Introduction to C and Unix

1
Introduction to C and Unix

• Vincent Levesque
• vleves_at_cim.mcgill.ca
• Based on Prof. Cooperstocks
• Crash Course in C
• http//www.cim.mcgill.ca/jer/courses/C/

2
UNIX in ECE dept.

• SUN workstations
• SSH room 502 catwoman, robin, obelix, cesar,
  asterix, idefix, falbala, getafix
• SSH only bruce, penguin, thor, wendee, wayne
• e.g catwoman.ece.mcgill.ca
• Windows PC
• connect to any workstation using SSH
• see instructions on course web page
• http//www.cim.mcgill.ca/sinclair/Ian20Sinclair_
  files/courses/os/OS20Course20Home20Page.htm

3
UNIX at Home

• Options
• SSH (putty, Cygwin, etc.)
• install Linux
• install Cygwin with Devel package
  (www.cygwin.com)
• Test your assignments on a SUN workstation!

4
Entering commands

• enter commands on the shells command prompt
• similar to DOS

5
Getting Help

• Manual pages
• man printf
• man 3 printf
• man k print
• Info
• info printf
• Other
• search on the web and in newsgroups (google)
• numerous books

6
Filesystem Navigation

• Wildcards
• (any group of characters), ? (a single
  character)
• Basic Commands

7
Redirection and Pipes

• Redirection
• sends output of a command to a file
• gt Creates or overwrites a file
• ls gt listing.txt
• gtgt Appends to a file
• prog gtgt log.txt
• Pipe
• sends output of a command to the input of another
  command
• cat readme.txt more

8
Viewing and Editing Files

• Viewing
• cat displays the content of a file
• cat readme.txt
• cat readme.txt more
• Editing
• many text editors are available
• emacs
• vi
• pico
• etc.

9
File Permissions

• -rwxr-xr--
• 3 user classes
• owner
• group
• world
• 3 permission types
• r (read)
• w (write)
• x (execute)
• use ls -l to see file permissions
• ls l foo.txt
• -rw-r-xr-x foo.txt

10
File Permissions (2)

• use chmod to change file permissions
• (owner gt u, group gt g, world gt o)
• chmod o-rx foo.txt
• ls l foo.txt
• -rw-r-x--- foo.txt
• chmod gw foo.txt
• ls l foo.txt
• -rw-rwx--- foo.txt
• use a private directory for your course work
• mkdir private
• chmod 700 private
• ls ld private
• drwx------ private

11
Compiling

• Steps to a running program
• vi prog.c
• gcc prog.c
• ./a.out

source code
object code
compiler
input data
executable object code
output data
12
Separate Compilation

• gcc -c part1.c
• gcc -c part2.c
• gcc part1.o part2.o
• a.out

part1.o
part1.c
preprocess -gt translate
part2.c
part2.o
preprocess -gt translate
part1.o part2.o
a.out
link
13
Makefiles

• Example
• Sample Makefile
• pgm prog.o sub.o
• gcc -o pgm prog.o sub.o
• prog.o incl.h prog.c
• gcc -c prog.c
• sub.o incl.h sub.c
• gcc -c sub.c
• Use make command to execute the Makefile
• make pgm
• make
• Note
• A tab character is required before gcc!
• http//www.cim.mcgill.ca/jer/courses/os/help/make
  files.html

14
Running your Program

• any file can be executed (remember file
  permissions!)
• no special extension such as .exe in dos
• you might need to specify the full path
• ./prog

15
Your First C Program

• "The only way to learn a new programming language
  is by writing programs in it" KR
• begin hello.c
• ---------------
• / A simple program that prints something /
• include ltstdio.hgt
• / main() is the entry point of any C program
  /
• main ()
• printf ("Hello, world!\n")
• ---------------
• end hello.c

16
Declarations

• Example
• float x
• double d 5
• int p, i, a100
• char s21
• Syntax
• type variable_name, ... value
• Rules
• declarations must precede executable statements
• int type may be modified by long, short,
  unsigned

17
Changing Variable Values

• Example
• int x, y, z
• x 2
• x x 1
• Getting Fancy
• y z 4 5
• x 1
• x
• y x--
• y x
• Note
• assignment statements return value, which may or
  may not be ignored same goes for increment
  statements

18
Formatted Output

• Example
• int i 10
• float f 2.5
• char s "hi"
• printf ("Jack\'s integer is d\n", i)
• printf ("Jill\0x27s float is f\n", f)
• printf ("My string s s\n", s)
• Syntax
• printf (string_with_formatting, var1, var2, ...)
• Formats d integer, f float, c character, s
  string,
• Note "escape sequences" \n newline, \' quote,
  \0x27, etc.
• include ltstdio.hgt is compulsory more about it
  later

