Ocaml Unix.pdf

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unix system programming in

objective caml

Xavier Leroy and Didier Rémy

April 4, 2010

Xavier Leroy and Didier Rémy,

inria

Rocquencourt.

Rights reserved. Distributed under the Creative Commons Attribution – Non commercial –

Share alike 2.0 France license. See

http://creativecommons.org/licenses/by-nc-sa/2.0/fr/

for

the legal terms.

Translation by

Daniel C. Bünzli, Eric Cooper, Eliot Handelman, Priya Hattiangdi, Thad Meyer,

Prashanth Mundkur, Richard Paradies, Till Varoquaux, Mark Wong-VanHaren

Proofread by

David Allsopp, Erik de Castro Lopo, John Clements, Anil Madhavapeddy, Prashanth

Mundkur

Translation coordination & layout by

Daniel C. Bünzli.

Please send corrections to

daniel.buenzl i@erratique.ch

Abstract

This document is an introductory course on Unix system program-

ming, with an emphasis on communications between processes. The main novelty

of this work is the use of the Objective Caml language, a dialect of the ML lan-

guage, instead of the C language that is customary in systems programming.

This gives an unusual perspective on systems programming and on the ML lan-

guage.

Contents

Introduction

1 Generalities

1.1 Modules

Sys

and

Unix

1.2 Interface with the calling program

1.3 Error handling

1.4 Library functions

2 Files

2.1 The ﬁle system

2.2 File names and ﬁle descriptors

2.3 Meta-attributes, types and permissions

2.4 Operations on directories

2.5 Complete example: search in a ﬁle hierarchy

2.6 Opening a ﬁle

2.7 Reading and writing

2.8 Closing a descriptor

2.9 Complete example: ﬁle copy

2.10 The cost of system calls and buﬀers

2.11 Complete example: a small input/output library

2.12 Positioning

2.13 Operations speciﬁc to certain ﬁle types

2.14 Locks on ﬁles

2.15 Complete example: recursive copy of ﬁles

2.16 Complete example:

ape

chive

3 Processes

3.1 Creation of processes

3.2 Complete Example: the command

leave

3.3 Awaiting the termination of a process

3.4 Launching a program

3.5 Complete example: a mini-shell

4 Signals

4.1 Default behavior

4.2 Using signals

4.3 Changing the eﬀect of a signal

4.4 How to mask signals

4.5 Signals and system calls

4.6 The passage of time

4.7 Problems with signals

5 Classical inter-process communication: pipes

5.1 Pipes

Contents

5.2

5.3

5.4

5.5

5.6

5.7

Complete example: parallel sieve of Eratosthenes

Named pipes

Descriptor redirections

Complete example: composing

commands

Input/output multiplexing

Miscellaneous:

write

6 Modern communication: sockets

6.1 Sockets

6.2 Socket creation

6.3 Addresses

6.4 Connection to a server

6.5 Disconnecting sockets

6.6 Complete example: the universal client

6.7 Establishing a service

6.8 Tuning sockets

6.9 Complete example: the universal server

6.10 Communication in connectionless mode

6.11 Low level reads and writes

6.12 High-level primitives

6.13 Examples of protocols

6.14 Complete example:

http

requests

7 Threads

101

7.1 Introduction

101

7.2 Creation and termination of threads

102

7.3 Waiting

103

7.4 Synchronization among threads: locks

106

7.5 Complete example:

http

relay

108

7.6 Conditions

110

7.7 Event-based synchronous communication

112

7.8 Implementation details

115

Going further

119

121

Exercise answers

References

Index

137

135

Introduction

These course notes originate from a system programming course Xavier Leroy

taught in 1994 to the ﬁrst year students of the Master’s program in fundamental

and applied mathematics and computer science at the École Normale Supérieure.

This earliest version used the Caml-Light [1] language. For a Master’s course in

computer science at the École Polytechnique taught from 2003 to 2006, Didier

Rémy adapted the notes to use the OCaml language. During these years, Gilles

Roussel, Fabrice Le Fessant and Maxence Guesdon helped to teach the course

and also contributed to this document. The new version also brought additions

and updates. In ten years, some orders of magnitude have shifted by a digit

and the web has left its infancy. For instance, the

http

relay example, now

commonplace, may have been a forerunner in 1994. But, most of all, the OCaml

language gained maturity and was used to program real system applications like

Unison [18].

Tradition dictates that Unix system programming must be done in C. For

this course we found it more interesting to use a higher-level language, namely

OCaml, to explain the fundamentals of Unix system programming.

The OCaml interface to Unix system calls is more abstract. Instead of en-

coding everything in terms of integers and bit ﬁelds as in C, OCaml uses the

whole power of the ML type system to clearly represent the arguments and re-

turn values of system calls. Hence, it becomes easier to explain the semantics of

the calls instead of losing oneself explaining how the arguments and the results

have to be en/decoded. (See, for example, the presentation of the system call

wait

, page 42.)

Furthermore, due to the static type system and the clarity of its primitives, it

is safer to program in OCaml than in C. The experienced C programmer may see

these beneﬁts as useless luxury, however they are crucial for the inexperienced

audience of this course.

A second goal of this exposition of system programming is to show OCaml

performing in a domain out of its usual applications in theorem proving, com-

pilation and symbolic computation. The outcome of the experiment is rather

positive, thanks to OCaml’s solid imperative kernel and its other novel aspects

like parametric polymorphism, higher-order functions and exceptions. It also

shows that instead of applicative and imperative programming being mutually

exclusive, their combination makes it possible to integrate in the same program

complex symbolic computations and a good interface with the operating system.

These notes assume the reader is familiar with OCaml and Unix shell com-

mands. For any question about the language, consult the Objective Caml System

documentation [2] and for questions about Unix, read section 1 of the Unix

man

ual

or introductory books on Unix like [5, 6].

This document describes only the programmatic interface to the Unix sys-

tem. It presents neither its implementation, neither its internal architecture.

The internal architecture of

bsd

4.3 is described in [8] and of System

in [9].

Tanenbaum’s books [13, 14] give an overall view of network and operating system

architecture.

The Unix interface presented in this document is part of the Objective Caml

System available as free software at

http://caml.inria.fr/ocaml/

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