detailed information about Pointers in c language

1

Pointers in C

Chapter 6

By:-Gourav Kottawar

Contents6.1 Introduction 6.2 Memory Organization 6.3 Basics of Pointer 6.4 Application of Pointer 6.5 Pointer Expressions Declaration of Pointer, Initializing Pointer, De-referencing Pointer Void Pointer Pointer Arithmetic 6.6 Precedence of &, * operators 6.7 Pointer to Pointer 6.8 Constant Pointe

2

6.9 Dynamic Memory Allocation 6.10 sizeof(), malloc(), calloc(), realloc(), free() 6.11 Pointers and Arrays 6.12 Pointers and character string 6.13 Array of pointers

3

Pre-requisite Basics of the C programming

language Data type Variable Array Function call Standard Input/Output

e.g. printf(), scanf()

Pointers - Introduction Pointer is a derived type. Pointers contain memory address

as their values. Pointers can be used to access and

manipulate data stored in memory.

4

5

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: 741004 a[0]: ‘a’ 1005 a[1]: ‘b’ 1006 a[2]: ‘c’ 1007 a[3]: ‘\

0’1008 ptr:

10011009 … 1010 1011

6

Computer Memory Revisited

Altering the value of a variable is indeed changing the content of the memory e.g. i = 40; a[2] = ‘z’;

Addr

Content

Addr

Content Addr

Content

Addr

Content

1000 i: 40 1001 j: 46 1002 k: 58 1003 m: 741004 a[0]: ‘a’ 1005 a[1]: ‘b’ 1006 a[2]: ‘z’ 1007 a[3]: ‘\

0’1008 ptr:

10011009 … 1010 1011

7

Addressing Concept Pointer stores the address of

another entity It refers to a memory locationint i = 5;

int *ptr; /* declare a pointer variable */

ptr = &i; /* store address-of i to ptr */

printf(“*ptr = %d\n”, *ptr); /* refer to referee of ptr */

8

Why do we need Pointer? Simply because it’s there! It is used in some circumstances in

CRemember this?

scanf(“%d”, &i);

Benefits of pointers More efficient in handling arrays and data

tables. Pointers can be used to return multiple

values from a function via function arguments.

Pointers permit references to functions and there by facilitating passing of functions as arguments to other functions.

The use of pointer arrays to character strings result in saving of data storage space in memory. 9

Benefits of Pointer Pointers allow C to support DMA. Pointers provides an efficient tool for

manipulating dynamic data structures such as structures, linked list, trees, etc.

Pointers reduce length and complexity program.

They increase execution speed and thus reduce the program execution time.

10

11

What actually ptr is? ptr is a variable storing an

address ptr is NOT storing the actual value

of iint i = 5;

int *ptr;

ptr = &i;

printf(“i = %d\n”, i);

printf(“*ptr = %d\n”, *ptr);

printf(“ptr = %p\n”, ptr);

5i

address of iptr

Output:i = 5

*ptr = 5

ptr = effff5e0

value of ptr = address of iin memory

12

Twin Operators

&: Address-of operator Get the address of an entity

e.g. ptr = &j;

Addr

Content

Addr

Content Addr

Content

Addr

Content

1000 i: 40 1001 j: 33 1002 k: 58 1003 m: 741004 ptr:

10011005 1006 1007

13

Twin Operators

*: De-reference operator Refer to the content of the referee

e.g. *ptr = 99;

Addr

Content

Addr

Content Addr

Content

Addr

Content

1000 i: 40 1001 j: 99 1002 k: 58 1003 m: 741004 ptr:

10011005 1006 1007

example Write a program to print address of

variable along with value.

14

Declaring pointer variables Syntax data_type *pt_name Ex- int* ptr;int *ptr;int * ptr;

15

Initialization of pointer variables

ptr= &a;

16

http://2.bp.blogspot.com/_uIwyaTjqYYw/SkuOlsbaKcI/AAAAAAAAA3g/TPnf2_wimrE/s1600-h/Concept1.jpeg

So, Pointer is a variable just like other

variables of c but only difference is unlike the other variable it stores the memory address of any other variables of c. This variable may be type of int, char, array, structure, function or any other pointers.