19
Formatted Input

• Example
• include ltstdio.hgt
• int i
• float f
• scanf ("d f\n", i, f)
• / inputs an integer and a float /
• Syntax
• scanf (string_with_formatting, var1, var2,...)
• Note
• The ampersand () is necessary because scanf
  modifies the values stored in the respective
  variables by comparison, printf only uses the
  values, without modifying them. More about this
  later

20
I/O Example

• include ltstdio.hgt
• main ()
• int n
• float x
• char mark
• scanf ("d f c", n, x, mark)
• printf ("Of d s,\nf got c\s\n",
• n, "students", x, mark)
• Input
• 86 85.999 A
• Output
• Of 86 students,
• 85.999001 got A's

21
Conditional Statements

• Example
• if (age lt 0)
• printf ("warning negative age\n")
• age -age
• Syntax
• if (condition) statement
• if (condition) statement else statement
• Rules
• the condition is an int ! (no booleans)
• parentheses required around conditional
  expression
• use curly braces to make a compound statement

22
More Conditionals

• Example
• if (x lt 0)
• printf ("x is less than 0\n")
• else if (x 0)
• printf ("x is equal to 0\n")
• else
• printf ("x is greater than 0\n")
• Whats wrong with this?
• if (x lt 0)
• if (y lt z)
• printf ("y is less than z\n")
• else
• printf ("x not less than 0\n")

23
While Loops

• Example
• / print "hi" forever /
• while (1)
• printf ("hi")
• Syntax
• while (condition)
• statement
• Rules (again)
• the condition is an int ! (no booleans)
• parentheses required around conditional
  expression
• use curly braces to make a compound statement

24
For Loops

• Example
• / print "hi" three times /
• int i / i continues to exist when loop ends /
• for (i 0 i lt 3 i)
• printf ("hi")
• Syntax
• for (statement1 condition statement2)
• statement3
• Equivalent to
• statement1
• while (condition)
• statement3
• statement2

25
Loop Example

• / print squares up to 100 /
• main ( )
• int i
• for (i 0 i lt 10 i)
• printf ("d \n", i i)
• Note
• cant do for (int i 0 ...

26
Arrays

• int years45
• float temperatures 11
• void main ()
• years0 2000
• temperatures11 -45.67 / illegal /
• Rules
• indices start at zero
• maximum valid index is the size of the array
  minus 1
• but C lets you go beyond the declared boundaries
• temperatures11 is illegal

27
Characters

• Characters
• char a, b, c1, c2
• a '0' b '\037' c1 'K' c2 c1 1
• Assigns values 48, 31, 75, 76
• The sequences '0',...,'9', 'a',...,'z',
  'A',...,'Z' contain characters numbered
  consecutively
• Casting
• printf ("c d\n", c1, (int) c1)
• Outputs K 75

28
Strings

• Strings are '\0'-terminated arrays of char
• char s3 "hi" / invisible '\0' /
• char t3 'h', 'i', '\0'
• char s "hi"
• String operations
• include ltstring.hgt
• strlen ("there") / returns 5 /
• strcpy (s, t) / copy t to s /
• strcmp (s, t) / alphabetical comparison /

29
Functions

• / Increment takes an integer argument and
• returns the argument plus one.
• /
• int incr (int i)
• int j
• j i 1
• return j
• main ()
• int k, m 4
• k incr(m)
• printf ("k d, m d\n", k, m)
• output k 5, m 4

30
More about Functions

• might have no return type, and no return
  statement
• void printhi ()
• printf ("hi\n")
• parameters are copied and can be modified
• int incr (int i)
• i
• return i

31
Variables within Functions

• But this does not work
• void no_incr (int i)
• i
• void main ()
• int x 5
• no_incr(x)
• printf ("d\n", x)
• beware that modifications are on internal copies
  of the parameters.
• note void main() since main does not return a
  value

32
C Preprocessor Directives

• Operations before compilation phase
• include "header" files that contain pre-supplied
  functions
• include ltstring.hgt
• this line will be substituted by the contents of
  the file
• define symbolic constants
• define MY_LARGE_NUMBER 12345
• define MY_NIFTY_STRING abc
• what happens if you add a by mistake?
• define MY_LARGE_NUMBER 12345
• int x MY_LARGE_NUMBER 1
• gt int x 1235 1 / error! /

33
Header Files

• / File myheader.h
• Definitions for program build.c
• /
• / Booleans /
• typedef int boolean_t
• define TRUE 1
• define FALSE 0
• / node type /
• typedef struct node_t
• ...
• / function prototypes /
• node_t NewNode(char label, int cost)
• void KillNode (node_t node)

