Intro to C Programming Winter 2013 COMP 2130 Intro Computer Systems Computing Science Thompson Rivers University.

Intro to C Programming

Winter 2013

COMP 2130 Intro Computer Systems

Computing ScienceThompson Rivers University

TRU-COMP2130 C Programming 2

Course Objectives

The better knowledge of computer systems, the better programing.Computer System C Programming Language

Computer architectureCPU (Central Processing Unit)IA32 assembly language

Introduction to C language

Compiling, linking, loading, executing

Physical main memoryMMU (Memory Management Unit)

Virtual memory space

Memory hierarchyCache

Dynamic memory management

Better coding – locality

Reliable and efficient programming for power programmers(to avoid strange errors, to optimize codes, to avoid security holes, …)


Course Contents

Introduction to computer systems: B&O 1 Introduction to C programming: K&R 1 – 4 Data representations: B&O 2.1 – 2.4 C: advanced topics: K&R 5.1 – 5.9, 5.11, 6 – 8 Introduction to IA32 (Intel Architecture 32): B&O 3.1 – 3.8, 3.13 Compiling, linking, loading, and executing: B&O 7 (except 7.12) Dynamic memory management – Heap: B&O 9.9.1 – 9.9.2, 9.9.4 –

9.9.5, 9.11 Code optimization: B&O 5.1 – 5.6, 5.13 Memory hierarchy, locality, caching: B&O 5.12, 6.1 – 6.3, 6.4.1 –

6.4.2, 6.5, 6.6.2 – 6.6.3, 6.7 Virtual memory (if time permits): B&O 9.4 – 9.5


Unit Learning Objectives

Use cc (or gcc) to compile C programs. Write a C program with the common (very similar) syntaxes with Java. Use printf() library function to print messages. Use define statements for constants. Use getchar() and putchar() for I/O. Use of integer values as Boolean values. Use a char array as a string. Use bit operators to manipulate bits in a given integer. Understand the scope of external identifiers. Distinguish signed integer types and unsigned integer types. Use register variables for indexing arrays. Use conditional inclusion statements with #if, #elif, … . …


Unit Contents

Introduction Types, Operators, and Expressions Control Flow Functions and Program Structure


1. Introduction

Only the topics that are different from Java will be discussed.


Getting Started

C program files should end with “.c”. hello.c

#include <stdio.h> // similar to import statements in Java // stdio.h includes the information about the

standard library

main() // similar to main method in Java{

printf (“hello, world\n”); // printf() defined in stdio.h}

How to compile? $ gcc hello.c or $ gcc hello.c -o hello If you do not have anything wrong, you will see a.out in the same

working directory.

How to run? $ ./a.out


printf (“hello, world\n”);“…” is a character string. This is also called a string constant.printf() is a library function to print a string to the terminal.

Very similar primitive data typeschar, short, int, long, float, double, … Same control structuresif, if-else, for, while, do-while, switch Same operators (we will discuss some new ones later.)+, -, *, /, %, =, ==, !=, &&, ||, …

Can you write a program to convert Fahrenheit temperature to Celsius temperature?

The conversion formula: C = 5 / 9 × (F – 32)


#include ...

/* print Fahrenheit-Celsius table for fahr = 0, 20, 40, ..., 300, using a loop */

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; // integer value?

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

fahr = fahr + step;

}

}


printf("%d\t%d\n", fahr, celsius); %d specifies an integer argument (d: decimal).

Output1 -17

20-6

404

...Can we print better? Like

1 -17

20 -6

40 4

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


1 -17

20 -6

40 4

... Is there any problem in the above output?

The Celsius temperatures in the previous out are not accurate. For example 0o F is -17.8o C.Then?We can use float data type for fahr and celsius instead of int.

float fahr, celsius;

Then how to print real numbers using printf()?celsius = 5 * (fahr-32) / 9; // okay?

printf("%3.0f\t%6.1f\n", fahr, celsius);%f specifies a floating point argument (f: floating point).6.1 means 1 decimal out of 6 digits.


Some easy errors: printf("%d\t%6.1f\n", fahr, celsius); ??? printf("%3.0f\t%6.1f\n", fahr); ??? printf("%6.1f\n", fahr, celsius); ???


Summary of printf() format specifiers %d print as decimal integer %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 characters wide and 2 after

decimal point Other format specifiers

%o for octal %x for hexadecimal %c for character %s for character string %% % itself


Symbolic Constants

#define name replacement_list

#include <...>

#define LOWER 0 // similar to final variable in Java

#define UPPER 300

#define STEP 20

main()

{ float fahr, celsius;

for (fahr = ... ; fahr <= ... ; fahr = fahr + ...) {

celsius = 5 * (fahr-32) / 9;

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

}

}


Symbolic Constants

#define name replacement_list

#include <stdio.h>

#define LOWER 0 // similar to final variable in Java

#define UPPER 300

#define STEP 20

main()

{ float fahr, celsius;

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

celsius = 5 * (fahr-32) / 9;

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

}

}


Character Input and Output

#include <stdio.h>

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

main()

{

int c;

c = getchar(); // in order to read a character

while (c != EOF) { // EOF is defined in stdio.h.

