C pointers

2
Pre-requisite
 Basics of the C programming language
 Data type
 Variable
 Array
 Function call
 Standard Input/Output
 e.g. printf(), scanf()

3
Outline
 Computer Memory Structure
 Addressing Concept
 Introduction to Pointer
 Pointer Manipulation
 Summary

4
Computer Memory Revisited
 Computers store data in memory slots
 Each slot has an unique address
 Variables store their values like this:
Addr Content Addr Content Addr Content Addr Content
1000 i: 37 1001 j: 46 1002 k: 58 1003 m: 74
1004 a[0]: ‘a’ 1005 a[1]: ‘b’ 1006 a[2]: ‘c’ 1007 a[3]: ‘0’
1008 ptr: 1001 1009 … 1010 1011

5
Computer Memory Revisited
 Altering the value of a variable is indeed
changing the content of the memory
 e.g. i = 40; a[2] = ‘z’;
1000 i: 40 1001 j: 46 1002 k: 58 1003 m: 74
1004 a[0]: ‘a’ 1005 a[1]: ‘b’ 1006 a[2]: ‘z’ 1007 a[3]: ‘0’
1008 ptr: 1001 1009 … 1010 1011

6
Addressing Concept
 Pointer stores the address of another
entity
 It refers to a memory location
int i = 5;
int *ptr; /* declare a pointer variable */
ptr = &i; /* store address-of i to ptr */
printf(“*ptr = %dn”, *ptr); /* refer to referee of ptr */

7
Why do we need Pointer?
 Simply because it’s there!
 It is used in some circumstances in C
Remember this?
scanf(“%d”, &i);

8
What actually ptr is?
 ptr is a variable storing an address
 ptr is NOT storing the actual value of i
int i = 5;
int *ptr;
ptr = &i;
printf(“i = %dn”, i);
printf(“*ptr = %dn”, *ptr);
printf(“ptr = %pn”, ptr);
5i
address of iptr
Output:
i = 5
*ptr = 5
ptr = effff5e0
value of ptr =
address of i
in memory

9
Twin Operators
 &: Address-of operator
 Get the address of an entity
 e.g. ptr = &j;
1000 i: 40 1001 j: 33 1002 k: 58 1003 m: 74
1004 ptr: 1001 1005 1006 1007

10
Twin Operators
 *: De-reference operator
 Refer to the content of the referee
 e.g. *ptr = 99;
1000 i: 40 1001 j: 99 1002 k: 58 1003 m: 74
1004 ptr: 1001 1005 1006 1007

11
Example: Pass by Reference
 Modify behaviour in argument passing
void f(int j)
{
j = 5;
}
void g()
{
int i = 3;
f(i);
}
void f(int *ptr)
{
*ptr = 5;
}
void g()
{
int i = 3;
f(&i);
} i = ?i = ?i = 3 i = 5

12
An Illustration
int i = 5, j = 10;
int *ptr;
int **pptr;
ptr = &i;
pptr = &ptr;
*ptr = 3;
**pptr = 7;
ptr = &j;
**pptr = 9;
*pptr = &i;
*ptr = -2;
Data Table
Name Type Description Value
i int integer variable 5
j int integer variable 10

13
An Illustration
int i = 5, j = 10;
int *ptr; /* declare a pointer-to-integer variable */
int **pptr;
ptr = &i;
pptr = &ptr;
*ptr = 3;
**pptr = 7;
ptr = &j;
**pptr = 9;
*pptr = &i;
*ptr = -2;
Data Table
ptr int * integer pointer variable

14
An Illustration
int i = 5, j = 10;
int *ptr;
int **pptr; /* declare a pointer-to-pointer-to-integer variable */
ptr = &i;
pptr = &ptr;
*ptr = 3;
**pptr = 7;
ptr = &j;
**pptr = 9;
*pptr = &i;
*ptr = -2;
Data Table
ptr int * integer pointer variable
pptr int ** integer pointer pointer variable
Double