34
Make your own booleans

• C has boolean operations on int
• ! !
• However, C has no boolean type. Make it!
• define boolean_t int
• define TRUE 1
• define FALSE 0
• Example
• boolean_t xIsFive, zAsBig, result
• xIsFive (x 5)
• zAsBig (z gt x)
• if (xIsFive zAsBig)
• result TRUE

35
Naming your own type

• Syntax
• typedef existing-type new-type
• Comments
• typedef does not create a new type
• only creates a new label for an existing type
• makes your code easier to read

36
typedef in action

• Example
• typedef char string_t
• typedef int boolean_t
• string_t msg "No more room"
• boolean_t full_class
• if (num_students gt MAX_SIZE)
• full_class TRUE

37
Pointers

• C relies heavily on pointers
• int i
• int pi
• pi i
• pi 3

i pi
3
pi i
38
Pointers and Arrays

• An array name by itself is a pointer to the first
  element
• int a100
• a 5 / same as a0 5 /
• (a 3) 10 / same as a3 10 /
• / Note a 3 means something else /
• Arrays are passed by reference
• void array_incr(int a10)
• int i
• for (i0 ilt10 i)
• ai
• array_incr(my_array)

39
Call by reference

• / increment i by 1 /
• void incr (int pi)
• pi pi 1
• main()
• int j
• j 4
• incr (j)
• printf ("j d\n", j)
• j and i are pointers to the same integer incr
  can change the integer (side effect), but not the
  pointer

4
j
i
40
Dynamic Memory Allocation

• Pointer errors can be nasty
• void main ()
• int p
• p 5
• Reserve space for the "pointed to" object
• include ltstdlib.hgt
• void main ()
• int p
• p (int )malloc(sizeof (int))
• p 5
• printf ("value at p d\n", p)
• free (p) / must do this yourself! /

41
Structures

• Use struct to create or define a new type
• struct str_data
• char string
• int length
• struct str_data s1, s2
• Syntax
• struct structure_name
• type1 member1
• type2 member2
• A member of a structure is referred to by
• structure_name.member

42
Structures, cont.

• Example
• struct str_data
• char string
• int length
• struct str_data s1, s2
• s1.string (char )malloc(80sizeof(char))
• strcpy (s1.string, "How now brown cow.")
• s1.length strlen(s1.string)
• s2 s1 / can copy structures, pass to fcns /
• free(s1.string)

43
Arrays of Structures

• Example
• define MAX_STUDENT 100
• typedef struct / define a new type /
• char name80
• int name_len
• int student_number
• student_t
• / create list of elements of type student_t /
• student_t classMAX_STUDENT
• for (i 0 i lt MAX_STUDENT i)
• gets(classi.name)
• classi.name_len strlen(classi.name)

44
Pointers to Structures

• These are all equivalent
• struct str_data list
• ...
• list0.length ...
• (list).length ...
• list-gtlength ...
• Rules
• p-gty is shorthand for (p).y
• means "follow the pointer p into structure member
  y
• more efficient to pass pointers to structs rather
  than the structs themselves

45
Self-Referential Structures

• Example Linked List
• This does not work
• typedef struct
• int data
• node_t next
• node_t
• Rules
• a structure cannot refer to its own name before
  it is defined
• but it can contain a pointer to itself
• how?

46
Naming your structure

• Give the structure a name
• typedef struct element
• int data
• struct element next
• node_t
• Rules
• next is a pointer to a structure of type node_t
• we can use this to create a pointer to the head
  of the list
• node_t head (node_t ) malloc (sizeof
  (node_t))
• How do we define the end of the list?

47
Traversing a Linked List

• node_t p
• p head
• while (p ! NULL)
• printf ("data d\n", p-gtdata)
• p p-gtnext

data
data
head
next
next
p
p-gtnext
48
Naming Conventions

• Consistency is bliss. Suggested format
• Constants use all upper case
• define TRUE 1
• define MAX_ELEMENTS 50
• Variables use all lower case
• char name int num_students
• Functions upper case for first letter of each
  world
• float GetCompressedValue() char GetNextWord()
• Types all lower case with a "_t" suffix (for
  type)
• typedef struct node_t ...

49
Programming Errors I

• include ltstdio.hgt
• void main ()
• int i
• scanf ("d", i)
• if (i 0)
• printf ("false")
• else
• printf ("true")
• Whats wrong?

50
Programming Errors I

• include ltstdio.hgt
• void main ()
• int i
• scanf ("d", i)
• if (i 0)
• puts ("false")
• else
• puts ("true")
• The assignment, i 0, will return a value of 0,
  so the program will always output "true"
• Use if(0 i) to be safe

51
Programming Errors II

• include ltstdio.hgt
• void main ()
• int i
• / echo one number /
• scanf ("d", i)
• printf ("input d\n", i)
• Whats wrong?
• Hint Segmentation fault / Bus error

52
Programming Errors II

• include ltstdio.hgt
• void main ()
• int i
• / echo one number /
• scanf ("d", i)
• printf ("input d\n", i)
• Must provide the address of an integer if the
  function is going to assign a value to it.

