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

2. Reliable and efficient programming for power programmers
Course Objectives
The better knowledge of computer systems, the better programing.
Computer System
C Programming Language
Computer architecture
CPU (Central Processing Unit)
IA32 assembly language
Introduction to C language
Compiling, linking, loading, executing
Physical main memory
MMU (Memory Management Unit)
Virtual memory space
Memory hierarchy
Cache
Dynamic memory management
Better coding – locality
Reliable and efficient programming for power programmers
(to avoid strange errors, to optimize codes, to avoid security holes, …)
Not the replacement of IPv6; Inter-transition mechanism; But if IPv6 would fail
TRU-COMP2130
C Programming

3. 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 – 9.9.2, – 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, – 6.4.2, 6.5, – 6.6.3, 6.7
Virtual memory (if time permits): B&O 9.4 – 9.5
Not the replacement of IPv6; Inter-transition mechanism; But if IPv6 would fail
TRU-COMP2130
C Programming

4. 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, … . …
TRU-COMP2130
C Programming

5. Unit Contents Introduction Types, Operators, and Expressions
Control Flow
Functions and Program Structure
TRU-COMP2130
C Programming

6. 1. Introduction
Only the topics that are different from Java will be discussed.
TRU-COMP2130
C Programming

7. Getting Started C program files should end with “.c”. How to compile?
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
TRU-COMP2130
C Programming

8. Very similar primitive data types Same control structures
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 types
char, short, int, long, float, double, …
Same control structures
if, 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)
TRU-COMP2130
C Programming

9. #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; }
TRU-COMP2130
C Programming

10. printf("%d\t%d\n", fahr, celsius);
%d specifies an integer argument (d: decimal).
Output
1 -17
20 -6
40 4
...
Can we print better? Like
1 -17
20 -6
40 4
printf("%3d\t%6d\n", fahr, celsius);
TRU-COMP2130
C Programming

11. Is there any problem in the above output?
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.
TRU-COMP2130
C Programming

12. 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); ???
TRU-COMP2130
C Programming

13. 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
TRU-COMP2130
C Programming

14. Symbolic Constants #define name replacement_list main() { TRU-COMP2130
#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); }
TRU-COMP2130
C Programming

15. Symbolic Constants #define name replacement_list main() { TRU-COMP2130
#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); }
TRU-COMP2130
C Programming

16. 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.
TRU-COMP2130
C Programming

17. What if we use the follow code instead?
#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 =.
TRU-COMP2130
C Programming

18. Operator precedence rules
() [] -> .
! ~ - (unary) + (unary) * & sizeof
* / %
+ -
<< >>
< <= > >=
== !=
& ^ |
&& ||
= += -= *= /= %= &= |= ^= <<= >>=
TRU-COMP2130
C Programming

19. 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.
TRU-COMP2130
C Programming

20. Arrays TRU-COMP2130 #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); }
TRU-COMP2130
C Programming

21. A character, e.g., ‘3’, is an integer value. int c = ‘3’; is valid.
ASCII table
TRU-COMP2130
C Programming

22. Functions TRU-COMP2130 #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?
TRU-COMP2130
C Programming

23. TRU-COMP2130 ... 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;
p = 1;
for (i = 1; i <= n; i++)
p = p * base;
return p;
TRU-COMP2130
C Programming

24. Arguments
/* 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; }
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.
TRU-COMP2130
C Programming

25. Character Arrays In the same file int getline(char s[], int lim)
#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') {
++i; }
s[i] = '\0'; // “...’\0’”
return i;
void copy(char to[], char from[])
int i;
i = 0;
while ((to[i] = from[i]) != '\0')

26. 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.
Main memory
addr
value
var …
def
abc
line
‘C’
line[0]
abc+1
‘O’
line[1]
TRU-COMP2130
C Programming

27. 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.
TRU-COMP2130
C Programming

28. 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’.
TRU-COMP2130
C Programming

29. What is the output? TRU-COMP2130 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?
TRU-COMP2130
C Programming

30. External Variables and Scope
In the same file
#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') {
++i; }
line[i] = '\0';
return i;
void copy(void)
int i;
// extern char line[], longest[];
i = 0;
while ((longest[i] = line[i]) != '\0')

31. Visible to every function in the 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
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C Programming

32. 2. Types, Operators, Expressions
TRU-COMP2130
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33. Data Types and Sizes Primitive data types C 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.
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C Programming

34. 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
TRU-COMP2130
C Programming

35. Operators Arithmetic operators Relational operators = assignment
+ addition
++ increment by one; sometimes confusing
- subtraction
-- decrement by one; sometimes confusing
* multiplication
/ division; integer division
% modulo
Relational operators
>
>=
<
<=
TRU-COMP2130
C Programming

36. Reference operators– will be discussed later.
Boolean operators == !=
&& AND not bitwise AND
|| OR not bitwise OR
Reference operators– will be discussed later.
TRU-COMP2130
C Programming

37. Bitwise Operators Examples | 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. …
TRU-COMP2130
C Programming

38. 3. Control Flow Statement Block, or compound 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.
TRU-COMP2130
C Programming

39. Goto and Labels Better not use goto statements
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?
TRU-COMP2130
C Programming

40. 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.
TRU-COMP2130
C Programming

41. 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.
TRU-COMP2130
C Programming

42. 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()?
TRU-COMP2130
C Programming

43. Can we make external variables declared in file1 be used in file2?
extern int sp;
extern double val[];
void push(double f) { ... }
double pop(void) { ... }
in file2:
int sp = 0;
double val[MAXVAL];
TRU-COMP2130
C Programming

44. 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.
TRU-COMP2130
C Programming

45. 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
TRU-COMP2130
C Programming

46. 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.
TRU-COMP2130
C Programming

47. The C Preprocessor File inclusion Macro substitution
#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
TRU-COMP2130
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48. #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
TRU-COMP2130
C Programming