Pointers

1
Pointers

• The best (and worst)
• thing about C

2
Best?

• Very Powerful
• Needed for any complex data structures
• Needed for dynamic memory management
• The Sharpest tool in your C toolkit.
• Makes a lot of things easier
• Faster, Smaller, etc

3
Worst?

• If used incorrectly it can be bad.
• Hand Power Drill Hospital Visit
• Pointers carelessness Core Dump
• Segmentation faults are common when you misuse
  pointers.
• But, errors dont always crash sometimes they
  mess upyour program later

4
What is a pointer anyway?

• It is a type (int, float, pointer to int..)
• There is a pointer type for every type
• A pointer to an int int
• A pointer to a float float
• They can even point to other pointers!
• A pointer to a pointer to an int int
• A pointer to a pointer to a pointer to a pointer
  to an int int
• Etc.. (You usually see 1 star occasionally 2
  Almost never more than that though.)

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Figure 9-12
Declaring and Initializing
Declare and initialize in one step
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Figure 9-5
Yeah, but what is a pointer?
int a -123 int p a
7
Yeah, but what is a pointer?

• Its a variable that points to another.
• int val / regular int /
• int ptr / pointer to an int /

val
ptr
int
int
8
Yeah, but what is it?

• Its a variable that points to another.
• int val / regular int /
• int ptr / pointer to an int /
• ptr val / ptr contains the
• address-of val /
• / ptr points to val /

val
ptr
int
int
9
Yeah, but what is it?

• Its a variable that points to another.
• ptr 42 / the target of ptr
• becomes 42, whatever
• that target may be
• indirection /

val
ptr
42
int
int
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Figure 9-7
int x, p, q p x q x
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Figure 9-7
int x, p, q p x q x
12
Figure 9-7
int x, p, q p x q x
13
Figure 9-7
int x, p, q p x q x
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Figure 9-21
15
And this is useful because

• Bear with me for a moment
• Lets say we have a program which has two
  integers (int i, j), and we want to be able to
  call a function to sort them such that i has
  the smaller, and j has the larger value
• How do we swap values in a function without using
  pointers?

16
And this is useful because

• Bear with me for a moment
• Lets say we have a program which has two
  integers (int i, j), and we want to be able to
  call a function to sort them such that i has
  the smaller, and j has the larger value
• How do we swap values in a function without using
  pointers?
• WE CANT!!!

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What about

• int main()
• int i2, j5
• swap(i, j)
• return 0

i
main
j
2
5
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What about

• void swap(int a, int b)
• int main()
• int i2, j5
• swap(i, j) // Pass by reference
• return 0

a
b
temp
swap
i
main
j
2
5
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swaps

• By using pointers we can make ONE more generic
  swap routine
• Using Pass by reference instead of Pass by
  value
• void swap(int a, int b) // Swaps any two
  ints!
• int tmp
• tmp a
• a b
• b tmp

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How do we use it?

• void swap(int a, int b)
• int tmp
• tmp a
• a b
• b tmp

array1

12
46
7
64
9
10
87
0
1
2
3
4
5
6
a
b
tmp
swap
i
main
j
swap( i, j )
2
5
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How do we use it?

• void swap(int a, int b)
• int tmp
• tmp a
• a b
• b tmp

a
b
temp
a
b
tmp
swap
swap
10
2
i
main
j
i
main
j
swap( i, j )
2
5
2
5
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How do we use it?

• void swap(int a, int b)
• int tmp
• tmp a
• a b
• b tmp

a
b
temp
a
b
temp
a
b
tmp
swap
swap
swap
10
10
2
i
main
j
i
main
j
i
main
j
swap( i, j )
2
5
2
5
5
5
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How do we use it?

• void swap(int a, int b)
• int tmp
• tmp a
• a b
• b tmp

a
b
temp
a
b
temp
a
b
temp
a
b
tmp
swap
swap
swap
swap
10
10
10
2
i
main
j
i
main
j
i
main
j
i
main
j
swap( i, j )
2
5
2
5
2
5
5
2
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When do we need Pointers?

• When passing an array, it automatically passes by
  reference so anything we do in the function
  changes the original no need to send the
  address for changing elements around
• What happens when we pass a struct to a function?
• Recall struct is passed by value we get a copy
  of the struct

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When do we need Pointers?

