C LANGUAGE

The C programming Language

By Brian W. Kernighan and Dennis M. Ritchie.

Published by Prentice−Hall in 1988

ISBN 0−13−110362−8 (paperback)

ISBN 0−13−110370−9

Contents · Preface · Preface to the first edition · Introduction Chapter 1: A Tutorial Introduction 1. Getting Started 2. Variables and Arithmetic Expressions 3. The for statement 4. Symbolic Constants 5.Character Input and Output 1. File Copying 2. Character Counting 3. Line Counting 4. Word Counting 5. 6. Arrays 7. Functions 8. Arguments − Call by Value 9. Character Arrays 10. External Variables and Scope Chapter 2: Types, Operators and Expressions 1. Variable Names 2. Data Types and Sizes 3. Constants 4. Declarations 5. Arithmetic Operators 6. Relational and Logical Operators 7. Type Conversions 8. Increment and Decrement Operators 9. Bitwise Operators 10. Assignment Operators and Expressions 11. Conditional Expressions 12. Precedence and Order of Evaluation Chapter 3: Control Flow 1. Statements and Blocks 2. If−Else

The C programming Language

3. Else−If 4. Switch 5. Loops − While and For 6. Loops − Do−While 7. Break and Continue 8. Goto and labels Chapter 4: Functions and Program Structure 1. Basics of Functions 2. Functions Returning Non−integers 3. External Variables 4. Scope Rules 5. Header Files 6. Static Variables 7. Register Variables 8. Block Structure 9. Initialization 10. Recursion 11.The C Preprocessor 1. File Inclusion 2. Macro Substitution 3. Conditional Inclusion Chapter 5: Pointers and Arrays 1. Pointers and Addresses 2. Pointers and Function Arguments 3. Pointers and Arrays 4. Address Arithmetic 5. Character Pointers and Functions 6. Pointer Arrays; Pointers to Pointers 7. Multi−dimensional Arrays 8. Initialization of Pointer Arrays 9. Pointers vs. Multi−dimensional Arrays 10. Command−line Arguments 11. Pointers to Functions 12. Complicated Declarations Chapter 6: Structures 1. Basics of Structures 2. Structures and Functions 3. Arrays of Structures 4. Pointers to Structures 5. Self−referential Structures 6. Table Lookup 7. Typedef 8. Unions 9. Bit−fields Chapter 7: Input and Output 1. Standard Input and Output 2. Formatted Output − printf

                                                         3. Variable−length Argument Lists

4. Formatted Input − Scanf

5. File Access

6. Error Handling − Stderr and Exit

7. Line Input and Output

8.Miscellaneous Functions

1. String Operations

2. Character Class Testing and Conversion

3. Ungetc

4. Command Execution

5. Storage Management

6. Mathematical Functions

7. Random Number generation

Chapter 8: The UNIX System Interface

1. File Descriptors

2. Low Level I/O − Read and Write

3. Open, Creat, Close, Unlink

4. Random Access − Lseek

5. Example − An implementation of Fopen and Getc

6. Example − Listing Directories

7. Example − A Storage Allocator

Appendix A: Reference Manual

1. Introduction

2. Lexical Conventions

3. Syntax Notation

4. Meaning of Identifiers

5. Objects and Lvalues

6. Conversions

7. Expressions

8. Declarations

9. Statements

10. External Declarations

11. Scope and Linkage

12. Preprocessor

13. Grammar

·

Appendix B: Standard Library

A.Input and Output: <stdio.h>

1. File Operations

2. Formatted Output

3. Formatted Input

4. Character Input and Output Functions

5. Direct Input and Output Func      tions

6. File Positioning Functions

7. Error Functions

B. Character Class Tests: <ctype.h>

C String Functions: <string.h>

D. Mathematical Functions: <math.h>

E. Utility Functions: <stdlib.h>                           

Index −−  Preface to the first edition

Preface

The computing world has undergone a revolution since the publication of The C Programming Language in

1978. Big computers are much bigger, and personal computers have capabilities that rival mainframes of a

decade ago. During this time, C has changed too, although only modestly, and it has spread far beyond its

origins as the language of the UNIX operating system.

The growing popularity of C, the changes in the language over the years, and the creation of compilers by

groups not involved in its design, combined to demonstrate a need for a more precise and more contemporary

definition of the language than the first edition of this book provided. In 1983, the American National

Standards Institute (ANSI) established a committee whose goal was to produce ``an unambiguous and

machine−independent definition of the language C'', while still retaining its spirit. The result is the ANSI

standard for C.

The standard formalizes constructions that were hinted but not described in the first edition, particularly

structure assignment and enumerations. It provides a new form of function declaration that permits

cross−checking of definition with use. It specifies a standard library, with an extensive set of functions for

performing input and output, memory management, string manipulation, and similar tasks. It makes precise

the behavior of features that were not spelled out in the original definition, and at the same time states

explicitly which aspects of the language remain machine−dependent.

This Second Edition of The C Programming Language describes C as defined by the ANSI standard.

Although we have noted the places where the language has evolved, we have chosen to write exclusively in

the new form. For the most part, this makes no significant difference; the most visible change is the new form

of function declaration and definition. Modern compilers already support most features of the standard.

We have tried to retain the brevity of the first edition. C is not a big language, and it is not well served by a

big book. We have improved the exposition of critical features, such as pointers, that are central to C

programming. We have refined the original examples, and have added new examples in several chapters. For

instance, the treatment of complicated declarations is augmented by programs that convert declarations into

words and vice versa. As before, all examples have been tested directly from the text, which is in

machine−readable form.

Appendix A, the reference manual, is not the standard, but our attempt to convey the essentials of the standard

in a smaller space. It is meant for easy comprehension by programmers, but not as a definition for compiler

writers −− that role properly belongs to the standard itself. Appendix B is a summary of the facilities of the

standard library. It too is meant for reference by programmers, not implementers. Appendix C is a concise

summary of the changes from the original version.

As we said in the preface to the first edition, C ``wears well as one's experience with it grows''. With a decade

more experience, we still feel that way. We hope that this book will help you learn C and use it well.

We are deeply indebted to friends who helped us to produce this second edition. Jon Bently, Doug Gwyn,

Doug McIlroy, Peter Nelson, and Rob Pike gave us perceptive comments on almost every page of draft

manuscripts. We are grateful for careful reading by Al Aho, Dennis Allison, Joe Campbell, G.R. Emlin, Karen

Fortgang, Allen Holub, Andrew Hume, Dave Kristol, John Linderman, Dave Prosser, Gene Spafford, and

Chris van Wyk. We also received helpful suggestions from Bill Cheswick, Mark Kernighan, Andy Koenig,

Preface5

Robin Lake, Tom London, Jim Reeds, Clovis Tondo, and Peter Weinberger. Dave Prosser answered many

detailed questions about the ANSI standard. We used Bjarne Stroustrup's C++ translator extensively for local

testing of our programs, and Dave Kristol provided us with an ANSI C compiler for final testing. Rich

Drechsler helped greatly with typesetting.

Our sincere thanks to all.

Brian W. Kernighan

Dennis M. Ritchie

Index −−  Preface to the first edition

The C programming Language

Preface6

Back to the Preface −−  Index −−  Introduction

Preface to the first edition

C is a general−purpose programming language with features economy of expression, modern flow control and

data structures, and a rich set of operators. C is not a ``very high level'' language, nor a ``big'' one, and is not

specialized to any particular area of application. But its absence of restrictions and its generality make it more

convenient and effective for many tasks than supposedly more powerful languages.

C was originally designed for and implemented on the UNIX operating system on the DEC PDP−11, by

Dennis Ritchie. The operating system, the C compiler, and essentially all UNIX applications programs

(including all of the software used to prepare this book) are written in C. Production compilers also exist for

several other machines, including the IBM System/370, the Honeywell 6000, and the Interdata 8/32. C is not

tied to any particular hardware or system, however, and it is easy to write programs that will run without

change on any machine that supports C.

This book is meant to help the reader learn how to program in C. It contains a tutorial introduction to get new

users started as soon as possible, separate chapters on each major feature, and a reference manual. Most of the

treatment is based on reading, writing and revising examples, rather than on mere statements of rules. For the

most part, the examples are complete, real programs rather than isolated fragments. All examples have been

tested directly from the text, which is in machine−readable form. Besides showing how to make effective use

of the language, we have also tried where possible to illustrate useful algorithms and principles of good style

and sound design.

