Pointers (1A) · 22-01-2018 · Pointers 3 Young Won Lim 1/22/18 Address and Data in a Memory...

Young Won Lim1/22/18

Pointers (1A)

Young Won Lim1/22/18

Copyright (c) 2010 - 2018 Young W. Lim.

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Series:2. Pointers

3 Young Won Lim1/22/18

Address and Data in a Memory

0x000

1K x 8Memory

address 10 bits

dataaddress

data 8 bits

0x0010x0020x003

0x3FC0x3FD0x3FE0x3FF

210 = 1024

HEXaddress

8 bits10 bits

data

Series:2. Pointers


Variables

&a

data int a;

a can hold an integer a

address

&a

a = 100;

a holds an integer 100a 100

dataaddress

Series:2. Pointers


Pointer Variables

&p

int * p; *p holds an integer

pint * p;

pointer to int

int

int * p;

p holds an address

*p

p holds an addressof a int type data

p

Series:2. Pointers


Dereferencing

p

The content of a pointed location :Dereferencing operator *

*p

p

The address of a variable :Address of operator &

&

p

&p

*

p

*p

*

p

Series:2. Pointers


Variables and their addresses

&a

data

int a; a

address

int * p; &p p

Series:2. Pointers


Assignment of a variable

&a

data

int a; a = 111

address

int b; &b b = ____

Series:2. Pointers


Assignment of an address

&a

data

int a; a = 111

address

int * p; &p p = ____

Series:2. Pointers


Variables with initializations

&a

data

int a; a

address

int * p = &a; &p p = &a

Series:2. Pointers


Pointed addresses : p

p

data

int a; a

address

int * p = &a; p &p

p ≡ &a

Series:2. Pointers


Dereferenced Variable : *p

p

data

int a; *p

address

int * p = &a; p

*p ≡ a

&p

p ≡ &a*p ≡ a

Series:2. Pointers


Another way to access a : *p

*p =100 ≡ a = 100

&a

data

a

address

&p p

1) Read/Write a2) Read/Write *p

Series:2. Pointers


1. Pass by Reference2. Arrays

Series:2. Pointers


Pass by Reference

Series:2. Pointers


Variable Scopes

int main (){ int x, int y; ... ...

func1 ( 10, 20 );

... ...}

int func1 (int a, int b){ int i, int j; ... ...

... ...}

int x, int y;

int a, int b

int i, int j;

i and j’s variable scope

x and y’s variable scope

Only top stack frame is activeand its variable can be accessed

Communications are performed only through the parameter variables

( 10, 20 )

cannot access each other

func1’s Stack Frame

main’sStackFrame

xy

ab

Series:2. Pointers


Pass by Reference

int main (){ int x, int y; ... ...

func1 ( &x, &y );

... ...}

int func1 (int* a, int* b){ int i, int j; ... ...

... ...}

func1’s Stack Frame

main’sStackFrame int x, int y;

int* a, int* b

int i, int j;

x and y’s variable scope ( &x, &y )

x and y are made known to func1func1 can read / write x and ythrough their addresses

xy

ab

*a*b

a=&x b=&y

*a *b

Series:2. Pointers


Swapping integers

a = 111

b = 222

&a

&b

a = 222

b = 111

&a

&b

swap( &a, &b );

swap( int *, int *);

function call

function prototype

int a, b;

Series:2. Pointers


Pass by integer reference

void swap(int *p, int *q) {int tmp;

tmp = *p; *p = *q;

*q = tmp;}

int a, b;…

swap( &a, &b );

int * mint *m

int * nint *n

int tmp

Series:2. Pointers


Arrays

Series:2. Pointers


Accessing array elements – using an address

0

4

123

int x[5];

x holds the starting addressof 5 consecutive int variables

5 int variablesaddress data

x

x + 1

x + 2

x + 3

x + 4

*x

*(x+2)

*(x+4)

*(x+1)

*(x+3)

x[0]

x[4]

x[1]x[2]x[3]

index datacannot change address x (constant)

Series:2. Pointers


Accessing an Array with a Pointer Variable

int x [5] = { 1, 2, 3, 4, 5 };

int *p = x;

x[0]

x[4]

x[1] x[2] x[3]

60

904070

*(x+0)

*(x+4)

*(x+1) *(x+2) *(x+3)

p[0]

p[4]

p[1] p[2] p[3]

*(p+0)

*(p+4)

*(p+1) *(p+2) *(p+3)

x&x p&px is a constant symbol cannot be changed

p is a variable can point to other addresses

80

Series:2. Pointers


Byte Address Little EndianBig Endian

Series:2. Pointers


Byte Address

long a; a

Increasing address

&a ?

