P10_PointersInC

8/3/2019 P10_PointersInC

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Pointers


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2.2

Roadmap

Overview

> What is C?

> C Features

> Memory layout

> Working with Pointers


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2.3

What is C?

C was designed as a general-purpose language with a very directmapping from data types and operators to machine instructions.

> cpp(C pre-processor) used for expanding macros and inclusion ofdeclaration header files

> explicit memory allocation (no garbage collection)

> memory manipulation throughpointers,pointer arithmetic and

typecasting

> used asportable, high-level assembler


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2.4

C Features

Developed in 1972 by Dennis Ritchie and Brian Kernighan as a systems language forUnix on the PDP-11. A successor to B [Thompson, 1970], in turn derived from BCPL.

C preprocessor: file inclusion,conditionalcompilation, macrosData types: char, short, int,long, double, float

Type constructors: pointer, array, struct, union

Basic operators: arithmetic,pointer manipulation, bit manipulation ...

Control abstractions: if/else, while/forloops, switch, goto ...

Functions: call-by-value, side-effects through pointers

Type operations: typedef, sizeof, explicit type-casting and coercion


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2.5

C Storage Classes

You must explicitly manage storage space for data

Static static objects exist for the entire life-time of theprocess

Automaticautomatic objects only live during function invocation

on the run-time stack

Dynamic dynamic objects live between calls to malloc and free theirlifetimes typically extend beyond their scope


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2.6

Memory Layout

The address space consists of(at least):

Text: executable program text (not writable)

Static: static data

Heap: dynamically allocated global memory (grows upward)Stack: local memory for function calls (grows downward)


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2.7

Where is memory?

#include

static int stat=0;

void dummy() { }

int main(void)

{

int local=1;

int *dynamic = (int*) malloc(sizeof(int));

printf("Text is here: %u\n", (unsigned) dummy);

/* function pointer */

printf("Static is here: %u\n", (unsigned) &stat);

printf("Heap is here: %u\n", (unsigned) dynamic);

printf("Stack is here: %u\n", (unsigned) &local);

}

Text is here: 7604

Static is here: 8216

Heap is here: 279216

Stack is here:

3221223448


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2.8

Roadmap

> Basics of Pointers

> Why Pointers?> Initialization

> Function pointers

> Typecasting


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2.9

Pointer Basics

> What is a pointer ?

> What is a variable ?

> Something with a name, the value of which can vary. int k;k=12;

> When we declare a variable, we inform the compiler of two things, the name of the variableand the type of the variable.

> lvalue &rvalue ?

> An objectis a named region of storage; an lvalue is an expression referring to such an

object.

> The term rvalue refers to a data value that is stored at some address in memory.

Cont.


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2.10

Pointer Basics

> Consider an example: int j,k;

k=2;

j = 7; In line 2, however,jis interpreted as its rvalue (since it is on the right hand side of the assignmentoperator '=').

> That is, here thejrefers to the value storedat the memory location set aside forj, in this case 7.

> So, the 7 is copied to the address designated by the lvalue ofk.

Cont.


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2.11

Pointer Basics

> Now,let's say that we have a reason for wanting a variable designed to hold an lvalue (an address).

> The size required to hold such a value depends on the system.

> Computers with more memory would require more bytes to hold an address.

> Such a variable is called apointer variable.

> In C a pointer variable is defined by preceding its name with an asterisk.

> Also,pointer has to be given a type which, in this case, refers to the type of data that will be stored atthe address stored in the pointer. int *ptr;

Cont.


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2.12

> With arrays:

> Extravagant waste of space.

> Strange to declare a 500 million character array to look at 100 lines ofcode.

> Have to declare it to have its maximum size in every dimension from the beginning, whilethere is no way to predict and handle the maximum line length of a text file, because,technically, that number is infinite.

Why pointers?

Cont.


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2.13

> With pointers:> You can create dynamic data structures.

> Allocation of memory from the heap while the program is running.

> Ability to use the exact amount of memory a document needs, with no waste.

> Ability to return the memory to the heap when you close a document.

> Memory can be recycled while the program is running.

Why pointers?


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2.14

Initialization of Pointers

> Can be initialized four different ways.

1. Using malloc statement2. Using statement such as p= q . If q is pointing at a valid block, p is

initialized.

3. Pointing to a known address, such as a global variable's address. Ex.p=&i

4. Using value zero i.e., p= 0; or: p= NULL;


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2.15

POINTERS- & AND * OPERATORS

> Consider the declaration,

> Int i=3;> Declaration tells the compiler:

Reserve space in memory to hold the integer value.

Associate the name i with this memory location.

Store the value 3 at this location.

3

i

6485

Location name

Value at location

Location no. (address)


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2.16

POINTERS

> Computer has selected 6485 as the place to store the value 3.

> Location number may vary when you compile the program differenttime on different machine.

> Important point is, is address in memory is a number.


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2.17

POINTERS Example

main()

{

Int I=3;

Printf( \n address of I =%u,&i);

Printf( \n value of I =%u , i);

}

> Output would be:

Address of I=6485

Value of I=

3;

In the first printf,&- address of operator

Otherpointer available in C is *,called value at address or indirection operator.


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2.18

POINTERS Example

main()

{

Int I=3;

Printf( \n address of I =%u,&i);

Printf( \n value of I =%u , i);

printf( \n value of i=%d,*(&i));

}

Output:address of I=6485

value of I=3

value of I=3

*(&i)- prints the value of i.


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2.19

POINTERExpressions

> We have seen that,&I returns the address of i.

> This address can be collected in a variable by saying,

> J=&I

> But,J is not an ordinary integer variable

> It is the variable which contains the address of the other variable.

> Compilerprovides space in the memory for the variable j as shown here:


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2.20

POINTERS

3 6485

I

6485

J

3276

J must be declared before used in the program. It is declared as,

Int *j;


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2.21

POINTERS

main()

{

Int i=3;

Int *j;J=&I;

Printf(\n address of i=%u,&i);

Printf(\n address of i=%u, j);

Printf(\n address of j=%u,&j);

Printf(\n value of j=%d, j);

Printf(\n value of i=%d, i);

Printf(\n value of i=%d, *(&i));Printf(\n value of i=%d, *j);

}

Good understanding of expressions &I, &j, *j and *(&i) is important to get a good

grip on pointers.


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2.22

POINTERS-Example

> Output of the above program:

Address of i=6485Address of j=6485

Address of j=3276

Value of j=6485

Value of i=3

Value of i=3

Value of i=3


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2.23

POINTERS

> Look at the following declarations,Int *alpha;

Char *ch;

Float *s;

> Here alpha,ch and s are declared as pointer variables (holdingaddresses).

> S-going to have the address of a floating point value> Ch-going to have a address ofchar value.


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2.24

POINTERS Example

Main()

{

Int i=3;

Int *j;

Int **k;

J=&I;

K=&j;

Printf(\n address of i=%u,&i);

Printf(\n address of i=%u, j);

Printf(\n address of i=%u,*k);

Printf(\n address of j=%u ,&j);Printf(\n address of j=%u ,k);

Printf(\n address ofk=%u,&k);

Continued..


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2.25

POINTERS

Printf( \n \n value of j =%u, j);

Printf(\n value ofk=%u,k);

Printf(\n value of i=%d, i);

Printf(\n value of i=%d,*(&i));

Printf(\n value of i=%d,*j);

Printf(\n value of i=%d,**k);}

3 6485 3276

i kj

6485 72343276


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2.26

POINTERS

> Output of the program would be:Address of i=6485

Address of i=6485

Address of i=6485

Address of j=3276

Address of j=3276

Address ofk=7234

Value of j=6485

Value ofk=3276

Value of i=3

Value of i=3

Value of i=3

Value of i=3


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2.27

POINTERS-Char, Int and float pointers

> Main()

> {

> Charc,*cc;

> Int I,*ii;

> Float a,*aa;

> c=A;

> i=54;

> a=3.14;

> cc=&c;

> ii=&I;

> aa=&a;

> Printf( \n address contained in cc=%u \t and value ofc=%c \n,cc,*cc);

> Printf( \n address contained in ii=%u \t and value of i=%d \n, ii,*ii);> Printf( \n address contained in aa=%u \t and value of a=%f \n, aa,*aa);

> }


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2.28

POINTERS

> Output of the program would be:> address contained in cc=1004 and value ofc=A

> address contained in ii=2008 and value of i=54

> address contained in aa=7006 and value ofc=3.140000

65

1004

Binary eq of 54

2008

Binary equivalent of3.14

7006

c i a

cc ii aa


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Function Pointers

> function itself is not a

variable,but it is possible to

define pointers to functions

which can be assigned,placed

in arrays,passed to

functions,returned from

functions and so on..


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2.30

Passing Addresses to functions

> Addresses of actual arguments in the calling function are copied intoformal arguments of the called function.

> Changes made in the formal arguments reflects on the actualarguments.

>Following program illustrates this fact:


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2.31

Pass by address-example

main( )

{

int a = 10, b =20 ;

swapr(&a,&b ) ;

printf( "\n a =%d b =%d", a, b ) ;}

swapr( int *x, int *y )

{

int t ;

t = *x ;

*x = *y ;

*y = t ;

}

Output would be:

a=20

b=10


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2.32

Pass by address

> By using pass by address intelligently,we can make a function return more than one value at a time.

> Example:main( ){

int radius ;float area,perimeter;

printf( "\n Enter radius of a circle " ) ;scanf( "%d",&radius ) ;areaperi ( radius,&area,&perimeter ) ;printf( "Area =% f", area ) ;printf( "\n Perimeter=% f",perimeter ) ;}

areaperi ( int r, float *a, float *p ){

*a =3.14 * r * r;

*p=2 * 3.14 * r;

}


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2.33

Pass by address

> Passing the value of radius but,addresses of area and perimeter.

> Any change we make to the values stored at addresses contained inthe variables,would make change effective even in main()


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2.34

Functions returning pointers

> Function can even return a pointer,like it return int,float,a double oranother data type.

> To make a function return a pointer,it has to be explicitly mentionedin the calling function as well as in the function definition.


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2.35

Functions returning pointers

/*Example*/

#include

main()

{

int *p;int *fun();

p=fun();

printf("\n%u\n",p);

printf("\n%d\n",*p);

printf("\n%u\n",&i);

}

int *fun()

{static int i=20;

return (&i);

}


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2.36

Typecasting

> To convert one type of variable to other type.

> General form:(type) expression

> Operation tellc to compute the value of the expression, and thenconvert it to specific type.

> This is useful when you work with integers and floating pointnumbers.


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2.37

Typecasting

int won,lost; /* # games won/lost so far */

float ratio; /* win/lose ratio */

won =5;

lost =3;

ratio = won / lost; /* ratio will get 1.0 (a wrong value) */

/* The following willcompute the correct ratio */

ratio =((float) won) / ((float) lost);

It is also used to convert pointers of one type to other.


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2.38

Roadmap

> Summary


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2.39

Pointers

A pointerholds theaddress of anobject

Use them to access and update variables: *ip = *ip + 1;

Array variables behave like pointers to their firstelement

int *ep = eightPrimes;

Pointers can be treated like arrays: ep[7] = 23;

But have different sizes: sizeof(eightPrimes) == 32)

sizeof(ep) == 4)

You may increment and decrement pointers: ep = ep+1;

Declare a pointer to an unknown data type asvoid*

void *vp = ep;

But typecast it properly before using it! ((int*)vp)[6] = 29;

int i = 10;

int *ip = &i; /* assign address of i to ip */


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