The book is not an introductory programming manual; it assumes some familiarity with basic programming

concepts like variables, assignment statements, loops, and functions. Nonetheless, a novice programmer

should be able to read along and pick up the language, although access to more knowledgeable colleague will

help.

In our experience, C has proven to be a pleasant, expressive and versatile language for a wide variety of

programs. It is easy to learn, and it wears well as on's experience with it grows. We hope that this book will

help you to use it well.

The thoughtful criticisms and suggestions of many friends and colleagues have added greatly to this book and

to our pleasure in writing it. In particular, Mike Bianchi, Jim Blue, Stu Feldman, Doug McIlroy Bill Roome,

Bob Rosin and Larry Rosler all read multiple volumes with care. We are also indebted to Al Aho, Steve

Bourne, Dan Dvorak, Chuck Haley, Debbie Haley, Marion Harris, Rick Holt, Steve Johnson, John Mashey,

Bob Mitze, Ralph Muha, Peter Nelson, Elliot Pinson, Bill Plauger, Jerry Spivack, Ken Thompson, and Peter

Weinberger for helpful comments at various stages, and to Mile Lesk and Joe Ossanna for invaluable

assistance with typesetting.

Brian W. Kernighan

Dennis M. Ritchie

Back to the Preface −−  Index −−  Introduction

The C programming Language

Preface to the first edition7

Back to the Preface to the First Edition −−  Index −−  Chapter 1

Introduction

C is a general−purpose programming language. It has been closely associated with the UNIX operating

system where it was developed, since both the system and most of the programs that run on it are written in C.

The language, however, is not tied to any one operating system or machine; and although it has been called a

``system programming language'' because it is useful for writing compilers and operating systems, it has been

used equally well to write major programs in many different domains.

Many of the important ideas of C stem from the language BCPL, developed by Martin Richards. The

influence of BCPL on C proceeded indirectly through the language B, which was written by Ken Thompson

in 1970 for the first UNIX system on the DEC PDP−7.

BCPL and B are ``typeless'' languages. By contrast, C provides a variety of data types. The fundamental types

are characters, and integers and floating point numbers of several sizes. In addition, there is a hierarchy of

derived data types created with pointers, arrays, structures and unions. Expressions are formed from operators

and operands; any expression, including an assignment or a function call, can be a statement. Pointers provide

for machine−independent address arithmetic.

C provides the fundamental control−flow constructions required for well−structured programs: statement

grouping, decision making (if−else), selecting one of a set of possible values (switch), looping with the

termination test at the top (while, for) or at the bottom (do), and early loop exit (break).

Functions may return values of basic types, structures, unions, or pointers. Any function may be called

recursively. Local variables are typically ``automatic'', or created anew with each invocation. Function

definitions may not be nested but variables may be declared in a block−structured fashion. The functions of a

C program may exist in separate source files that are compiled separately. Variables may be internal to a

function, external but known only within a single source file, or visible to the entire program.

A preprocessing step performs macro substitution on program text, inclusion of other source files, and

conditional compilation.

C is a relatively ``low−level'' language. This characterization is not pejorative; it simply means that C deals

with the same sort of objects that most computers do, namely characters, numbers, and addresses. These may

be combined and moved about with the arithmetic and logical operators implemented by real machines.

C provides no operations to deal directly with composite objects such as character strings, sets, lists or arrays.

There are no operations that manipulate an entire array or string, although structures may be copied as a unit.

The language does not define any storage allocation facility other than static definition and the stack discipline

provided by the local variables of functions; there is no heap or garbage collection. Finally, C itself provides

no input/output facilities; there are no READ or WRITE statements, and no built−in file access methods. All

of these higher−level mechanisms must be provided by explicitly called functions. Most C implementations

have included a reasonably standard collection of such functions.

Similarly, C offers only straightforward, single−thread control flow: tests, loops, grouping, and subprograms,

but not multiprogramming, parallel operations, synchronization, or coroutines.

The C programming Language

Introduction8

Although the absence of some of these features may seem like a grave deficiency, (``You mean I have to call

a function to compare two character strings?''), keeping the language down to modest size has real benefits.

Since C is relatively small, it can be described in small space, and learned quickly. A programmer can

reasonably expect to know and understand and indeed regularly use the entire language.

For many years, the definition of C was the reference manual in the first edition of The C Programming

Language. In 1983, the American National Standards Institute (ANSI) established a committee to provide a

modern, comprehensive definition of C. The resulting definition, the ANSI standard, or ``ANSI C'', was

completed in late 1988. Most of the features of the standard are already supported by modern compilers.

The standard is based on the original reference manual. The language is relatively little changed; one of the

goals of the standard was to make sure that most existing programs would remain valid, or, failing that, that

compilers could produce warnings of new behavior.

For most programmers, the most important change is the new syntax for declaring and defining functions. A

function declaration can now include a description of the arguments of the function; the definition syntax

changes to match. This extra information makes it much easier for compilers to detect errors caused by

mismatched arguments; in our experience, it is a very useful addition to the language.

There are other small−scale language changes. Structure assignment and enumerations, which had been

widely available, are now officially part of the language. Floating−point computations may now be done in

single precision. The properties of arithmetic, especially for unsigned types, are clarified. The preprocessor is

more elaborate. Most of these changes will have only minor effects on most programmers.

A second significant contribution of the standard is the definition of a library to accompany C. It specifies

functions for accessing the operating system (for instance, to read and write files), formatted input and output,

memory allocation, string manipulation, and the like. A collection of standard headers provides uniform

access to declarations of functions in data types. Programs that use this library to interact with a host system

are assured of compatible behavior. Most of the library is closely modeled on the ``standard I/O library'' of the

UNIX system. This library was described in the first edition, and has been widely used on other systems as

well. Again, most programmers will not see much change.

Because the data types and control structures provided by C are supported directly by most computers, the

run−time library required to implement self−contained programs is tiny. The standard library functions are

only called explicitly, so they can be avoided if they are not needed. Most can be written in C, and except for

the operating system details they conceal, are themselves portable.

Although C matches the capabilities of many computers, it is independent of any particular machine

architecture. With a little care it is easy to write portable programs, that is, programs that can be run without

change on a variety of hardware. The standard makes portability issues explicit, and prescribes a set of

constants that characterize the machine on which the program is run.

C is not a strongly−typed language, but as it has evolved, its type−checking has been strengthened. The

original definition of C frowned on, but permitted, the interchange of pointers and integers; this has long since

been eliminated, and the standard now requires the proper declarations and explicit conversions that had

already been enforced by good compilers. The new function declarations are another step in this direction.

Compilers will warn of most type errors, and there is no automatic conversion of incompatible data types.

Nevertheless, C retains the basic philosophy that programmers know what they are doing; it only requires that

they state their intentions explicitly.

The C programming Language

Introduction9

C, like any other language, has its blemishes. Some of the operators have the wrong precedence; some parts of

the syntax could be better. Nonetheless, C has proven to ben an extremely effective and expressive language

for a wide variety of programming applications.

The book is organized as follows. Chapter 1 is a tutorial on the central part of C. The purpose is to get the

reader started as quickly as possible, since we believe strongly that the way to learn a new language is to write

programs in it. The tutorial does assume a working knowledge of the basic elements of programming; there is

no explanation of computers, of compilation, nor of the meaning of an expression like n=n+1. Although we

have tried where possible to show useful programming techniques, the book is not intended to be a reference

work on data structures and algorithms; when forced to make a choice, we have concentrated on the language.

Chapters 2 through 6 discuss various aspects of C in more detail, and rather more formally, than does Chapter

1, although the emphasis is still on examples of complete programs, rather than isolated fragments. Chapter 2

deals with the basic data types, operators and expressions. Chapter 3 threats control flow: if−else,

switch, while, for, etc. Chapter 4 covers functions and program structure − external variables, scope

rules, multiple source files, and so on − and also touches on the preprocessor. Chapter 5 discusses pointers and

address arithmetic. Chapter 6 covers structures and unions.

Chapter 7 describes the standard library, which provides a common interface to the operating system. This

library is defined by the ANSI standard and is meant to be supported on all machines that support C, so

programs that use it for input, output, and other operating system access can be moved from one system to

another without change.