Examples17

18

(1)Pointer p which is storing memory address of a int type variable:int i=50;

int *p=&i; (2)Pointer p which is storing memory address of an array:

int arr[20];int (*p)[20]=&arr;

(3)Pointer p which is storing memory address of a function:

char display(void);char(*p)(void)=&display;

19

(4)Pointer p which is storing memory address of struct type variable:

struct abc{int a;float b;}var;struct abc *p=&var;

20

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

21

An Illustrationint 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 TableName Type Description Value

i int integer variable 5j int integer variable 10

22


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;



ptr int * integer pointer variable

23


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;



ptr int * integer pointer variablepptr int

**integer pointer pointer variable

Double Indirection

24


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;



ptr int * integer pointer variable address of ipptr int


*ptr int de-reference of ptr 5

25


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;





address of ptr

*pptr int * de-reference of pptr value of ptr(address of

i)

26


int *ptr;

int **pptr;

ptr = &i;

pptr = &ptr;

*ptr = 3;

**pptr = 7;

ptr = &j;

**pptr = 9;

*pptr = &i;

*ptr = -2;





address of ptr


27


int *ptr;

int **pptr;

ptr = &i;

pptr = &ptr;

*ptr = 3;

**pptr = 7;

ptr = &j;

**pptr = 9;

*pptr = &i;

*ptr = -2;





address of ptr

**pptr int de-reference of de-reference of pptr

7

28


int *ptr;

int **pptr;

ptr = &i;

pptr = &ptr;

*ptr = 3;

**pptr = 7;

ptr = &j;

**pptr = 9;

*pptr = &i;

*ptr = -2;



ptr int * integer pointer variable address of jpptr int


address of ptr


29


int *ptr;

int **pptr;

ptr = &i;

pptr = &ptr;

*ptr = 3;

**pptr = 7;

ptr = &j;

**pptr = 9;

*pptr = &i;

*ptr = -2;



ptr int * integer pointer variable address of jpptr int


address of ptr

**pptr int de-reference of de-reference of pptr

9

30


int *ptr;

int **pptr;

ptr = &i;

pptr = &ptr;

*ptr = 3;

**pptr = 7;

ptr = &j;

**pptr = 9;

*pptr = &i;

*ptr = -2;





address of ptr

*pptr int * de-reference of pptr value of ptr(address of

i)

31


int *ptr;

int **pptr;

ptr = &i;

pptr = &ptr;

*ptr = 3;

**pptr = 7;

ptr = &j;

**pptr = 9;

*pptr = &i;

*ptr = -2;


i int integer variable -2j int integer variable 9



address of ptr

*ptr int de-reference of ptr -2

Chain of pointers

32

Address 2

Address 1 value

P2 p1 variable

The pointer variable p2 contains the address of the pointer variable p1 which points to the location that contains the desired value. This is known as multiple indirection.

•Example

Pointer ExpressionY = *p1 * *p2;Sum=sum + *p1;Z=5* - *p2/ *p1;*p2= *p2+10;P1 + 4;P1 -2P1-p2

33

examplemain(){ int a,b,*p1,*p2,x,y,z;

a=12;b=4; p1=&a;p2=&b;x=*p1 * *p2 – 6;y=4* - *p2 / * p1+10;printf(“Address of a = %u”,p1);printf(“\n Address of b = %u”,p2);printf(“a= %d, b= %d”.a,b); printf(“x= %d , y= %d”,x,y);*p2 = *p2 +3;*p1= *p2 – 5;z= *p1 * *p2 – 6;printf(“\n a = %d , b = %d”, a,b);printf(“\n z= %d”,z);

}

34

Pointer Increment and scale factor

characters 1 byte Integers 2 bytes Floats 4 bytes Long integers 4 bytes Doubles 8 bytes

35

Rules of pointer variable A pointer variable can be assigned the address of another

varaible. A pointer variable can be assigned the values of another

pointer variable. A pointer variable can be initialized with NULL or zero

value. A pointer variable can be pre-fixed or post-fixed with

increment or decrement operators. An integer value may be added or subtracted from a

pointer variable. When two pointers point to the same array, one pointer

variable can be subtracted from other. When two pointers point to the same objects of the same

data types, they can compared using relational operators. Two pointer variable can not be added A value can not be assigned to an arbitrary address. i.e

x= &10;

36

37

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!!!

