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C Programming Tutorial
4th Edition (K&R version)
Mark Burgess
Faculty of Engineering, Oslo College
Copyright (cid:13)c 1987,1999 Mark Burgess
Permission is granted to make and distribute verbatim copies of this manual
provided the copyright notice and this permissionnotice are preserved on all
copies.
Preface xi
Preface
Every program is limited by the language which is used to write it. C is a
programmer’s language. Unlike BASIC or Pascal, C was not written as a
teaching aid, but as an implementation language. C is a computer language
and aprogrammingtool which has grown popularbecauseprogrammers like
it! It is a tricky language but a masterful one. Sceptics have said that it is
a language in which everything which can go wrong does go wrong. True, it
does not do much hand holding, but also it does not hold anything back. If
you have come to C in the hope of finding a powerful language for writing
everyday computerprograms,thenyouwillnotbedisappointed. Cisideally
suited to modern computers and modern programming.
This book is a tutorial. Its aim is to teach C to a beginner, but with
enough of the details so as not be outgrown as the years go by. It presumes
that you have some previous aquaintance with programming — you need to
know what a variable is and what a function is — butyou do not need much
experience. It is not essential to follow the order of the chapters rigorously,
but if you are a beginner to C it is recommended. When it comes down to
it, most languages have basically the same kinds of features: variables, ways
of making loops, ways of making decisions, ways of accessing files etc. If you
want to plan your assault on C, think about what you already know about
programmingand whatyou expect to look for in C.You willmost likely find
all of those things and more, as you work though the chapters.
The examples programs range from quick one-function programs, which
do no more than illustrate the sole use of one simple feature, to complete
applicationexamplesoccupyingseveralpages. Inplacestheseexamplesmake
use of features before they have properly been explained. These programs
serve as a taster of what is to come.
Mark Burgess. 1987, 1999
This book was first written in 1987; this new edition was updated and
rewritten in 1999. The book was originally published by Dabs Press. Since
the book has gone out of print, David Atherton of Dabs and I agreed to
release the manuscript, as per the original contract. This new edition is
written in Texinfo, which isa documentation system that usesa single source
file to produce both on-line information and printed output. You can read
this tutorial online, using either the Emacs Info reader, the standalone Info
reader, or a World Wide Web browser, or you can read this same text as a
typeset, printed book.
High Levels and Low Levels 1
1 Introduction
What is C? What is it for? Why is it special?
1.1 High Levels and Low Levels
Any kind of object that is sufficiently complicated can be thought of as
havinglevels of detail; theamountof detail weseedependsuponhowclosely
we scrutinize it. A computer falls definitely into the category of complex
objects and it can be thought of as working at many different levels. The
terms low level and high level are often used to describe these onion-layers
of complexity in computers. Low level is perhaps the easiest to understand:
it describes a level of detail which is burieddown amongst the working parts
of the machine: the low level is the level at which the computer seems most
primitive and machine-like. A higher level describes the same object, but
with the detail left out. Imagine stepping back from the complexity of the
machine level pieces and grouping together parts which work together, then
covering up all the details. (For instance, in a car, a group of nuts, bolts,
pistons can be grouped together to make up a new basic object: an engine.)
At a high level a computer becomes a group of black boxes which can then
be thought of as the basic components of the computer.
C is called a high level, compiler language. The aim of any high level
computer language is to provide an easy and natural way of giving a pro-
gramme of instructions to a computer (a computer program). The language
of the raw computer is a stream of numbers called machine code. As you
might expect, the action which results from a single machine code instruc-
tion is very primitive and many thousands of them are required to make a
program which does anything substantial. It is therefore the job of a high
level language to provide a new set of black box instructions, which can be
given to the computer without us needing to see what happens inside them
– and it is the job of a compiler to fill in the details of these "black boxes"
2 Chapter 1: Introduction
so that the final product is a sequence of instructions in the language of the
computer.