• What happens when we pass a struct to a function?
• Pass by value, get a duplicate
• Instead, pass a pointer to it
• typedef struct
• int x, y
• Pt
• void func( Pt p )
• int main( )
• Pt mypoint 1, 2
• printf( BEFORE x d, y d\n, mypoint.x,
  mypoint.y )
• func( mypoint )
• printf( BEFORE x d, y d\n, mypoint.x,
  mypoint.y )
• return 0

Must wrap p inside ( )
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Nuthin much about NULL
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NULL
NULL

• NULL is a pointer to NOTHING!
• ptr NULL // NULL is a pointer to
• // address 0 (on most compilers)
• What is in ptr?

ptr
int
28
Some pointer secrets

• pointer int gives you a pointer
• It moves the pointer forward
• How much depends on the type of the pointer
• Example
• int array5, ptr
• ptr array2
• ptr ptr 2

The thing ptr points to is 4 bytes So ptr 1
will move you forward 4 bytes in memory. So, ptr
2 will give you a pointer to the int two cells
after ptr.
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Some pointer secrets

• pointer - int gives you a pointer
• It moves the pointer backward
• pointer - pointer gives you a int
• Gives you the difference between the memory cells
• pointer pointer not valid
• same with / etc

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Some pointer secrets

• int i, a100
• An array IS a pointer!
• It points to the beginning of the cells
• All three of these are the same
• a a a0
• ai is equivalent to (ai)
• what is the difference between ai and ia?
• Is 4a legal?

31
Arrays are pointers

• include ltstdio.hgt
• include ltstdlib.hgt

void swap (int a, int b) int temp temp
a a b b temp
void sort(int arr, int len) int p,i for
(p1pltlenp) for (i0iltlen-pi)
if (arri gt arri1) swap(arri,arri
1)
int main() int array1100, array250,
array3500 ... // fill arrays
sort(array1,100) sort(array2,50)
sort(array3,500) ...
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Arrays are pointers

• include ltstdio.hgt
• include ltstdlib.hgt

void swap (int a, int b) int temp temp
a a b b temp
void sort(int arr, int len) int p,i
for(p1pltlenp) for(i0iltlen-pi)
if(arri gt arri1) swap(arri,arri
1)
swap(arri,arri1)
int main() int array1100, array250,
array3500 ... // fill arrays
sort(array1,100) sort(array2,50)
sort(array3,500) ...
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Strings

• Strings are char arrays
• But, arrays are just pointers
• So a string is just a pointer to char(s)

34
String functions you may want

• strcpy(char dest, char src)
• copy a string from src to dest
• strcat(char dest, char src)
• copy src onto the end of dest
• strncpy(char dest, char src, int max)
• copy up to max chars from src to dest
• strncat(char dest, char src, int max)
• copy up to max chars from src onto the end of dest

35
String functions you may want

• strchr(char s, char c)
• Returns a pointer to the first occurrence of the
  char c in the string s, or NULL if it isnt
  there!
• strstr(char haystack, char needle)
• Returns a pointer to the first occurrence of the
  string needle in the string haystack, or NULL if
  it isnt there!

36
String Funky Funk

• String Equality
• Considerchar p abc, q abcif ( p
  q ) printf( Same address!\n )else
  printf( Different addresses\n )
• What does it print?
• Depends on your compiler
• String constants MAY be stored in the same
  location therefore optimized by pointing to the
  same spot
• But, some compilers will store them separately!

37
Returning Values
38
Returning values

• Pointers can also be used to return MULTIPLE
  values instead of just one
• void func(int a, int b, int c)
• a 1
• b 4
• c 9
• int main()
• int number, thingy, value
• func(number, thingy, value)
• func(number, number, number)
• return 0

39
Returning values

• How can you return TWO (or more) values from a
  function?
• Example Write a function that takes two numbers
  and returns the quotient AND the remainder!

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Returning values

• void long_division( int dividend, int divisor,
  int quotientp, int remainderp)
• quotientp dividend / divisor
• remainderp dividend divisor

• Example Write a function that takes two numbers
  and returns the quotient AND the remainder!

Dividend and divisor are input only
Dividend and divisor are input only
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Returning values

• void long_division( int dividend, int divisor,
  int quotientp, int remainderp)
• quotientp dividend / divisor
• remainderp dividend divisor

• Example Write a function that takes two numbers
  and returns the quotient AND the remainder!

Values can be returned to calling function using
pointers
Values can be returned to calling function using
pointers
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Returning values

• include ltstdio.hgt
• void long_division(int dividend, int divisor, int
  quotientp, int remainderp)
• int main( )
• int quot, rem
• long_division(40, 3, quot, rem)
• printf("40 divided by 3 yields quotient d ",
  quot)
• printf("and remainder d\n", rem)
• return 0
• void long_division( int dividend, int divisor,
  int quotientp, int remainderp)
• quotientp dividend / divisor
• remainderp dividend divisor

Send the address to the function
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Ack! POW! Boom!
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PROBLEMS!

• Dereferencing a NULL pointer
• BOOM!
• Incorrectly applying address and indirection
  operators. Example pt is a pointer
• pt 45 // invalid attempt to take address of
  a value
• pt (miles10) // invalid attempt to take
  address of a value
• pt miles 10 // valid 10 is added to
  address of miles
• Addresses of pointer constants cannot be taken
• Int nums25int ptpt nums // invalid
  nums is an address, so no address of the
  address!pt nums // valid

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PROBLEM! faults

• Dont dereference a variable that is not a
  pointer!int num 7printf( d, num )
• Fastest way to a crash
• ((int ) NULL) 0