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Brian W. Kernighan: Programming in C: A Tutorial
Programming in C: A Tutorial
Brian W. Kernighan
Bell Laboratories, Murray Hill, N. J.
Disclaimer:
This ``tutorial'' is presented as a historical document, not as a tutorial. Although it has lost
little of its didactic value, it describes a language that C compilers today do no longer understand: the C
of 1974, four years before
Kernighan
and
Ritchie
published the first edition of ``The C Programming
Language''.
Table of Contents:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
Introduction
A Simple C Program
A Working C Program; Variables; Types and Type Declarations
Constants
Simple I/O
-- getchar, putchar, printf
If; relational operators; compound statements
While Statement; Assignment within an Expression; Null Statement
Arithmetic
Else Clause; Conditional Expressions
Increment and Decrement Operators
Arrays
Character Arrays; Strings
For Statement
Functions; Comments
Local and External Variables
Pointers
Function Arguments
Multiple Levels of Pointers; Program Arguments
The Switch Statement; Break; Continue
Structures
Initialization
of Variables
Scope Rules:
Who Knows About What
#define, #include
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Brian W. Kernighan: Programming in C: A Tutorial
24.
25.
26.
27.
28.
Bit Operators
Assignment Operators
Floating Point
Horrors!
goto's and labels
Acknowledgements
1. Introduction.
C is a computer language available on the GCOS and UNIX operating systems at Murray Hill and (in
preliminary form) on OS/360 at Holmdel. C lets you write your programs clearly and simply it has
decent control flow facilities so your code can be read straight down the page, without labels or
GOTO's; it lets you write code that is compact without being too cryptic; it encourages modularity and
good program organization; and it provides good data-structuring facilities.
This memorandum is a tutorial to make learning C as painless as possible. The first part concentrates
on the central features of C; the second part discusses those parts of the language which are useful
(usually for getting more efficient and smaller code) but which are not necessary for the new user. This
is
not
a reference manual. Details and special cases will be skipped ruthlessly, and no attempt will be
made to cover every language feature. The order of presentation is hopefully pedagogical instead of
logical. Users who would like the full story should consult the "C Reference Manual" by D. M. Ritchie
[1],
which should be read for details anyway. Runtime support is described in
[2]
and
[3];
you will
have to read one of these to learn how to compile and run a C program.
We will assume that you are familiar with the mysteries of creating files, text editing, and the like in the
operating system you run on, and that you have programmed in some language before.
2. A Simple C Program
main( ) {
printf("hello, world");
}
A C program consists of one or more functions, which are similar to the functions and subroutines of a
Fortran program or the procedures of PL/I, and perhaps some external data definitions.
main
is such a
function, and in fact all C programs must have a
main.
Execution of the program begins at the first
statement of
main. main
will usually invoke other functions to perform its job, some coming from the
same program, and others from libraries.
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One method of communicating data between functions is by arguments. The parentheses following the
function name surround the argument list; here
main
is a function of no arguments, indicated by ( ).
The {} enclose the statements of the function. Individual statements end with a semicolon but are
otherwise free-format.
printf
is a library function which will format and print output on the terminal (unless some other
destination is specified). In this case it prints
hello, world
A function is invoked by naming it, followed by a list of arguments in parentheses. There is no CALL
statement as in Fortran or PL/I.
3. A Working C Program; Variables; Types and Type
Declarations
Here's a bigger program that adds three integers and prints their sum.
main( ) {
int a, b, c, sum;
a = 1; b = 2; c = 3;
sum = a + b + c;
printf("sum is %d", sum);
}
Arithmetic and the assignment statements are much the same as in Fortran (except for the semicolons) or
PL/I. The format of C programs is quite free. We can put several statements on a line if we want, or
we can split a statement among several lines if it seems desirable. The split may be between any of the
operators or variables, but
not
in the middle of a name or operator. As a matter of style, spaces, tabs,
and newlines should be used freely to enhance readability.
C has four fundamental types of variables:
q
q
q
q
int integer (PDP-11: 16 bits; H6070: 36 bits; IBM360: 32 bits)
char one byte character (PDP-11, IBM360: 8 bits; H6070: 9 bits)
float single-precision floating point
double double-precision floating point
There are also arrays and structures of these basic types, pointers to them and functions that return them,
all of which we will meet shortly.
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All variables in a C program must be declared, although this can sometimes be done implicitly by
context. Declarations must precede executable statements. The declaration
int a, b, c, sum;
declares
a, b, c,
and
sum
to be integers.
Variable names have one to eight characters, chosen from A-Z, a-z, 0-9, and _, and start with a non-
digit. Stylistically, it's much better to use only a single case and give functions and external variables
names that are unique in the first six characters. (Function and external variable names are used by
various assemblers, some of which are limited in the size and case of identifiers they can handle.)
Furthermore, keywords and library functions may only be recognized in one case.
4. Constants
We have already seen decimal integer constants in the
previous example--
1, 2, and 3. Since C is often
used for system programming and bit-manipulation, octal numbers are an important part of the
language. In C, any number that begins with 0 (zero!) is an octal integer (and hence can't have any 8's
or 9's in it). Thus
0777
is an octal constant, with decimal value 511.
A ``character'' is one byte (an inherently machine-dependent concept). Most often this is expressed as a
character constant, which is one character enclosed in single quotes. However, it may be any quantity
that fits in a byte, as in flags below:
char quest, newline, flags;
quest = '?';
newline = '\n';
flags = 077;
The sequence `\n' is C notation for ``newline character'', which, when printed, skips the terminal to the
beginning of the next line. Notice that `\n' represents only a single character. There are several other
``escapes'' like `\n' for representing hard-to-get or invisible characters, such as `\t' for tab, `\b' for
backspace, `\0' for end of file, and `\\' for the backslash itself.
float
and
double
constants are discussed in
section 26.
5. Simple I/O -- getchar, putchar, printf
main( ) {
char c;
c = getchar( );
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Brian W. Kernighan: Programming in C: A Tutorial
putchar(c);
}
getchar
and
putchar
are the basic I/O library functions in C.
getchar
fetches one character from
the standard input (usually the terminal) each time it is called, and returns that character as the value of
the function. When it reaches the end of whatever file it is reading, thereafter it returns the character
represented by `\0' (ascii NUL, which has value zero). We will see how to use this very shortly.
putchar
puts one character out on the standard output (usually the terminal) each time it is called. So
the program above reads one character and writes it back out. By itself, this isn't very interesting, but
observe that if we put a loop around this, and add a test for end of file, we have a complete program for
copying one file to another.
printf
is a more complicated function for producing formatted output. We will talk about only the
simplest use of it. Basically,
printf
uses its first argument as formatting information, and any
successive arguments as variables to be output. Thus
printf ("hello, world\n");
is the simplest use. The string ``hello, world\n'' is printed out. No formatting information, no
variables, so the string is dumped out verbatim. The newline is necessary to put this out on a line by
itself. (The construction
"hello, world\n"
is really an array of chars.
More about this shortly.)
More complicated, if
sum
is 6,
printf ("sum is %d\n", sum);
prints
sum is 6
Within the first argument of
printf,
the characters ``%d'' signify that the next argument in the
argument list is to be printed as a base 10 number.
Other useful formatting commands are ``%c'' to print out a single character, ``%s'' to print out an entire
string, and ``%o'' to print a number as octal instead of decimal (no leading zero). For example,
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