Chapter 8 describes an interface between C programs and the UNIX operating system, concentrating on

input/output, the file system, and storage allocation. Although some of this chapter is specific to UNIX

systems, programmers who use other systems should still find useful material here, including some insight

into how one version of the standard library is implemented, and suggestions on portability.

Appendix A contains a language reference manual. The official statement of the syntax and semantics of the C

language is the ANSI standard itself. That document, however, is intended foremost for compiler writers. The

reference manual here conveys the definition of the language more concisely and without the same legalistic

style. Appendix B is a summary of the standard library, again for users rather than implementers. Appendix C

is a short summary of changes from the original language. In cases of doubt, however, the standard and one's

own compiler remain the final authorities on the language.

Back to the Preface to the First Edition −−  Index −−  Chapter 1

The C programming Language

Introduction10

Back to Introduction −−  Index −−  Chapter 2

Chapter 1 − A Tutorial Introduction

Let us begin with a quick introduction in C. Our aim is to show the essential elements of the language in real

programs, but without getting bogged down in details, rules, and exceptions. At this point, we are not trying to

be complete or even precise (save that the examples are meant to be correct). We want to get you as quickly as

possible to the point where you can write useful programs, and to do that we have to concentrate on the

basics: variables and constants, arithmetic, control flow, functions, and the rudiments of input and output. We

are intentionally leaving out of this chapter features of C that are important for writing bigger programs. These

include pointers, structures, most of C's rich set of operators, several control−flow statements, and the

standard library.

This approach and its drawbacks. Most notable is that the complete story on any particular feature is not found

here, and the tutorial, by being brief, may also be misleading. And because the examples do not use the full

power of C, they are not as concise and elegant as they might be. We have tried to minimize these effects, but

be warned. Another drawback is that later chapters will necessarily repeat some of this chapter. We hope that

the repetition will help you more than it annoys.

In any case, experienced programmers should be able to extrapolate from the material in this chapter to their

own programming needs. Beginners should supplement it by writing small, similar programs of their own.

Both groups can use it as a framework on which to hang the more detailed descriptions that begin in Chapter

2.

1.1 Getting Started

The only way to learn a new programming language is by writing programs in it. The first program to write is

the same for all languages:

Print the words

hello, world

This is a big hurdle; to leap over it you have to be able to create the program text somewhere, compile it

successfully, load it, run it, and find out where your output went. With these mechanical details mastered,

everything else is comparatively easy.

In C, the program to print ``hello, world'' is

#include <stdio.h>

main()

{

printf("hello, world\n");

}

Just how to run this program depends on the system you are using. As a specific example, on the UNIX

operating system you must create the program in a file whose name ends in ``.c'', such as hello.c, then

compile it with the command

cc hello.c

The C programming Language

Chapter 1 − A Tutorial Introduction11

If you haven't botched anything, such as omitting a character or misspelling something, the compilation will

proceed silently, and make an executable file called a.out. If you run a.out by typing the command

a.out

it will print

hello, world

On other systems, the rules will be different; check with a local expert.

Now, for some explanations about the program itself. A C program, whatever its size, consists of functions

and variables. A function contains statements that specify the computing operations to be done, and variables

store values used during the computation. C functions are like the subroutines and functions in Fortran or the

procedures and functions of Pascal. Our example is a function named main. Normally you are at liberty to

give functions whatever names you like, but ``main'' is special − your program begins executing at the

beginning of main. This means that every program must have a main somewhere.

main will usually call other functions to help perform its job, some that you wrote, and others from libraries

that are provided for you. The first line of the program,

#include <stdio.h>

tells the compiler to include information about the standard input/output library; the line appears at the

beginning of many C source files. The standard library is described in Chapter 7 and Appendix B.

One method of communicating data between functions is for the calling function to provide a list of values,

called arguments, to the function it calls. The parentheses after the function name surround the argument list.

In this example, main is defined to be a function that expects no arguments, which is indicated by the empty

list ( ).

#include <stdio.h>                 include information about standard library

main()                                          define a function called main

that received no argument values

{                                   statements of main are enclosed in braces

printf("hello, world\n");              main calls library function printf

to print this sequence of characters

}                                         \n represents the newline character

The first C program

The statements of a function are enclosed in braces { }. The function main contains only one statement,

printf("hello, world\n");

A function is called by naming it, followed by a parenthesized list of arguments, so this calls the function

printf with the argument "hello, world\n". printf is a library function that prints output, in this

case the string of characters between the quotes.

A sequence of characters in double quotes, like "hello, world\n", is called a character string or string

constant. For the moment our only use of character strings will be as arguments for printf and other

functions.

The C programming Language

Chapter 1 − A Tutorial Introduction12

The sequence \n in the string is C notation for the newline character, which when printed advances the

output to the left margin on the next line. If you leave out the \n (a worthwhile experiment), you will find that

there is no line advance after the output is printed. You must use \n to include a newline character in the

printf argument; if you try something like

printf("hello, world

");

the C compiler will produce an error message.

printf never supplies a newline character automatically, so several calls may be used to build up an output

line in stages. Our first program could just as well have been written

#include <stdio.h>

main()

{

printf("hello, ");

printf("world");

printf("\n");

}

to produce identical output.

Notice that \n represents only a single character. An escape sequence like \n provides a general and

extensible mechanism for representing hard−to−type or invisible characters. Among the others that C provides

are \t for tab, \b for backspace, \" for the double quote and \\ for the backslash itself. There is a complete

list in Section 2.3.

Exercise 1−1. Run the ``hello, world'' program on your system. Experiment with leaving out parts of the

program, to see what error messages you get.

Exercise 1−2. Experiment to find out what happens when prints's argument string contains \c, where c is

some character not listed above.

1.2 Variables and Arithmetic Expressions

The next program uses the formula oC=(5/9)(oF−32) to print the following table of Fahrenheit temperatures

and their centigrade or Celsius equivalents:

1    −17

20   −6

40   4

60   15

80   26

100  37

120  48

140  60

160  71

180  82

200  93

220  104

240  115

260  126

1.2 Variables and Arithmetic Expressions13

280  137

300  148

The program itself still consists of the definition of a single function named main. It is longer than the one

that printed ``hello, world'', but not complicated. It introduces several new ideas, including comments,

declarations, variables, arithmetic expressions, loops , and formatted output.

#include <stdio.h>

/* print Fahrenheit−Celsius table

for fahr = 0, 20, ..., 300 */

main()

{

int fahr, celsius;

int lower, upper, step;

lower = 0;      /* lower limit of temperature scale */

upper = 300;    /* upper limit */

step = 20;      /* step size */

fahr = lower;

while (fahr <= upper) {

celsius = 5 * (fahr−32) / 9;

printf("%d\t%d\n", fahr, celsius);

fahr = fahr + step;

}

}

The two lines

/* print Fahrenheit−Celsius table

for fahr = 0, 20, ..., 300 */

are a comment, which in this case explains briefly what the program does. Any characters between /* and */

are ignored by the compiler; they may be used freely to make a program easier to understand. Comments may

appear anywhere where a blank, tab or newline can.

In C, all variables must be declared before they are used, usually at the beginning of the function before any

executable statements. A declaration announces the properties of variables; it consists of a name and a list of

variables, such as

int fahr, celsius;

int lower, upper, step;

The type int means that the variables listed are integers; by contrast with float, which means floating

point, i.e., numbers that may have a fractional part. The range of both int and float depends on the

machine you are using; 16−bits ints, which lie between −32768 and +32767, are common, as are 32−bit

ints. A float number is typically a 32−bit quantity, with at least six significant digits and magnitude

generally between about 10−38 and 1038.

C provides several other data types besides int and float, including:

char character − a single byte

short short integer

1.2 Variables and Arithmetic Expressions14

long long integer

double double−precision floating point

The size of these objects is also machine−dependent. There are also arrays, structures and unions of these

basic types, pointers to them, and functions that return them, all of which we will meet in due course.

Computation in the temperature conversion program begins with the assignment statements

lower = 0;

upper = 300;

step = 20;

which set the variables to their initial values. Individual statements are terminated by semicolons.

Each line of the table is computed the same way, so we use a loop that repeats once per output line; this is the

purpose of the while loop

while (fahr <= upper) {

...