38

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 TableName Type Description Valuea[0] float float array element

(variable)?

a[1] float float array element (variable)

?


?


?

ptr float *

float pointer variable

*ptr float de-reference of float pointer variable

?

39


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;


(variable)?


?


?


?

ptr float *

float pointer variable address of a[2]


?

40


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;


(variable)?


?


3.14


?

ptr float *



3.14

41


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;


(variable)?


?


3.14


?

ptr float *



?

42


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;


(variable)?


?


3.14


9.0

ptr float *



9.0

43


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;


(variable)?


?


3.14


9.0

ptr float *



?

44


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;


(variable)6.0


?


3.14


9.0

ptr float *



6.0

45


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;


(variable)6.0


?


3.14


9.0

ptr float *



3.14

46


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;


(variable)6.0


?


7.0


9.0

ptr float *



7.0

47


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;

Pointers and arrays Example Pointers and character strings example

48

49

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?

Addr

Content Addr

Content Addr

Content

Addr

Content

1000 a[0]: 37.9

1001 … 1002 … 1003 …

1004 a[1]: 1.23

1005 … 1006 … 1007 …

1008 a[2]: 3.14

1009 … 1010 … 1011 …

50

More Pointer Arithmetic Arithmetic operators + and – auto-

adjust the address offset According to the type of the pointer:

1000 + sizeof(double) = 1000 + 4 = 1004

Addr

Content Addr

Content Addr

Content

Addr

Content

1000 a[0]: 37.9

1001 … 1002 … 1003 …

1004 a[1]: 1.23

1005 … 1006 … 1007 …

1008 a[2]: 3.14

1009 … 1010 … 1011 …

Array of string Pointer to array of string: A pointer which pointing to

an array which content is string, is known as pointer to array of strings.#include<stdio.h>int main(){char *array[4]={"c","c++","java","sql"};char *(*ptr)[4]=&array;printf("%s ",++(*ptr)[2]);return 0;}

51

Array of string Pointer to array of string: A pointer which pointing to

an array which content is string, is known as pointer to array of strings.#include<stdio.h>int main(){\char *array[4]={"c","c++","java","sql"};char *(*ptr)[4]=&array;printf("%s ",++(*ptr)[2]);return 0;}

52

Output: ava

example#include<stdio.h>int main(){

static char *s[3]={"math","phy","che"};typedef char *( *ppp)[3];static ppp p1=&s,p2=&s,p3=&s;char * (*(*array[3]))[3]={&p1,&p2,&p3};char * (*(*(*ptr)[3]))[3]=&array;p2+=1;p3+=2;printf("%s",(***ptr[0])[2]);

return 0;}

54

example#include<stdio.h>int main(){

static char *s[3]={"math","phy","che"};typedef char *( *ppp)[3];static ppp p1=&s,p2=&s,p3=&s;char * (*(*array[3]))[3]={&p1,&p2,&p3};char * (*(*(*ptr)[3]))[3]=&array;p2+=1;p3+=2;printf("%s",(***ptr[0])[2]);

return 0;}

55

Output: che

As we know p[i]=*(p+i)(***ptr[0])[2]=(*(***ptr+0))[2]=(***ptr)[2]

=(***(&array))[2] //ptr=&array=(**array)[2] //From rule *&p=p=(**(&p1))[2] //array=&p1=(*p1)[2]=(*&s)[2] //p1=&s

=s[2]=”che”

56

Exampleint main(){ int i,j,temp1,temp2; int arr[8]={5,3,0,2,12,1,33,2}; int *ptr; for(i=0;i<7;i++){ for(j=0;j<7-i;j++){if(*(arr+j)>*(arr+j+1)){ ptr=arr+j; temp1=*ptr++; temp2=*ptr; *ptr--=temp1; *ptr=temp2;} } }

57

exampleint main(){static float farray[]

[3]={0.0f,1.0f,2.0f,3.0f,4.0f,5.0f,6.0f,7.0f,8.0f};float (*array[3])[3]={&farray[0],&farray[1],&farray[2]};float (*(*ptr)[])[3]=&array;printf("%f ",2[(*(**ptr+1))]);return 0;}

58

exampleint main(){static float farray[]

[3]={0.0f,1.0f,2.0f,3.0f,4.0f,5.0f,6.0f,7.0f,8.0f};float (*array[3])[3]={&farray[0],&farray[1],&farray[2]};float (*(*ptr)[])[3]=&array;printf("%f ",2[(*(**ptr+1))]);return 0;}

59

Output: 5.000000

Dynamic memory Allocation

Malloc –Use - The C library function void *malloc(size_t

size) allocates the requested memory and returns a pointer to it

Syntax - void *malloc(size_t size)Where size -- This is the size of the memory

block, in bytes.Return Value - This function returns a pointer to

the allocated memory, or NULL if the request fails.

60

exampleint main() { char *str; /* Initial memory allocation */

str = (char *) malloc(15); strcpy(str, "tutorialspoint"); printf("String = %s, Address = %u\n", str, str); return(0); }

61

exampleint main() { char *str; /* Initial memory allocation */

str = (char *) malloc(15); strcpy(str, "tutorialspoint"); printf("String = %s, Address = %u\n", str, str); return(0); }

62

Output - String = tutorialspoint, Address = 355090448 String = tutorialspoint.com, Address = 355090448

Dynamic Memory Allocation Calloc () –

The C library function void *calloc(size_t nitems, size_t size) allocates the requested memory and returns a pointer to it. The difference in malloc and calloc is that malloc does not set the memory to zero where as calloc sets allocated memory to zero.

Syntax - void *calloc(size_t nitems, size_t size) Parametersnitems -- This is the number of elements to be allocated.size -- This is the size of elements. Return ValueThis function returns a pointer to the allocated memory, or

NULL if the request fails.

63

exampleint main() { int i, n; int *a; printf("Number of elements to be entered:"); scanf("%d",&n); a = (int*)calloc(n, sizeof(int)); printf("Enter %d numbers:\

n",n); for( i=0 ; i < n ; i++ ) { scanf("%d",&a[i]); } printf("The numbers entered are: "); for( i=0 ; i < n ; i++ ) { printf("%d ",a[i]); } return(0); }

64

Number of elements to be entered:3 Enter 3 numbers: 22 55 14 The numbers entered are: 22 55 14

65

Difference

66

Free()The C library function void

free(void *ptr) deallocates the memory previously allocated by a call to calloc, malloc, or realloc.

67

DMA functions realloc()Use- can reduce or maximize previously

allocated memorySyntax – ptr = realloc(ptr,newsize);

68

Generic pointervoid pointer in c is known as generic pointer.

Literal meaning of generic pointer is a pointer which can point type of data.

Example:

void *ptr;Here ptr is generic pointer.

69

Generic pointerint main(){void *ptr;printf("%d",*ptr);return 0;}

70

Generic pointerint main(){void *ptr;printf("%d",*ptr);return 0;}

71

Output: Compiler error

1. We cannot dereference generic pointer.

Generic pointer

int main(){void *ptr;printf("%d",sizeof(ptr));return 0;}

72

Output: 2Explanation: Size of any type of near pointer in c

is two byte.

We can find the size of generic pointer using sizeof operator.

Generic Pointerint main(){char c='A';int i=4;void *p;char *q=&c;int *r=&i;p=q;printf("%c",*(char *)p);p=r;printf("%d",*(int *)p);

return 0;}

73

Output: A4

Generic pointer can hold any type of pointers like char

pointer, struct pointer, array of pointer etc

without any typecasting.

Generic Pointer

4. Any type of pointer can hold generic pointer without any typecasting.

5. Generic pointers are used when we want to return such pointer which is applicable to all types of pointers. For example return type of malloc function is generic pointer because it can dynamically allocate the memory space to stores integer, float, structure etc. hence we type cast its return type to appropriate pointer type

74

Null pointerLiteral meaning of NULL pointer is a pointer which is

pointing to nothing. NULL pointer points the base address of segment.

Examples of NULL pointer:

1. int *ptr=(char *)0;2. float *ptr=(float *)0;3. char *ptr=(char *)0;4. double *ptr=(double *)0;5. char *ptr=’\0’;6. int *ptr=NULL;

75

Null PointerWhat is meaning of NULL?

NULL is macro constant which has been defined in the heard file stdio.h, alloc.h, mem.h, stddef.h and stdlib.h as

76

int main(){if(!NULL)printf("I know preprocessor");elseprintf("I don't know preprocessor");return 0;

}Output: I know preprocessor

Explanation:!NULL = !0 = 1

77

int main(){int i;

static int count;for(i=NULL;i<=5;){

count++;i+=2;}

printf("%d",count);return 0;

}

78

Output: 3

Example – Null pointerint main(){#ifndef NULL#define NULL 5#endif

printf("%d",NULL+sizeof(NULL));return 0;

}

79

Output: 2Explanation:

NULL+sizeof(NULL)=0+sizeoof(0)

=0+2 //size of int data type is two byte.

int main(){char *str=NULL;

strcpy(str,"c-pointer.blogspot.com");printf("%s",str);

return 0;}

80

Output: (null)

We cannot copy any thing

in the NULL pointer.

Wild pointerWild pointer:

A pointer in c which has not been initialized is known as wild pointer.

81

Output: Any addressGarbage value#include<stdio.h>

int main(){int *ptr;printf("%u\n",ptr);printf("%d",*ptr);return 0;}

Wild Vs Null

82

There is difference between the NULL pointer and wild pointer. Null pointer points the base address of segment while wild pointer doesn’t point any specific memory location.

Dangling pointer:

If any pointer is pointing the memory address of any variable but after some variable has deleted from that memory location whilepointer is still pointing such memory location. Such pointer is known as dangling pointer and this problem is known as dangling pointer problem.

83

Initial

Later :

84

Near pointer:

The pointer which can points only 64KB data segment or segment number 8 is known as near pointer.

85

Near Pointer

86

•Near pointer cannot access beyond the data segment like graphics video memory, text video memory etc. •Size of near pointer is two byte. •With help keyword near, we can make any pointer as near pointer.

Near pointerint main(){int x=25;int near* ptr;ptr=&x;printf(“%d”,sizeof ptr)

;return 0;

}

87

Output: 2

Far pointer

The pointer which can point or access whole the residence memory of RAM i.e. which can access all 16 segments is known as far pointer

88

89

Size of far pointer is 4 byte or 32 bit

Far pointerint main(){

int x=10;int far *ptr;ptr=&x;printf("%d",sizeof ptr)

;return 0;

}

90

Output: 4

Limitation of far pointer:We cannot change or modify the segment address of given far address by applying any arithmetic operation on it. That is by using arithmetic operator we cannot jump from one segment to other segment. If you will increment the far address beyond the maximum value of its offset address instead of incrementing segment address it will repeat its offset address in cyclic order.

Huge Pointer The pointer which can point or

access whole the residence memory of RAM i.e. which can access all the 16 segments is known as huge pointer.

91

92

Size of huge pointer is 4

byte or 32 bit.

93

Summary A pointer stores the address

(memory location) of another entity Address-of operator (&) gets the

address of an entity De-reference operator (*) makes a

reference to the referee of a pointer Pointer and array Pointer arithmetic

detailed information about Pointers in c language

Education

Transcript of detailed information about Pointers in c language