53
Programming Errors III

• include ltstdio.hgt
• void main ()
• int pc
• scanf ("d", pc)
• printf ("d\n", pc)
• Whats wrong?

54
Programming Errors III

• include ltstdio.hgt
• void main ()
• int pc
• scanf ("d", pc)
• printf ("d\n", pc)
• pc is an uninitialized pointer so results are
  unpredictable --gt use malloc

55
Programming Errors IV

• include ltstdlib.hgt
• void main ()
• double p
• p malloc (sizeof (double ))
• Whats wrong?

56
Programming Errors IV

• include ltstdlib.hgt
• void main ()
• double p
• p malloc (sizeof (double ))
• Insufficient memory allocated for a double
  (typically, 8 bytes)
• Use malloc (sizeof (double))

57
Programming Errors V

• include ltstdio.hgt
• void main ()
• char s "hi"
• if (s "hi")
• printf ("Strings are equal")
• else
• printf ("Strings are not equal")
• What happens?

58
Programming Errors V

• include ltstdio.hgt
• main ()
• char s "hi"
• if (s "hi")
• puts ("Strings are equal")
• else
• puts ("Strings are not equal")
• Use strcmp() to compare strings, otherwise you
  are comparing addresses.

59
Finding Bugs

• Insert "print" statements
• printf ("At point A, s d\n", "x", x)
• Use a debugger to help you catch bugs in the act
• run step-by-step
• insert breakpoints
• display variables

60
The GNU Debugger GDB

• Allows you to
• Make your program stop on specified conditions
• Step through your programs execution line by
  line
• Examine what has happened when your program has
  stopped or crashed.

61
Using GDB

• Compile your program with the -g option
• gcc -g -o prog prog.c
• To run your program in GDB
• gdb prog
• (gdb) run
• To examine your program after a crash (core
  dump)
• prog
• Segmentation fault (core dumped)
• gdb prog core
• Core was generated by prog'.
• Program terminated with signal 11, Segmentation
  fault.
• (gdb) ...

62
Some GDB Commands

• Set a breakpoint
• (gdb) break filefunction
• Run your program
• (gdb) run arglist
• List 10 lines of code
• (gdb) list
• Display the value of an expression
• (gdb) print expr
• Display a value after each stoppage
• (gdb) display expr

63
More GDB Commands

• Execute next line (skipping function calls)
• (gdb) next
• Execute next line stepping into fcn calls
• (gdb) step
• Continue after a break
• (gdb) cont
• Getting help
• (gdb) help name
• Exiting
• (gdb) quit

64
Debugging A Program I

• Example (searching a linked list)
• / search for data, return pointer to the node /
• node_t Search(node_t head, int data)
• node_t p
• p head
• do
• if (p-gtdata data)
• return (p)
• else
• p p-gtnext
• while (p ! NULL)
• return (NULL)

65
Debugging A Program II

• There is a bug
• Input
• 1 2 3 4 -1 (values for linked list)
• 17 (value to search for)
• Output
• 17 is found (! but it's not in the list)
• 17 3 2 1 (! 4 is missing)
• Steps to find the bug
• start up GDB
• gdb linklist

66
Debugging A Program III

• set a breakpoint at the Search function
• (gdb) break Search
• Breakpoint 1 at 0x10ab0 file linklist.c, line
  13.
• (gdb)
• run the program and provide some input
• (gdb) run
• Starting program linklist
• 1 2 3 4 -1 17
• Breakpoint 1, Search (head0x21b50, data17)
• 13 p head
• (gdb)
• display some variables
• (gdb) display p
• 1 p (node_t ) 0x20d58
• (gdb) display p
• 2 p data 4, next 0x20d48
• (gdb)

67
Debugging A Program IV

• step through the code and watch the variables
  change
• (gdb) step
• 15 if (p-gtdata data)
• (gdb) display p
• 1 p (node_t ) 0x21b50
• (gdb) display p
• 2 p data 4, next 0x21b40
• (gdb) step
• 16 return (p)
• 2 p data 17, next 0x21b40
• 1 p (node_t ) 0x21b50
• We found the bug

68
Debugging A Program V

• But there is another bug
• Input
• -1 (empty list)
• 17 (value to search for)
• Output
• Segmentation fault (core dumped)
• See if you can find the bug.

69
References

• Prof. Jeremy Cooperstocks Crash Course in C,
  http//www.cim.mcgill.ca/jer/courses/C/
• Kernighan Ritchie "The C programming
  language", 2nd edition. Prentice-Hall, 1988.