}

The while loop operates as follows: The condition in parentheses is tested. If it is true (fahr is less than or

equal to upper), the body of the loop (the three statements enclosed in braces) is executed. Then the

condition is re−tested, and if true, the body is executed again. When the test becomes false (fahr exceeds

upper) the loop ends, and execution continues at the statement that follows the loop. There are no further

statements in this program, so it terminates.

The body of a while can be one or more statements enclosed in braces, as in the temperature converter, or a

single statement without braces, as in

while (i < j)

i = 2 * i;

In either case, we will always indent the statements controlled by the  while by one tab stop (which we

have shown as four spaces) so you can see at a glance which statements are inside the loop. The indentation

emphasizes the logical structure of the program. Although C compilers do not care about how a program

looks, proper indentation and spacing are critical in making programs easy for people to read. We recommend

writing only one statement per line, and using blanks around operators to clarify grouping. The position of

braces is less important, although people hold passionate beliefs. We have chosen one of several popular

styles. Pick a style that suits you, then use it consistently.

Most of the work gets done in the body of the loop. The Celsius temperature is computed and assigned to the

variable celsius by the statement

celsius = 5 * (fahr−32) / 9;

The reason for multiplying by 5 and dividing by 9 instead of just multiplying by 5/9 is that in C, as in many

other languages, integer division truncates: any fractional part is discarded. Since 5 and 9 are integers. 5/9

would be truncated to zero and so all the Celsius temperatures would be reported as zero.

This example also shows a bit more of how printf works. printf is a general−purpose output formatting

function, which we will describe in detail in Chapter 7. Its first argument is a string of characters to be printed,

with each % indicating where one of the other (second, third, ...) arguments is to be substituted, and in what

1.2 Variables and Arithmetic Expressions15

form it is to be printed. For instance, %d specifies an integer argument, so the statement

printf("%d\t%d\n", fahr, celsius);

causes the values of the two integers fahr and celsius to be printed, with a tab (\t) between them.

Each % construction in the first argument of printf is paired with the corresponding second argument, third

argument, etc.; they must match up properly by number and type, or you will get wrong answers.

By the way, printf is not part of the C language; there is no input or output defined in C itself. printf is

just a useful function from the standard library of functions that are normally accessible to C programs. The

behaviour of printf is defined in the ANSI standard, however, so its properties should be the same with any

compiler and library that conforms to the standard.

In order to concentrate on C itself, we don't talk much about input and output until chapter 7. In particular, we

will defer formatted input until then. If you have to input numbers, read the discussion of the function scanf

in Section 7.4. scanf is like printf, except that it reads input instead of writing output.

There are a couple of problems with the temperature conversion program. The simpler one is that the output

isn't very pretty because the numbers are not right−justified. That's easy to fix; if we augment each %d in the

printf statement with a width, the numbers printed will be right−justified in their fields. For instance, we

might say

printf("%3d %6d\n", fahr, celsius);

to print the first number of each line in a field three digits wide, and the second in a field six digits wide, like

this:

0     −17

20      −6

40       4

60      15

80      26

100      37

...

The more serious problem is that because we have used integer arithmetic, the Celsius temperatures are not

very accurate; for instance, 0oF is actually about −17.8oC, not −17. To get more accurate answers, we should

use floating−point arithmetic instead of integer. This requires some changes in the program. Here is the

second version:

#include <stdio.h>

/* print Fahrenheit−Celsius table

for fahr = 0, 20, ..., 300; floating−point version */

main()

{

float fahr, celsius;

float lower, upper, step;

lower = 0;      /* lower limit of temperatuire scale */

upper = 300;    /* upper limit */

step = 20;      /* step size */

fahr = lower;

1.2 Variables and Arithmetic Expressions16

while (fahr <= upper) {

celsius = (5.0/9.0) * (fahr−32.0);

printf("%3.0f %6.1f\n", fahr, celsius);

fahr = fahr + step;

}

}

This is much the same as before, except that fahr and celsius are declared to be float and the formula

for conversion is written in a more natural way. We were unable to use 5/9 in the previous version because

integer division would truncate it to zero. A decimal point in a constant indicates that it is floating point,

however, so 5.0/9.0 is not truncated because it is the ratio of two floating−point values.

If an arithmetic operator has integer operands, an integer operation is performed. If an arithmetic operator has

one floating−point operand and one integer operand, however, the integer will be converted to floating point

before the operation is done. If we had written (fahr−32), the 32 would be automatically converted to

floating point. Nevertheless, writing floating−point constants with explicit decimal points even when they

have integral values emphasizes their floating−point nature for human readers.

The detailed rules for when integers are converted to floating point are in Chapter 2. For now, notice that the

assignment

fahr = lower;

and the test

while (fahr <= upper)

also work in the natural way − the int is converted to float before the operation is done.

The printf conversion specification %3.0f says that a floating−point number (here fahr) is to be printed

at least three characters wide, with no decimal point and no fraction digits. %6.1f describes another number

(celsius) that is to be printed at least six characters wide, with 1 digit after the decimal point. The output

looks like this:

0   −17.8

20    −6.7

40     4.4

...

Width and precision may be omitted from a specification: %6f says that the number is to be at least six

characters wide; %.2f specifies two characters after the decimal point, but the width is not constrained; and

%f merely says to print the number as floating point.

%6d print as decimal integer, at least 6 characters wide

%f print as floating point

%6f print as floating point, at least 6 characters wide

%.2f print as floating point, 2 characters after decimal point

%6.2f print as floating point, at least 6 wide and 2 after decimal point

Among others, printf also recognizes %o for octal, %x for hexadecimal, %c for character, %s for character

string and %% for itself.

The C programming Language

1.2 Variables and Arithmetic Expressions17

Exercise 1−3. Modify the temperature conversion program to print a heading above the table.

Exercise 1−4. Write a program to print the corresponding Celsius to Fahrenheit table.

1.3 The for statement

There are plenty of different ways to write a program for a particular task. Let's try a variation on the

temperature converter.

#include <stdio.h>

/* print Fahrenheit−Celsius table */

main()

{

int fahr;

for (fahr = 0; fahr <= 300; fahr = fahr + 20)

printf("%3d %6.1f\n", fahr, (5.0/9.0)*(fahr−32));

}

This produces the same answers, but it certainly looks different. One major change is the elimination of most

of the variables; only fahr remains, and we have made it an int. The lower and upper limits and the step

size appear only as constants in the for statement, itself a new construction, and the expression that computes

the Celsius temperature now appears as the third argument of printf instead of a separate assignment

statement.

This last change is an instance of a general rule − in any context where it is permissible to use the value of

some type, you can use a more complicated expression of that type. Since the third argument of printf must

be a floating−point value to match the %6.1f, any floating−point expression can occur here.

The for statement is a loop, a generalization of the while. If you compare it to the earlier while, its

operation should be clear. Within the parentheses, there are three parts, separated by semicolons. The first

part, the initialization

fahr = 0

is done once, before the loop proper is entered. The second part is the test or condition that controls the loop:

fahr <= 300

This condition is evaluated; if it is true, the body of the loop (here a single ptintf) is executed. Then the

increment step

fahr = fahr + 20

is executed, and the condition re−evaluated. The loop terminates if the condition has become false. As with

the while, the body of the loop can be a single statement or a group of statements enclosed in braces. The

initialization, condition and increment can be any expressions.

The choice between while and for is arbitrary, based on which seems clearer. The for is usually

appropriate for loops in which the initialization and increment are single statements and logically related,

since it is more compact than while and it keeps the loop control statements together in one place.

The C programming Language

1.3 The for statement18

Exercise 1−5. Modify the temperature conversion program to print the table in reverse order, that is, from 300

degrees to 0.

1.4 Symbolic Constants

A final observation before we leave temperature conversion forever. It's bad practice to bury ``magic

numbers'' like 300 and 20 in a program; they convey little information to someone who might have to read the

program later, and they are hard to change in a systematic way. One way to deal with magic numbers is to

give them meaningful names. A #define line defines a symbolic name or symbolic constant to be a

particular string of characters:

#define name replacement list

Thereafter, any occurrence of name (not in quotes and not part of another name) will be replaced by the

corresponding replacement text. The name has the same form as a variable name: a sequence of letters and

digits that begins with a letter. The replacement text can be any sequence of characters; it is not limited to

numbers.