// EOF means End of File, ^D.

putchar(c); // in order to print a character

c = getchar();

}

}

Where did getchar(), EOF, and putchar() come from? Let’s run this program to understand I/O better.


#include <stdio.h>

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

main()

{

int c;

while ((c = getchar()) != EOF) // okay?

putchar(c);

}

What if we use the follow code instead? while (c = getchar() != EOF)

The above code is equivalent to while (c = (getchar() != EOF))

This is because the precedence of != is higher than =.


Operator precedence rules() [] -> .

! ~ - (unary) + (unary) * & sizeof ++ --

* / %

+ -

<< >>

< <= > >=

== !=

&

^

|

&&

||

= += -= *= /= %= &= |= ^= <<= >>=


Boolean values?

Syntax error? while (c = getchar() != EOF)

Let’s assume getchar() returns ‘A’. Then getchar() != EOF becomes TRUE.

The data type of c is int. Is there a boolean data type in C? No. 0 is FALSE and non-zero value is considered as TRUE in C. Hence getchar() != EOF becomes 1, and c has 1. while(c) -> while(1). The loop repeats.


Arrays

#include <stdio.h>

/* count digits, white space, others */main(){ int c, i, nwhite, nother, ndigit[10]; // very similar to Java

nwhite = nother = 0; // really necessary? 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);}


A character, e.g., ‘3’, is an integer value. int c = ‘3’; is valid. ASCII table


Functions

#include <stdio.h>

int power(int m, int n); // function declaration

// What if it is not declared before // main()?

main()

{

int i;

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

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

return 0;

}

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

int power(int base, int n)

{

int i, p;

... // How to implement?


...

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

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

return 0;

}

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


{

int i, p;

p = 1;

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

p = p * base;

return p;

}


Arguments

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


{

int p;

for (p = 1; n > 0; n--)

p = p * base;

return p;

}

base, n and p are local variables that are used only in power(). When the function is called (or also called invoked), the values will be

copied (oac passed) into base and n. Even if those variables are updated with other values, the new values

will not be passed back to the caller function. How is this implemented in main memory?

This type of function calling is called call by value.

Character Arrays

#include <stdio.h>

#define MAXLINE 1000

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

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

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

max = 0;

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

if (len > max) {

max = len;

copy(longest, line);

}

if (max > 0) printf("%s", longest);

return 0;

}

...

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'; // “...’\0’”

return i;

}

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

{

int i;

i = 0;

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

++i;

}

In the same file


In main(),while ((len = getline(line, MAXLINE)) > 0)The data type of line[] is an array.getline() can change the contents in line[], and the change in line[] can be used in main(). Very similar to Java.This type of function calling is called call by reference.How is this implemented in main memory?

line represents MAXLINE variables - line[0], line[1], ...,

line[MAXLINE-1]. How? Address should be passed. line contains the address of the first byte of line[0]. line[i] is the memory area pointed by the address line +

sizeof(char) * i.

addr value var

… … …

def abc line

… … …

abc ‘C’ line[0]

abc+1 ‘O’ line[1]

… … …

Main memory


In getline(),int getline(char s[], int lim)The data type of the first parameter is an array. The address stored in line in main() is passed into s. Therefore s[i] will become equal to line[i]. Any change in this variable will be preserved so that the caller can use the change.This type of function calling is called call by reference.

s[i] = '\0';After all the characters are stored in s[] till EOF or ‘\n’,

‘\0’ is stored to mark the end of the character string.That is, a string is a list of characters with the end of ‘\0’, and a char array is usually used to keep a string.This is very important.


In main(),if (max > 0)

printf("%s", longest);%s is used to print a character string. The argument is a char array, not longest[], nor longest[0], and …What is the data type of longest?

char longest[MAXLINE];What value is stored in longest?

The address of the first byte of longest[0] to represent all longest[0], longest[1], ….

Can a char array be a character string?Yes. printf() with %s will try to print characters pointed by longest(, i.e., longest[0], longest[1], …,) until the character becomes ‘\0’.


char name[256], tmp[256];

name[0] = ‘C’;

name[1] = ‘O’;

name[2] = ‘M’;

name[3] = ‘P’;

name[4] = ‘\0’; // it is very important.

printf(“course number = %s\n”, name);

name[5] = ‘ ’;

name[6] = ‘2’;


name[7] = ‘1’;

name[8] = ‘3’;

name[9] = ‘0’;

name[10] = ‘\0’; // it is very important.


What is the output?

External Variables and Scope#include <stdio.h>

#define MAXLINE 1000

int max; // similar to instance in Java

char line[MAXLINE];

char longest[MAXLINE];

int getline(void);

void copy(void);

main()

{

int len;

extern int max,

extern char longest[];

max = 0;

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

if (len > max) {

max = len;

copy();

}

if (max > 0) /* there was a line */

printf("%s", longest);

return 0;

}

int getline(void)

{

int c, i;

// extern char line[]; // no need to declare

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;

}

void copy(void)

{

int i;

// extern char line[], longest[];

i = 0;

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

++i;

}

In the same file


#include …

…

#define …

…

// declaration of global variables

…

// declaration (i.e., definitions) of functions

…

// implementation of functions

{

…

}

Any identifier declared early is visible in the same block or sub-blocks.