15
An Illustration
int i = 5, j = 10;
int *ptr;
int **pptr;
ptr = &i; /* store address-of i to ptr */
pptr = &ptr;
*ptr = 3;
**pptr = 7;
ptr = &j;
**pptr = 9;
*pptr = &i;
*ptr = -2;
Data Table
ptr int * integer pointer variable address of i
pptr int ** integer pointer pointer variable
*ptr int de-reference of ptr 5

16
An Illustration
int i = 5, j = 10;
int *ptr;
int **pptr;
ptr = &i;
pptr = &ptr; /* store address-of ptr to pptr */
*ptr = 3;
**pptr = 7;
ptr = &j;
**pptr = 9;
*pptr = &i;
*ptr = -2;
Data Table
pptr int ** integer pointer pointer variable address of ptr
*pptr int * de-reference of pptr value of ptr
(address of i)

17
An Illustration
int i = 5, j = 10;
int *ptr;
int **pptr;
ptr = &i;
pptr = &ptr;
*ptr = 3;
**pptr = 7;
ptr = &j;
**pptr = 9;
*pptr = &i;
*ptr = -2;
Data Table

18
An Illustration
int i = 5, j = 10;
int *ptr;
int **pptr;
ptr = &i;
pptr = &ptr;
*ptr = 3;
**pptr = 7;
ptr = &j;
**pptr = 9;
*pptr = &i;
*ptr = -2;
Data Table
**pptr int de-reference of de-reference of
pptr
7

19
An Illustration
int i = 5, j = 10;
int *ptr;
int **pptr;
ptr = &i;
pptr = &ptr;
*ptr = 3;
**pptr = 7;
ptr = &j;
**pptr = 9;
*pptr = &i;
*ptr = -2;
Data Table
ptr int * integer pointer variable address of j

20
An Illustration
int i = 5, j = 10;
int *ptr;
int **pptr;
ptr = &i;
pptr = &ptr;
*ptr = 3;
**pptr = 7;
ptr = &j;
**pptr = 9;
*pptr = &i;
*ptr = -2;
Data Table
ptr int * integer pointer variable address of j
**pptr int de-reference of de-reference of
pptr
9

21
An Illustration
int i = 5, j = 10;
int *ptr;
int **pptr;
ptr = &i;
pptr = &ptr;
*ptr = 3;
**pptr = 7;
ptr = &j;
**pptr = 9;
*pptr = &i;
*ptr = -2;
Data Table
*pptr int * de-reference of pptr value of ptr
(address of i)

22
An Illustration
int i = 5, j = 10;
int *ptr;
int **pptr;
ptr = &i;
pptr = &ptr;
*ptr = 3;
**pptr = 7;
ptr = &j;
**pptr = 9;
*pptr = &i;
*ptr = -2;
Data Table
i int integer variable -2
*ptr int de-reference of ptr -2

23
Pointer Arithmetic
 What’s ptr + 1?
The next memory location!
 What’s ptr - 1?
The previous memory location!
 What’s ptr * 2 and ptr / 2?
Invalid operations!!!

Operations possible on
pointers
 Subtraction of pointers pointing to same
array
 Increment or decrement Operations
 Addition with integer value
 Subtraction with integer value.
24

Operations not possible on
pointers
 Addition
 Multiplication
 Division
 Modulo Division
25

Arrays and pointers
 An array is a collection of similar elements .It
is also known as a subscripted variable.
 Before using an array its type and size must
be declared.
int arr[10]
float a[60];
 However big an array may be, its elements
are always stored in contiguous memory
locations.
26

Arrays and pointers
 In c there is no check to see if the
subscript exceeding the array size.
 It will simply be placed in memory
outside the array.
 This will leads to unpredictable results.
27

29
Pointer Arithmetic and Array
float a[4];
float *ptr;
ptr = &(a[2]);
*ptr = 3.14;
ptr++;
*ptr = 9.0;
ptr = ptr - 3;
*ptr = 6.0;
ptr += 2;
*ptr = 7.0;
Data Table
a[0] float float array element (variable) ?
ptr float * float pointer variable
*ptr float de-reference of float pointer
variable
?