#include <stdio.h>

#define LOWER  0     /* lower limit of table */

#define UPPER  300   /* upper limit */

#define STEP   20    /* step size */

/* print Fahrenheit−Celsius table */

main()

{

int fahr;

for (fahr = LOWER; fahr <= UPPER; fahr = fahr + STEP)

printf("%3d %6.1f\n", fahr, (5.0/9.0)*(fahr−32));

}

The quantities LOWER, UPPER and STEP are symbolic constants, not variables, so they do not appear in

declarations. Symbolic constant names are conventionally written in upper case so they can ber readily

distinguished from lower case variable names. Notice that there is no semicolon at the end of a #define

line.

1.5 Character Input and Output

We are going to consider a family of related programs for processing character data. You will find that many

programs are just expanded versions of the prototypes that we discuss here.

The model of input and output supported by the standard library is very simple. Text input or output,

regardless of where it originates or where it goes to, is dealt with as streams of characters. A text stream is a

sequence of characters divided into lines; each line consists of zero or more characters followed by a newline

character. It is the responsibility of the library to make each input or output stream confirm this model; the C

programmer using the library need not worry about how lines are represented outside the program.

The standard library provides several functions for reading or writing one character at a time, of which

getchar and putchar are the simplest. Each time it is called, getchar reads the next input character

from a text stream and returns that as its value. That is, after

The C programming Language

1.4 Symbolic Constants19

c = getchar();

the variable c contains the next character of input. The characters normally come from the keyboard; input

from files is discussed in Chapter 7.

The function putchar prints a character each time it is called:

putchar(c);

prints the contents of the integer variable c as a character, usually on the screen. Calls to putchar and

printf may be interleaved; the output will appear in the order in which the calls are made.

1.5.1 File Copying

Given getchar and putchar, you can write a surprising amount of useful code without knowing anything

more about input and output. The simplest example is a program that copies its input to its output one

character at a time:

read a character

while (charater is not end−of−file indicator)

output the character just read

read a character

Converting this into C gives:

#include <stdio.h>

/* copy input to output; 1st version  */

main()

{

int c;

c = getchar();

while (c != EOF) {

putchar(c);

c = getchar();

}

}

The relational operator != means ``not equal to''.

What appears to be a character on the keyboard or screen is of course, like everything else, stored internally

just as a bit pattern. The type char is specifically meant for storing such character data, but any integer type

can be used. We used int for a subtle but important reason.

The problem is distinguishing the end of input from valid data. The solution is that getchar returns a

distinctive value when there is no more input, a value that cannot be confused with any real character. This

value is called EOF, for ``end of file''. We must declare c to be a type big enough to hold any value that

getchar returns. We can't use char since c must be big enough to hold EOF in addition to any possible

char. Therefore we use int.

EOF is an integer defined in <stdio.h>, but the specific numeric value doesn't matter as long as it is not the

same as any char value. By using the symbolic constant, we are assured that nothing in the program depends

on the specific numeric value.

The C programming Language

1.5.1 File Copying20

The program for copying would be written more concisely by experienced C programmers. In C, any

assignment, such as

c = getchar();

is an expression and has a value, which is the value of the left hand side after the assignment. This means that

a assignment can appear as part of a larger expression. If the assignment of a character to c is put inside the

test part of a while loop, the copy program can be written this way:

#include <stdio.h>

/* copy input to output; 2nd version  */

main()

{

int c;

while ((c = getchar()) != EOF)

putchar(c);

}

The while gets a character, assigns it to c, and then tests whether the character was the end−of−file signal. If

it was not, the body of the while is executed, printing the character. The while then repeats. When the end

of the input is finally reached, the while terminates and so does main.

This version centralizes the input − there is now only one reference to getchar − and shrinks the program.

The resulting program is more compact, and, once the idiom is mastered, easier to read. You'll see this style

often. (It's possible to get carried away and create impenetrable code, however, a tendency that we will try to

curb.)

The parentheses around the assignment, within the condition are necessary. The precedence of != is higher

than that of =, which means that in the absence of parentheses the relational test != would be done before the

assignment =. So the statement

c = getchar() != EOF

is equivalent to

c = (getchar() != EOF)

This has the undesired effect of setting c to 0 or 1, depending on whether or not the call of getchar returned

end of file. (More on this in Chapter 2.)

Exercsise 1−6. Verify that the expression getchar() != EOF is 0 or 1.

Exercise 1−7. Write a program to print the value of EOF.

1.5.2 Character Counting

The next program counts characters; it is similar to the copy program.

#include <stdio.h>

/* count characters in input; 1st version */

1.5.2 Character Counting21

main()

{

long nc;

nc = 0;

while (getchar() != EOF)

++nc;

printf("%ld\n", nc);

}

The statement

++nc;

presents a new operator, ++, which means increment by one. You could instead write nc = nc + 1 but

++nc is more concise and often more efficient. There is a corresponding operator −− to decrement by 1. The

operators ++ and −− can be either prefix operators (++nc) or postfix operators (nc++); these two forms have

different values in expressions, as will be shown in Chapter 2, but ++nc and nc++ both increment nc. For

the moment we will will stick to the prefix form.

The character counting program accumulates its count in a long variable instead of an int. long integers are

at least 32 bits. Although on some machines, int and long are the same size, on others an int is 16 bits,

with a maximum value of 32767, and it would take relatively little input to overflow an int counter. The

conversion specification %ld tells printf that the corresponding argument is a long integer.

It may be possible to cope with even bigger numbers by using a double (double precision float). We will

also use a for statement instead of a while, to illustrate another way to write the loop.

#include <stdio.h>

/* count characters in input; 2nd version */

main()

{

double nc;

for (nc = 0; gechar() != EOF; ++nc)

;

printf("%.0f\n", nc);

}

printf uses %f for both float and double; %.0f suppresses the printing of the decimal point and the

fraction part, which is zero.

The body of this for loop is empty, because all the work is done in the test and increment parts. But the

grammatical rules of C require that a for statement have a body. The isolated semicolon, called a null

statement, is there to satisfy that requirement. We put it on a separate line to make it visible.

Before we leave the character counting program, observe that if the input contains no characters, the while

or for test fails on the very first call to getchar, and the program produces zero, the right answer. This is

important. One of the nice things about while and for is that they test at the top of the loop, before

proceeding with the body. If there is nothing to do, nothing is done, even if that means never going through

the loop body. Programs should act intelligently when given zero−length input. The while and for

statements help ensure that programs do reasonable things with boundary conditions.

1.5.2 Character Counting22

1.5.3 Line Counting

The next program counts input lines. As we mentioned above, the standard library ensures that an input text

stream appears as a sequence of lines, each terminated by a newline. Hence, counting lines is just counting

newlines:

#include <stdio.h>

/* count lines in input */

main()

{

int c, nl;

nl = 0;

while ((c = getchar()) != EOF)

if (c == '\n')

++nl;

printf("%d\n", nl);

}

The body of the while now consists of an if, which in turn controls the increment ++nl. The if statement

tests the parenthesized condition, and if the condition is true, executes the statement (or group of statements in

braces) that follows. We have again indented to show what is controlled by what.

The double equals sign == is the C notation for ``is equal to'' (like Pascal's single = or Fortran's .EQ.). This

symbol is used to distinguish the equality test from the single = that C uses for assignment. A word of caution:

newcomers to C occasionally write = when they mean ==. As we will see in Chapter 2, the result is usually a

legal expression, so you will get no warning.

A character written between single quotes represents an integer value equal to the numerical value of the

character in the machine's character set. This is called a character constant, although it is just another way to

write a small integer. So, for example, 'A' is a character constant; in the ASCII character set its value is 65,

the internal representation of the character A. Of course, 'A' is to be preferred over 65: its meaning is

obvious, and it is independent of a particular character set.

The escape sequences used in string constants are also legal in character constants, so '\n' stands for the

value of the newline character, which is 10 in ASCII. You should note carefully that '\n' is a single

character, and in expressions is just an integer; on the other hand, '\n' is a string constant that happens to

contain only one character. The topic of strings versus characters is discussed further in Chapter 2.

Exercise 1−8. Write a program to count blanks, tabs, and newlines.

