myinterests shifted from computing and programming metho dol-

ogy to mathematical metho dology in general. A return to the University Campus

seemed appropriate, and when the UniversityofTexas at Austin o ered me the

Schlumb erger Centennial Chair in Computer Sciences, the o er was actually

welcome. The o er was not so surprising, nor my acceptance, for I knew UT and

they knew me. All through myyears at Burroughs I had made it a rule to visit,

where ever I went, the lo cal university and since the late 70s Burroughs had an

Austin Research Centre a visit to whichwas included in most of my trips to the

USA.

In my interest in mathematical metho dology I had b een encouraged by a

grant that I received thanks to the e orts of Dr. Donald W. Brab en of the Ven-

ture Research Unit of BP, a grant that enabled me to o er a p osition to Netty

van Gasteren who thus b ecame the rst BP Venture ResearchFellow. Another

strong stimulus came from my e orts in whichI would so on b e joined by Carel S.

Scholten to givemy \predicate transformer semantics" of the 70s the necessary

mathematical underpinning; these e orts b ecame an exciting exp erience when

we discovered after few notational innovations the surprising utility of calcu-

lational pro ofs. In 1988, Netty van Gasterned successfully defended her Ph.D.

thesis \On the shap e of mathematical arguments".

In the summer of 1984 wemoved to Austin, Texas, where wehave b een happy

ever since, partly by the simple device of having warned ourselves b eforehand

that The Universityof Texas at Austin would not be Heaven-on-Earth. It is

not. With the prolonged recession, the pressures to do the wrong things, always

already strong, are mounting, and part of the university gives in. And in com-

puting, the excessive preo ccupation with sp eed is, of course, a little bit vulgar.

Austin, 29 Novemb er 1993