30
float a[4];
float *ptr;
ptr = &(a[2]);
*ptr = 3.14;
ptr++;
*ptr = 9.0;
ptr = ptr - 3;
*ptr = 6.0;
ptr += 2;
*ptr = 7.0;
Data Table
ptr float * float pointer variable address of a[2]
variable
?

31
float a[4];
float *ptr;
ptr = &(a[2]);
*ptr = 3.14;
ptr++;
*ptr = 9.0;
ptr = ptr - 3;
*ptr = 6.0;
ptr += 2;
*ptr = 7.0;
Data Table
a[2] float float array element (variable) 3.14
variable
3.14

32
float a[4];
float *ptr;
ptr = &(a[2]);
*ptr = 3.14;
ptr++;
*ptr = 9.0;
ptr = ptr - 3;
*ptr = 6.0;
ptr += 2;
*ptr = 7.0;
Data Table
variable
?

33
float a[4];
float *ptr;
ptr = &(a[2]);
*ptr = 3.14;
ptr++;
*ptr = 9.0;
ptr = ptr - 3;
*ptr = 6.0;
ptr += 2;
*ptr = 7.0;
Data Table
variable
9.0

34
float a[4];
float *ptr;
ptr = &(a[2]);
*ptr = 3.14;
ptr++;
*ptr = 9.0;
ptr = ptr - 3;
*ptr = 6.0;
ptr += 2;
*ptr = 7.0;
Data Table
variable
?

35
float a[4];
float *ptr;
ptr = &(a[2]);
*ptr = 3.14;
ptr++;
*ptr = 9.0;
ptr = ptr - 3;
*ptr = 6.0;
ptr += 2;
*ptr = 7.0;
Data Table
variable
6.0

36
float a[4];
float *ptr;
ptr = &(a[2]);
*ptr = 3.14;
ptr++;
*ptr = 9.0;
ptr = ptr - 3;
*ptr = 6.0;
ptr += 2;
*ptr = 7.0;
Data Table
variable
3.14

37
float a[4];
float *ptr;
ptr = &(a[2]);
*ptr = 3.14;
ptr++;
*ptr = 9.0;
ptr = ptr - 3;
*ptr = 6.0;
ptr += 2;
*ptr = 7.0;
Data Table
variable
7.0

38
 Type of a is float *
 a[2]  *(a + 2)
ptr = &(a[2])
 ptr = &(*(a + 2))
 ptr = a + 2
 a is a memory address
constant
 ptr is a pointer variable
float a[4];
float *ptr;
ptr = &(a[2]);
*ptr = 3.14;
ptr++;
*ptr = 9.0;
ptr = ptr - 3;
*ptr = 6.0;
ptr += 2;
*ptr = 7.0;

39
More Pointer Arithmetic
 What if a is a double array?
 A double may occupy more memory slots!
 Given double *ptr = a;
 What’s ptr + 1 then?
1000 a[0]: 37.9 1001 … 1002 … 1003 …
1004 a[1]: 1.23 1005 … 1006 … 1007 …
1008 a[2]: 3.14 1009 … 1010 … 1011 …

40
More Pointer Arithmetic
 Arithmetic operators + and – auto-adjust
the address offset
 According to the type of the pointer:
 1000 + sizeof(double) = 1000 + 4 = 1004
1000 a[0]: 37.9 1001 … 1002 … 1003 …
1004 a[1]: 1.23 1005 … 1006 … 1007 …
1008 a[2]: 3.14 1009 … 1010 … 1011 …

Increment & Decrement
int a[]={1,2,3,4,5,6,7,8,9,10};
int *p1,*p2;
p1=&a[0];
p2=&a[5];
*p1++;
++*p2;
printf(“%d”,*p1);
printf(“%d”,*p2); 41

42
Generic pointer
 Also known as void pointer
void *ptr;
 It’s a pointer which points any type of
data.
 Arithmetic on void pointers not
permitted.

int a=10,*j;
void *k;
j=&a; k=&a;
j++;
k++;
printf(“%p”,j);
printf(“%p”,k);
43
OUTPUT ?

C pointers

C pointers

Editor's Notes

C pointers

Editor's Notes

C pointers

C pointers

Editor's Notes

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