Exercise 1−9. Write a program to copy its input to its output, replacing each string of one or more blanks by a

single blank.

Exercise 1−10. Write a program to copy its input to its output, replacing each tab by \t, each backspace by

\b, and each backslash by \\. This makes tabs and backspaces visible in an unambiguous way.

1.5.4 Word Counting

The fourth in our series of useful programs counts lines, words, and characters, with the loose definition that a

word is any sequence of characters that does not contain a blank, tab or newline. This is a bare−bones version

1.5.3 Line Counting23

of the UNIX program wc.

#include <stdio.h>

#define IN   1  /* inside a word */

#define OUT  0  /* outside a word */

/* count lines, words, and characters in input */

main()

{

int c, nl, nw, nc, state;

state = OUT;

nl = nw = nc = 0;

while ((c = getchar()) != EOF) {

++nc;

if (c == '\n')

++nl;

if (c == ' ' || c == '\n' || c = '\t')

state = OUT;

else if (state == OUT) {

state = IN;

++nw;

}

}

printf("%d %d %d\n", nl, nw, nc);

}

Every time the program encounters the first character of a word, it counts one more word. The variable

state records whether the program is currently in a word or not; initially it is ``not in a word'', which is

assigned the value OUT. We prefer the symbolic constants IN and OUT to the literal values 1 and 0 because

they make the program more readable. In a program as tiny as this, it makes little difference, but in larger

programs, the increase in clarity is well worth the modest extra effort to write it this way from the beginning.

You'll also find that it's easier to make extensive changes in programs where magic numbers appear only as

symbolic constants.

The line

nl = nw = nc = 0;

sets all three variables to zero. This is not a special case, but a consequence of the fact that an assignment is an

expression with the value and assignments associated from right to left. It's as if we had written

nl = (nw = (nc = 0));

The operator || means OR, so the line

if (c == ' ' || c == '\n' || c = '\t')

says ``if c is a blank orc is a newline orc is a tab''. (Recall that the escape sequence \t is a visible

representation of the tab character.) There is a corresponding operator && for AND; its precedence is just

higher than ||. Expressions connected by && or || are evaluated left to right, and it is guaranteed that

evaluation will stop as soon as the truth or falsehood is known. If c is a blank, there is no need to test whether

it is a newline or tab, so these tests are not made. This isn't particularly important here, but is significant in

more complicated situations, as we will soon see.

1.5.3 Line Counting24

The example also shows an else, which specifies an alternative action if the condition part of an if

statement is false. The general form is

if (expression)

statement1

else

statement2

One and only one of the two statements associated with an if−else is performed. If the expression is true,

statement1 is executed; if not, statement2 is executed. Each statement can be a single statement or several in

braces. In the word count program, the one after the else is an if that controls two statements in braces.

Exercise 1−11. How would you test the word count program? What kinds of input are most likely to uncover

bugs if there are any?

Exercise 1−12. Write a program that prints its input one word per line.

1.6 Arrays

Let is write a program to count the number of occurrences of each digit, of white space characters (blank, tab,

newline), and of all other characters. This is artificial, but it permits us to illustrate several aspects of C in one

program.

There are twelve categories of input, so it is convenient to use an array to hold the number of occurrences of

each digit, rather than ten individual variables. Here is one version of the program:

#include <stdio.h>

/* count digits, white space, others */

main()

{

int c, i, nwhite, nother;

int ndigit[10];

nwhite = nother = 0;

for (i = 0; i < 10; ++i)

ndigit[i] = 0;

while ((c = getchar()) != EOF)

if (c >= '0' &c <= '9')

++ndigit[c−'0'];

else if (c == ' ' || c == '\n' || c == '\t')

++nwhite;

else

++nother;

printf("digits =");

for (i = 0; i < 10; ++i)

printf(" %d", ndigit[i]);

printf(", white space = %d, other = %d\n",

nwhite, nother);

}

The output of this program on itself is

1.6 Arrays25

digits = 9 3 0 0 0 0 0 0 0 1, white space = 123, other = 345

The declaration

int ndigit[10];

declares ndigit to be an array of 10 integers. Array subscripts always start at zero in C, so the elements are

ndigit[0], ndigit[1], ..., ndigit[9]. This is reflected in the for loops that initialize and

print the array.

A subscript can be any integer expression, which includes integer variables like i, and integer constants.

This particular program relies on the properties of the character representation of the digits. For example, the

test

if (c >= '0' &c <= '9')

determines whether the character in c is a digit. If it is, the numeric value of that digit is

c − '0'

This works only if '0', '1', ..., '9' have consecutive increasing values. Fortunately, this is true for

all character sets.

By definition, chars are just small integers, so char variables and constants are identical to ints in

arithmetic expressions. This is natural and convenient; for example c−'0' is an integer expression with a

value between 0 and 9 corresponding to the character '0' to '9' stored in c, and thus a valid subscript for

the array ndigit.

The decision as to whether a character is a digit, white space, or something else is made with the sequence

if (c >= '0' &c <= '9')

++ndigit[c−'0'];

else if (c == ' ' || c == '\n' || c == '\t')

++nwhite;

else

++nother;

The pattern

if (condition1)

statement1

else if (condition2)

statement2

...

...

else

statementn

occurs frequently in programs as a way to express a multi−way decision. The conditions are evaluated in

order from the top until some condition is satisfied; at that point the corresponding statement part is executed,

and the entire construction is finished. (Any statement can be several statements enclosed in braces.) If none

of the conditions is satisfied, the statement after the final else is executed if it is present. If the final else

and statement are omitted, as in the word count program, no action takes place. There can be any number of

The C programming Language

1.6 Arrays26

else if(condition)

statement

groups between the initial if and the final else.

As a matter of style, it is advisable to format this construction as we have shown; if each if were indented

past the previous else, a long sequence of decisions would march off the right side of the page.

The switch statement, to be discussed in Chapter 4, provides another way to write a multi−way branch that

is particulary suitable when the condition is whether some integer or character expression matches one of a set

of constants. For contrast, we will present a switch version of this program in Section 3.4.

Exercise 1−13. Write a program to print a histogram of the lengths of words in its input. It is easy to draw the

histogram with the bars horizontal; a vertical orientation is more challenging.

Exercise 1−14. Write a program to print a histogram of the frequencies of different characters in its input.

1.7 Functions

In C, a function is equivalent to a subroutine or function in Fortran, or a procedure or function in Pascal. A

function provides a convenient way to encapsulate some computation, which can then be used without

worrying about its implementation. With properly designed functions, it is possible to ignore how a job is

done; knowing what is done is sufficient. C makes the sue of functions easy, convinient and efficient; you will

often see a short function defined and called only once, just because it clarifies some piece of code.

So far we have used only functions like printf, getchar and putchar that have been provided for us;

now it's time to write a few of our own. Since C has no exponentiation operator like the ** of Fortran, let us

illustrate the mechanics of function definition by writing a function power(m,n) to raise an integer m to a

positive integer power n. That is, the value of power(2,5) is 32. This function is not a practical

exponentiation routine, since it handles only positive powers of small integers, but it's good enough for

illustration.(The standard library contains a function pow(x,y) that computes xy.)

Here is the function power and a main program to exercise it, so you can see the whole structure at once.

#include <stdio.h>

int power(int m, int n);

/* test power function */

main()

{

int i;

for (i = 0; i < 10; ++i)

printf("%d %d %d\n", i, power(2,i), power(−3,i));

return 0;

}

/* power:  raise base to n−th power; n >= 0 */

int power(int base, int n)

{

int i,  p;

p = 1;

1.7 Functions27

for (i = 1; i <= n; ++i)

p = p * base;

return p;

}

A function definition has this form:

return−type function−name(parameter declarations, if any)

{

declarations

statements

}

Function definitions can appear in any order, and in one source file or several, although no function can be

split between files. If the source program appears in several files, you may have to say more to compile and

load it than if it all appears in one, but that is an operating system matter, not a language attribute. For the

moment, we will assume that both functions are in the same file, so whatever you have learned about running

C programs will still work.

The function power is called twice by main, in the line

printf("%d %d %d\n", i, power(2,i), power(−3,i));

Each call passes two arguments to power, which each time returns an integer to be formatted and printed. In

an expression, power(2,i) is an integer just as 2 and i are. (Not all functions produce an integer value; we

will take this up in Chapter 4.)