Visible to every function in the file


2. Types, Operators, Expressions


Data Types and Sizes

Primitive data typesC Java Size

char, unsigned char byte 1B

char in Java uses 2Bs.

short, unsigned short short 2Bs

int, unsigned int int 4Bs

long, unsigned long long 8Bs

// there is no unsigned in Java

float float 4Bs

double double 8Bs

// boolean? TRUE: any non-zero value, boolean in Java

FALSE: zero

// string? “...”: with ‘\0’ at the end String in Java

sizeof(data_type) or sizeof(variable) gives the number of bytes used.


Constants

int

1234, long

12345678L double

123.4, 1e-3 float

123.4F printf() format specifiers

%u unsigned integers %l long integers %lu unsigned long integers %e float, double


Operators

Arithmetic operators= assignment

+ addition

++ increment by one; sometimes confusing

- subtraction

-- decrement by one; sometimes confusing

* multiplication

/ division; integer division

% modulo Relational operators

>

>=

<

<=


Boolean operators==

!=

&& AND not bitwise AND

|| OR not bitwise OR Reference operators– will be discussed later.


Bitwise Operators

| bitwise OR

& bitwise AND

^ XOR

~ 1’s complement

<< shift left

>> arithmetic shift right (the left most bit will be copied.)

Examples Exercise 2-6 setbits(x, p, n, y) Exercise 2-7 invert(x, p, n) Exercise 2-8 rightrot(x, n) IPv4 address conversion Check if a target bit in an integer is 1. Get the first three bytes in an integer. …


3. Control Flow

Statement …;

Block, or compound statement { … }

The same control structures as Java if, … for, while, do-while switch break, continue

You must know them all already.


Goto and Labels

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

for (j = 0; j < m; j++)

if (a[i] == b[j])

goto found;

/* didn't find any common element */

...

found:

/* got one: a[i] == b[j] */

...

Better not use goto statements Can you convert the above code so that you would not use goto?


4. Functions and Program Structure

Function in C is the same as method in Java.

return-type function-name(argument declarations) Various parts may be absent.


External Variables

If a large number of variables must be shared among functions, external variables (or also called global variables) are more convenient and efficient than long argument lists.

External variables are declared outside of any function, usually with initial values.

Automatic variables (local variables and parameters) are internal to a function; they come into existence when the function is entered, and disappear when it is left.

External variables, on the other hand, are permanent, so they can retain values from one function invocation to the next. Thus if two functions must share some data, yet neither calls the other, it is often most convenient if the shared data is kept in external variables rather than being passed in and out via arguments.


Scope Rules

The scope of a name is the part of the program within which the name can be used.

The scope of an external variable or a function lasts from the point at which it is declared to the end of the file being compiled.

main() { ... }

int sp = 0;double val[MAXVAL];

void push(double f) { ... }

double pop(void) { ... }

Is sp visible in push()? Can sp and pop() be used in main()?


Can we make external variables declared in file1 be used in file2?

in file1:

extern int sp;

extern double val[];

void push(double f) { ... }

double pop(void) { ... }

in file2:

int sp = 0;

double val[MAXVAL];


Static Variables

The static declaration, applied to an external variable or function, limits the scope of that object to the rest of the source file being compiled.

External static thus provides a way to hide names from other files.

static char buf[BUFSIZE]; // only in this file

static int bufp = 0; // only in this file

int getch(void) { ... }

void ungetch(int c) { ... }

Static in Java has a bit different meaning.


Register Variables

A register declaration advises the compiler that the variable in question will be heavily used.

The idea is that register variables are to be placed in machine registers, which may result in smaller and faster programs. But compilers are free to ignore the advice.

register int x;

register char c;

f(register unsigned m, register long n) {

register int i;

...

}

Usually for index variables used in loop structures


Initialization

Very similar to Java

In the absence of explicit initialization, external and static variables are guaranteed to be initialized to zero; but automatic and register variables have undefined (i.e., garbage) initial values.


The C Preprocessor

File inclusion#include “filename” // from the working dir

or

#include <filename> // from the standard dir Macro substitution

#define name replacement_text

#define max(A, B) ((A) > (B) ? (A) : (B)) Conditional inclusion

#if !defined(HDR) // or #ifndef HDR

#define HDR

/* contents of hdr.h go here */

#endif


#if SYSTEM == SYSV

#define HDR "sysv.h"

#elif SYSTEM == BSD

#define HDR "bsd.h"

#elif SYSTEM == MSDOS

#define HDR "msdos.h"

#else

#define HDR "default.h"

#endif

#include HDR

Intro to C Programming Winter 2013 COMP 2130 Intro Computer Systems Computing Science Thompson Rivers University.

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