Series:2. Pointers


Little / Big Endian

long a;

LSByteMSByte Little Endian

a

Increasing address

MSByteLSByte Big Endian

Series:2. Pointers


Little Endian Byte Address Example

&a

long a;a7 a6 a5 a4 a3 a2 a1 a0

0x3

00

7

0x3

00

6

0x3

00

5

0x3

00

4

0x3

00

3

0x3

00

2

0x3

00

1

0x3

00

0

Increasing address

LSByteMSByte Little Endian

Series:2. Pointers


Big Endian Byte Address Example

&a

long a;

0x3

00

7

0x3

00

6

0x3

00

5

0x3

00

4

0x3

00

3

0x3

00

2

0x3

00

1

0x3

00

0

Increasing address

a0 a1 a2 a3 a4 a5 a6 a7

MSByteLSByte Big Endian

Series:2. Pointers


Representations of Endianness

0x3000

0x3001

0x3002

0x3003

0x3004

0x3005

0x3006

0x3007

a7

a6

a5

a4

a3

a2

a1

a0

0x3000

0x3001

0x3002

0x3003

0x3004

0x3005

0x3006

0x3007

a1

a2

a3

a4

a5

a6

a7

a0 0x3007

0x3006

0x3005

0x3004

0x3003

0x3002

0x3001

0x3000

a7

a6

a5

a4

a3

a2

a1

a0

0x3007

0x3006

0x3005

0x3004

0x3003

0x3002

0x3001

0x3000

a1

a2

a3

a4

a5

a6

a7

a0

Little Endian

Big Endian

Little Endian

Big Endian

&a &a

&a &a

https://stackoverflow.com/questions/15620673/which-bit-is-the-address-of-an-integer

Series:2. Pointers


Little / Big Endian Processors

Processor Endianness

Motorola 68000 Big Endian

PowerPC (PPC) Big Endian

Sun Sparc Big Endian

IBM S/390 Big Endian

Intel x86 (32 bit) Little Endian

Intel x86_64 (64 bit) Little Endian

Dec VAX Little Endian

Alpha (Big/Little) Endian

ARM (Big/Little) Endian

IA-64 (64 bit) (Big/Little) Endian

MIPS (Big/Little) Endian

http://www.yolinux.com/TUTORIALS/Endian-Byte-Order.html

Series:2. Pointers


Pointer Types

Series:2. Pointers


Integer Pointer Types

&a

long a;

int b;

short c;&b

&cchar d;

&d

d

a

b

c

Increasing address

Little Endian

Series:2. Pointers


Sizes of Integer Types

sizeof(long) 8 (bytes)

sizeof(int) 4 (bytes)

sizeof(short) 2 (bytes)

sizeof(char) 1 (bytes) d

a

b

c

Series:2. Pointers



int *p;

short *q;

&p p

&q q

char *r; &r r

Little Endian

Increasing address

Series:2. Pointers


Pointer to integer values

0x3

00

7

0x3

00

6

0x3

00

5

0x3

00

4

0x3

00

3

0x3

00

2

0x3

00

1

0x3

00

0

int *p;

Series:2. Pointers


Pointer to short values

0x3

00

7

0x3

00

6

0x3

00

5

0x3

00

4

0x3

00

3

0x3

00

2

0x3

00

1

0x3

00

0

short *q;

Series:2. Pointers


Pointer to char values

0x3

00

7

0x3

00

6

0x3

00

5

0x3

00

4

0x3

00

3

0x3

00

2

0x3

00

1

0x3

00

0

char *r;

Series:2. Pointers


Initial pointer values

int *p;

short *q;

char *r;

p

q

r

Increasing address

Series:2. Pointers


Incrementing / decrementing pointer values

int *p;

short *q;

char *r;

p

q

r

p+1

q+1

r+1

p-1

q-1

r-1

Increasing address

Series:2. Pointers


Pointer Type Cast

Series:2. Pointers


Re-interpretation of memory data

int a;

int *p;

short *q;

char *r; 80

0x7080

0x50607080

0x1020304050607080

&a

q = (short *) &a

r = (char *) &a

p = (int *) &a

Series:2. Pointers


Pointer Type Cast

long a;

int *p;

short *q;

p

char *r;