The first line of power itself,

int power(int base, int n)

declares the parameter types and names, and the type of the result that the function returns. The names used

by power for its parameters are local to power, and are not visible to any other function: other routines can

use the same names without conflict. This is also true of the variables i and p: the i in power is unrelated to

the i in main.

We will generally use parameter for a variable named in the parenthesized list in a function. The terms formal

argument and actual argument are sometimes used for the same distinction.

The value that power computes is returned to main by the return: statement. Any expression may follow

return:

return expression;

A function need not return a value; a return statement with no expression causes control, but no useful value,

to be returned to the caller, as does ``falling off the end'' of a function by reaching the terminating right brace.

And the calling function can ignore a value returned by a function.

You may have noticed that there is a return statement at the end of main. Since main is a function like

any other, it may return a value to its caller, which is in effect the environment in which the program was

executed. Typically, a return value of zero implies normal termination; non−zero values signal unusual or

erroneous termination conditions. In the interests of simplicity, we have omitted return statements from our

main functions up to this point, but we will include them hereafter, as a reminder that programs should return

1.7 Functions28

status to their environment.

The declaration

int power(int base, int n);

just before main says that power is a function that expects two int arguments and returns an int. This

declaration, which is called a function prototype, has to agree with the definition and uses of power. It is an

error if the definition of a function or any uses of it do not agree with its prototype.

parameter names need not agree. Indeed, parameter names are optional in a function prototype, so for the

prototype we could have written

int power(int, int);

Well−chosen names are good documentation however, so we will often use them.

A note of history: the biggest change between ANSI C and earlier versions is how functions are declared and

defined. In the original definition of C, the power function would have been written like this:

/* power:  raise base to n−th power; n >= 0 */

/*         (old−style version) */

power(base, n)

int base, n;

{

int i, p;

p = 1;

for (i = 1; i <= n; ++i)

p = p * base;

return p;

}

The parameters are named between the parentheses, and their types are declared before opening the left brace;

undeclared parameters are taken as int. (The body of the function is the same as before.)

The declaration of power at the beginning of the program would have looked like this:

int power();

No parameter list was permitted, so the compiler could not readily check that power was being called

correctly. Indeed, since by default power would have been assumed to return an int, the entire declaration

might well have been omitted.

The new syntax of function prototypes makes it much easier for a compiler to detect errors in the number of

arguments or their types. The old style of declaration and definition still works in ANSI C, at least for a

transition period, but we strongly recommend that you use the new form when you have a compiler that

supports it.

Exercise 1.15. Rewrite the temperature conversion program of Section 1.2 to use a function for conversion.

1.7 Functions29

1.8 Arguments − Call by Value

One aspect of C functions may be unfamiliar to programmers who are used to some other languages,

particulary Fortran. In C, all function arguments are passed ``by value.'' This means that the called function is

given the values of its arguments in temporary variables rather than the originals. This leads to some different

properties than are seen with ``call by reference'' languages like Fortran or with var parameters in Pascal, in

which the called routine has access to the original argument, not a local copy.

Call by value is an asset, however, not a liability. It usually leads to more compact programs with fewer

extraneous variables, because parameters can be treated as conveniently initialized local variables in the called

routine. For example, here is a version of power that makes use of this property.

/* power:  raise base to n−th power; n >= 0; version 2 */

int power(int base, int n)

{

int p;

for (p = 1; n > 0; −−n)

p = p * base;

return p;

}

The parameter n is used as a temporary variable, and is counted down (a for loop that runs backwards) until

it becomes zero; there is no longer a need for the variable i. Whatever is done to n inside  power has no

effect on the argument that power was originally called with.

When necessary, it is possible to arrange for a function to modify a variable in a calling routine. The caller

must provide the address of the variable to be set (technically a pointer to the variable), and the called

function must declare the parameter to be a pointer and access the variable indirectly through it. We will cover

pointers in Chapter 5.

The story is different for arrays. When the name of an array is used as an argument, the value passed to the

function is the location or address of the beginning of the array − there is no copying of array elements. By

subscripting this value, the function can access and alter any argument of the array. This is the topic of the

next section.

1.9 Character Arrays

The most common type of array in C is the array of characters. To illustrate the use of character arrays and

functions to manipulate them, let's write a program that reads a set of text lines and prints the longest. The

outline is simple enough:

while (there's another line)

if (it's longer than the previous longest)

(save it)

(save its length)

print longest line

This outline makes it clear that the program divides naturally into pieces. One piece gets a new line, another

saves it, and the rest controls the process.

1.8 Arguments − Call by Value30

Since things divide so nicely, it would be well to write them that way too. Accordingly, let us first write a

separate function getline to fetch the next line of input. We will try to make the function useful in other

contexts. At the minimum, getline has to return a signal about possible end of file; a more useful design

would be to return the length of the line, or zero if end of file is encountered. Zero is an acceptable

end−of−file return because it is never a valid line length. Every text line has at least one character; even a line

containing only a newline has length 1.

When we find a line that is longer than the previous longest line, it must be saved somewhere. This suggests a

second function, copy, to copy the new line to a safe place.

Finally, we need a main program to control getline and copy. Here is the result.

#include <stdio.h>

#define MAXLINE 1000   /* maximum input line length */

int getline(char line[], int maxline);

void copy(char to[], char from[]);

/* print the longest input line */

main()

{

int len;            /* current line length */

int max;            /* maximum length seen so far */

char line[MAXLINE];    /* current input line */

char longest[MAXLINE]; /* longest line saved here */

max = 0;

while ((len = getline(line, MAXLINE)) > 0)

if (len > max) {

max = len;

copy(longest, line);

}

if (max > 0)  /* there was a line */

printf("%s", longest);

return 0;

}

/* getline:  read a line into s, return length  */

int getline(char s[],int lim)

{

int c, i;

for (i=0; i < lim−1 &(c=getchar())!=EOF &c!='\n'; ++i)

s[i] = c;

if (c == '\n') {

s[i] = c;

++i;

}

s[i] = '\0';

return i;

}

/* copy:  copy 'from' into 'to'; assume to is big enough */

void copy(char to[], char from[])

{

int i;

i = 0;

while ((to[i] = from[i]) != '\0')

1.8 Arguments − Call by Value31

++i;

}

The functions getline and copy are declared at the beginning of the program, which we assume is

contained in one file.

main and getline communicate through a pair of arguments and a returned value. In getline, the

arguments are declared by the line

int getline(char s[], int lim);

which specifies that the first argument, s, is an array, and the second, lim, is an integer. The purpose of

supplying the size of an array in a declaration is to set aside storage. The length of an array s is not necessary

in getline since its size is set in main. getline uses return to send a value back to the caller, just as

the function power did. This line also declares that getline returns an int; since int is the default return

type, it could be omitted.

Some functions return a useful value; others, like copy, are used only for their effect and return no value. The

return type of copy is void, which states explicitly that no value is returned.

getline puts the character '\0' (the null character, whose value is zero) at the end of the array it is

creating, to mark the end of the string of characters. This conversion is also used by the C language: when a

string constant like

"hello\n"

appears in a C program, it is stored as an array of characters containing the characters in the string and

terminated with a '\0' to mark the end.

The %s format specification in printf expects the corresponding argument to be a string represented in this

form. copy also relies on the fact that its input argument is terminated with a '\0', and copies this character

into the output.

It is worth mentioning in passing that even a program as small as this one presents some sticky design

problems. For example, what should main do if it encounters a line which is bigger than its limit? getline

works safely, in that it stops collecting when the array is full, even if no newline has been seen. By testing the

length and the last character returned, main can determine whether the line was too long, and then cope as it

wishes. In the interests of brevity, we have ignored this issue.

There is no way for a user of getline to know in advance how long an input line might be, so getline

checks for overflow. On the other hand, the user of copy already knows (or can find out) how big the strings

are, so we have chosen not to add error checking to it.

Exercise 1−16. Revise the main routine of the longest−line program so it will correctly print the length of

arbitrary long input lines, and as much as possible of the text.

Exercise 1−17. Write a program to print all input lines that are longer than 80 characters.

1.8 Arguments − Call by Value32

Exercise 1−18. Write a program to remove trailing blanks and tabs from each line of input, and to delete

entirely blank lines.