C is one of a large number of high level languages which can be used for
generalpurposeprogramming,thatis,anythingfromwritingsmallprograms
for personal amusement to writing complex applications. It is unusual in
several ways. Before C, high level languages were criticized by machine
code programmers because they shielded the user from the working details
of the computer, with their black box approach, to such an extent that
the languages become inflexible: in other words, they did not not allow
programmers to use all the facilities which the machine has to offer. C, on
theother hand,was designed to give access to any level of the machinedown
High Levels and Low Levels 3
to raw machine code and because of this it is perhaps the most flexible of
all high level languages.
Surprisingly, programming books often ignore an important role of high
level languages: high level programs are not only a way to express instruc-
tions to the computer, they are also a means of communication among hu-
man beings. They are not merely monologues to the machine, they are a
way to express ideas and a way to solve problems. The C language has
been equipped with features that allow programs to be organized in an easy
and logical way. This is vitally important for writing lengthy programs
because complex problems are only manageable with a clear organization
and program structure. C allows meaningful variable names and meaningful
function names to be used in programs without any loss of efficiency and it
gives a complete freedom of style; it has a set of very flexible loop construc-
4 Chapter 1: Introduction
tions (for, while, do) and neat ways of making decisions. These provide an
excellent basis for controlling the flow of programs.
Another unusual feature of C is the way it can express ideas concisely.
The richness of a language shapes what it can talk about. C gives us the
apparatus to build neat and compact programs. This sounds, first of all,
either like a great bonus or something a bit suspect. Its conciseness can
be a mixed blessing: the aim is to try to seek a balance between the often
conflictinginterestsofreadabilityofprogramsandtheirconciseness. Because
The Compiler 5
this side of programming is so often presumed to be understood, we shall
try to develop a style which finds the right balance.
Callows things which aredisallowed in other languages: this is nodefect,
buta very powerfulfreedom which, whenused with caution, opens uppossi-
bilities enormously. It does mean however that there are aspects of C which
can run away with themselves unless some care is taken. The programmer
carries an extra responsibility to write a careful and thoughtful program.
The reward for this care is that fast, efficient programs can be produced.
C tries to make the best of a computer by linking as closely as possi-
ble to the local environment. It is no longer necessary to have to put up
with hopelessly inadequate input/output facilities anymore (a legacy of the
timesharing/mainframe computer era): one can use everything that a com-
puterhas tooffer. Above all itis flexible. Clearly nolanguage can guarantee
intrinsically good programs: thereis always a responsibility on theprogram-
mer, personally, to ensurethat a program is neat, logical and well organized,
but it can give a framework in which it is easy to do so.
The aim of this book is to convey some of the C philosophy in a prac-
tical way and to provide a comprehensive introduction to the language by
appealing to a number of examples and by sticking to a strict structuring
scheme. It is hoped that this will give a flavour of the kind of programming
which C encourages.
1.2 Basic ideas about C
What to do with a compiler. What can go wrong.
Using a compiler language is not the same as using an interpreted lan-
guage like BASIC or a GNU shell. It differs in a number of ways. To begin
with, a C program has to be created in two stages:
• Firstly, theprogramiswrittenintheformofanumberoftextfilesusing
a screen editor. This form of the program is called the source program.
It is not possible to execute this file directly.
• Secondly, the completed source file is passed to a compiler—a program
which generates a new file containing a machine code translation of the
source text. This file is called an object file or executable file. The
executable file is said to have been compiled from the source text.
Compiler languages do not usually contain their own editor, nor do they
have words like ‘RUN’ with which to execute a finished program. You use a
screen editor to create the words of a program (program text) and run the
final program in its compiled form usually by simply typing the name of the
executable file.
6 Chapter 1: Introduction
1.3 The Compiler
A C program is made by running a compiler which takes the typed source
programandconverts itinto anobjectfilethatthecomputercan execute. A
compiler usually operates in two or more phases (and each phase may have
stages within it). These phases must be executed one after the other. As we