0x10203040 0x50607080

p = (int *) &a

starting address

&a

Series:2. Pointers



long a;

int *p;

short *q; q

char *r;

0x10200x30400x50600x7080

q = (short *) &a

&a

Series:2. Pointers



long a;

int *p;

short *q;

char *r; r

10 20 30 40 50 60 70 80

r = (char *) &a

&a

Series:2. Pointers


Memory Alignment

10 20 30 40 50 60 70 80

0x3

00

7

0x3

00

6

0x3

00

5

0x3

00

4

0x3

00

3

0x3

00

2

0x3

00

1

0x3

00

0

int *p;

short *q;

char *r;

pp+1

qq+2 q+1q+3r+

7r+

6r+

5r+

4r+

3r+

2r+

1r

Series:2. Pointers


const pointers

Series:2. Pointers


const type, const pointer type (1)

const int * p;

int * const q ;

const int * const r ;

Series:2. Pointers



qqp

integerinteger

read only

integerwr

read only

wr

const int * p; int * const q ;

address address

Series:2. Pointers



address

rr

read only

integerinteger

read only

const int * const r ;

wr

wr

Series:2. Pointers


Pointer Types and Associated Data

data

pc

char *pc;

data

ps

short *ps;

data

pi

int *pi;

short valchar val

int val

address address address

8 bits

incr

easi

ng a

ddre

ss

Series:2. Pointers


Pointer Types

data

pc

char *pc;

data

ps

short *ps;

data

pi

int *pi;


8 bits

incr

easi

ng a

ddre

ss

Series:2. Pointers


Little Endian Example

data

a

b

c int a;short b;char c;

&a

&b

&c

incr

easi

ng a

ddre

ss

8 bits

data

a

b

c

&a

&b

&c

8 bits

incr

easi

ng a

ddre

ss

the order of definition

Series:2. Pointers


int *, short *, char * type variables

int * pi;short * ps;char * pc;

pc

ps

pi

address

Not a sized representation

Series:2. Pointers


Pointer Variable Assignment

data

char * pc; short * ps;int * pi;

int a;short b;char c;

pc

ps

pi

pi = &a;ps = &b;pc = &c;

address

8 bits

a

b

c

&a

&b

&c

Series:2. Pointers


a

&a

a

&a

Pointer Type Casting

data

pc

char *pc;pc = (char *) &a

data

ps

data

pi


short *ps;ps = (short *) &a

int *pi;pi = (int *) &a

a

&a*ps

*pc

*pi

8 bits

Series:2. Pointers


Accessing bytes of a variable

data

pc


address

a

&a

pc+3

pc+2

pc+1

pc

*(pc+3)

data

*(pc+2)

*(pc+1)

*(pc+0)

pc


address

&a

pc+3

pc+2

pc+1

pc

8 bits

Series:2. Pointers


32-bit and 64-bit Address

32-bit machine : address : 4 bytes

64-bit machine : address : 8 bytes

pc

ps

pi

32-bit machine address :

4 bytes64-bit

machine address :

8 bytes

8 bits 8 bits

Series:2. Pointers


64-bit machine : 8-byte address

pi ps pc

char *pc; short *ps; int *pi;

Series:2. Pointers


64-bit machine : 8-byte address & data buses

char *pc; pc

8 bits

short *ps; ps

8 bits

int *pi; pi

8 bits

Series:2. Pointers


32-bit machine : 4-byte address

pi ps pc

char *pc; short *ps; int *pi;

Series:2. Pointers


64-bit machine : 8-byte address and data buses

char *pc;pc

8 bits

short *ps;ps

8 bits

int *pi;pi

8 bits

Series:2. Pointers


References

[1] Essential C, Nick Parlante[2] Efficient C Programming, Mark A. Weiss[3] C A Reference Manual, Samuel P. Harbison & Guy L. Steele Jr.[4] C Language Express, I. K. Chun

Pointers (1A) · 22-01-2018 · Pointers 3 Young Won Lim 1/22/18 Address and Data in a Memory...

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