Exercise 1−19. Write a function reverse(s) that reverses the character string s. Use it to write a program

that reverses its input a line at a time.

1.10 External Variables and Scope

The variables in main, such as line, longest, etc., are private or local to main. Because they are

declared within main, no other function can have direct access to them. The same is true of the variables in

other functions; for example, the variable i in  getline is unrelated to the i in copy. Each local variable in

a function comes into existence only when the function is called, and disappears when the function is exited.

This is why such variables are usually known as automatic variables, following terminology in other

languages. We will use the term automatic henceforth to refer to these local variables. (Chapter 4 discusses

the static storage class, in which local variables do retain their values between calls.)

Because automatic variables come and go with function invocation, they do not retain their values from one

call to the next, and must be explicitly set upon each entry. If they are not set, they will contain garbage.

As an alternative to automatic variables, it is possible to define variables that are external to all functions, that

is, variables that can be accessed by name by any function. (This mechanism is rather like Fortran COMMON

or Pascal variables declared in the outermost block.) Because external variables are globally accessible, they

can be used instead of argument lists to communicate data between functions. Furthermore, because external

variables remain in existence permanently, rather than appearing and disappearing as functions are called and

exited, they retain their values even after the functions that set them have returned.

An external variable must be defined, exactly once, outside of any function; this sets aside storage for it. The

variable must also be declared in each function that wants to access it; this states the type of the variable. The

declaration may be an explicit extern statement or may be implicit from context. To make the discussion

concrete, let us rewrite the longest−line program with line, longest, and max as external variables. This

requires changing the calls, declarations, and bodies of all three functions.

#include <stdio.h>

#define MAXLINE 1000    /* maximum input line size */

int max;                /* maximum length seen so far */

char line[MAXLINE];     /* current input line */

char longest[MAXLINE];  /* longest line saved here */

int getline(void);

void copy(void);

/* print longest input line; specialized version */

main()

{

int len;

extern int max;

extern char longest[];

max = 0;

while ((len = getline()) > 0)

if (len > max) {

max = len;

1.10 External Variables and Scope33

copy();

}

if (max > 0)  /* there was a line */

printf("%s", longest);

return 0;

}

/* getline:  specialized version */

int getline(void)

{

int c, i;

extern char line[];

for (i = 0; i < MAXLINE − 1

&(c=getchar)) != EOF &c != '\n'; ++i)

line[i] = c;

if (c == '\n') {

line[i] = c;

++i;

}

line[i] = '\0';

return i;

}

/* copy: specialized version */

void copy(void)

{

int i;

extern char line[], longest[];

i = 0;

while ((longest[i] = line[i]) != '\0')

++i;

}

The external variables in main, getline and copy are defined by the first lines of the example above,

which state their type and cause storage to be allocated for them. Syntactically, external definitions are just

like definitions of local variables, but since they occur outside of functions, the variables are external. Before

a function can use an external variable, the name of the variable must be made known to the function; the

declaration is the same as before except for the added keyword extern.

In certain circumstances, the extern declaration can be omitted. If the definition of the external variable

occurs in the source file before its use in a particular function, then there is no need for an extern

declaration in the function. The extern declarations in main, getline and copy are thus redundant. In

fact, common practice is to place definitions of all external variables at the beginning of the source file, and

then omit all extern declarations.

If the program is in several source files, and a variable is defined in file1 and used in file2 and file3, then

extern declarations are needed in file2 and file3 to connect the occurrences of the variable. The usual

practice is to collect extern declarations of variables and functions in a separate file, historically called a

header, that is included by #include at the front of each source file. The suffix .h is conventional for

header names. The functions of the standard library, for example, are declared in headers like <stdio.h>.

This topic is discussed at length in Chapter 4, and the library itself in Chapter 7 and Appendix B.

Since the specialized versions of getline and copy have no arguments, logic would suggest that their

prototypes at the beginning of the file should be getline() and copy(). But for compatibility with older

1.10 External Variables and Scope34

C programs the standard takes an empty list as an old−style declaration, and turns off all argument list

checking; the word void must be used for an explicitly empty list. We will discuss this further in Chapter 4.

You should note that we are using the words definition and declaration carefully when we refer to external

variables in this section.``Definition'' refers to the place where the variable is created or assigned storage;

``declaration'' refers to places where the nature of the variable is stated but no storage is allocated.

By the way, there is a tendency to make everything in sight an extern variable because it appears to

simplify communications − argument lists are short and variables are always there when you want them. But

external variables are always there even when you don't want them. Relying too heavily on external variables

is fraught with peril since it leads to programs whose data connections are not all obvious − variables can be

changed in unexpected and even inadvertent ways, and the program is hard to modify. The second version of

the longest−line program is inferior to the first, partly for these reasons, and partly because it destroys the

generality of two useful functions by writing into them the names of the variables they manipulate.

At this point we have covered what might be called the conventional core of C. With this handful of building

blocks, it's possible to write useful programs of considerable size, and it would probably be a good idea if you

paused long enough to do so. These exercises suggest programs of somewhat greater complexity than the ones

earlier in this chapter.

Exercise 1−20. Write a program detab that replaces tabs in the input with the proper number of blanks to

space to the next tab stop. Assume a fixed set of tab stops, say every n columns. Should n be a variable or a

symbolic parameter?

Exercise 1−21. Write a program entab that replaces strings of blanks by the minimum number of tabs and

blanks to achieve the same spacing. Use the same tab stops as for detab. When either a tab or a single blank

would suffice to reach a tab stop, which should be given preference?

Exercise 1−22. Write a program to ``fold'' long input lines into two or more shorter lines after the last

non−blank character that occurs before the n−th column of input. Make sure your program does something

intelligent with very long lines, and if there are no blanks or tabs before the specified column.

Exercise 1−23. Write a program to remove all comments from a C program. Don't forget to handle quoted

strings and character constants properly. C comments don't nest.

Exercise 1−24. Write a program to check a C program for rudimentary syntax errors like unmatched

parentheses, brackets and braces. Don't forget about quotes, both single and double, escape sequences, and

comments. (This program is hard if you do it in full generality.)

Back to Introduction −−  Index −−  Chapter 2

1.10 External Variables and Scope35

Back to Chapter 1 −−  Index −−  Chapter 3

Chapter 2 − Types, Operators and Expressions

Variables and constants are the basic data objects manipulated in a program. Declarations list the variables to

be used, and state what type they have and perhaps what their initial values are. Operators specify what is to

be done to them. Expressions combine variables and constants to produce new values. The type of an object

determines the set of values it can have and what operations can be performed on it. These building blocks are

the topics of this chapter.

The ANSI standard has made many small changes and additions to basic types and expressions. There are

now signed and unsigned forms of all integer types, and notations for unsigned constants and

hexadecimal character constants. Floating−point operations may be done in single precision; there is also a

long double type for extended precision. String constants may be concatenated at compile time.

Enumerations have become part of the language, formalizing a feature of long standing. Objects may be

declared const, which prevents them from being changed. The rules for automatic coercions among

arithmetic types have been augmented to handle the richer set of types.

2.1 Variable Names

Although we didn't say so in Chapter 1, there are some restrictions on the names of variables and symbolic

constants. Names are made up of letters and digits; the first character must be a letter. The underscore ``_''

counts as a letter; it is sometimes useful for improving the readability of long variable names. Don't begin

variable names with underscore, however, since library routines often use such names. Upper and lower case

letters are distinct, so x and X are two different names. Traditional C practice is to use lower case for variable

names, and all upper case for symbolic constants.

At least the first 31 characters of an internal name are significant. For function names and external variables,

the number may be less than 31, because external names may be used by assemblers and loaders over which

the language has no control. For external names, the standard guarantees uniqueness only for 6 characters and

a single case. Keywords like if, else, int, float, etc., are reserved: you can't use them as variable

names. They must be in lower case.

It's wise to choose variable names that are related to the purpose of the variable, and that are unlikely to get

mixed up typographically. We tend to use short names for local variables, especially loop indices, and longer

names for external variables.

2.2 Data Types and Sizes

There are only a few basic data types in C:

intan integer, typically reflecting the natural size of integers on the host machine

floatsingle−precision floating point

doubledouble−precision floating point

In addition, there are a number of qualifiers that can be applied to these basic types. short and long apply

to integers: