The C programming language.

The C programming Language
7. Variable Argument Lists: <stdarg.h>
8.
Non-local Jumps: <setjmp.h>
9.
Signals: <signal.h>
10.
Date and Time Functions: <time.h>
11.
Implementation-defined Limits: <limits.h> and <float.h>
●
Appendix C: Summary of Changes
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Preface
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
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Preface
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, 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
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Preface to the first edition
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
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Preface to the first edition
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
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Introduction
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.
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Introduction
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.
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
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Introduction
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.
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.
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Introduction
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
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Chapter 1 - A Tutorial Introduction
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");
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