Pointers (1A) Young Won Lim 1/5/18

Transcription

1 Pointers (1A)

2 Copyright (c) Young W. Lim. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled "GNU Free Documentation License". Please send corrections (or suggestions) to youngwlim@hotmail.com. This document was produced by using OpenOffice.

3 Address and Data in a Memory 10 bits 1K x 8 Memory 8 bits 0x3FF 0x3FE 0x3FD 0x3FC 2 10 = x003 0x002 0x001 0x000 HEX 10 bits 8 bits 3

4 Variables int a; a can hold an integer &a a a = 100; a holds an integer 100 &a a 100 4

5 Pointer Variables int * p; p holds an int * p; p holds an of a int type &p p pointer to int int * p; *p holds an integer int p *p 5

6 Dereferencing The of a variable : Address of operator & The content of a pointed location : Dereferencing operator * &p p p & p p * *p p * *p 6

7 Variables and their es int a; &a a int * p; &p p 7

8 Assignment of a variable int a; &a a = 111 int b; &b b = 8

9 Assignment of an int a; &a a = 111 int * p; &p p = 9

10 Variables with initializations int a; &a a int * p = &a; &p p = &a 10

11 Pointed es : p int a; p a int * p = &a; &p p p &a 11

12 Dereferenced Variable : *p int a; p *p *p a int * p = &a; &p p p &a *p a 12

13 Another way to access a : *p &a &p a p *p =100 a = 100 1) Read/Write a 2) Read/Write *p 13

14 1. Pass by Reference 2. Arrays 14

15 Pass by Reference 15

16 Variable Scopes int func1 (int a, int b) { int i, int j; } i and j s variable scope cannot access each other Only top stack frame is active and its variable can be accessed Communications are performed only through the parameter variables int main () { int x, int y; x and y s variable scope func1 s Stack Frame int i, int j; int a, int b a b } func1 ( 10, 20 ); main s Stack Frame ( 10, 20 ) int x, int y; x y 16

17 Pass by Reference int func1 (int* a, int* b) { int i, int j; x and y are made known to func1 func1 can read / write x and y through their es } *a *b a=&x b=&y int main () { int x, int y; } func1 ( &x, &y ); func1 s Stack Frame int i, int j; int* a, int* b x and y s variable scope ( &x, &y ) main s Stack Frame int x, int y; a b x y *a *b 17

18 Swapping integers &a a = 111 &a a = 222 &b b = 222 &b b = 111 int a, b; swap( &a, &b ); swap( int *, int *); function call function prototype 18

19 Pass by integer reference void swap(int *p, int *q) { int tmp; } tmp = *p; *p = *q; *q = tmp; int * m int *m int * n int *n int tmp int a, b; swap( &a, &b ); 19

20 Arrays 20

21 Accessing array elements using an int x[5]; x holds the starting of 5 consecutive int variables 5 int variables index cannot change x (constant) x x + 1 *x *(x+1) x[0] x[1] x[2] x[3] x[4] x + 2 x + 3 x + 4 *(x+2) *(x+3) *(x+4) 21

22 Accessing an Array with a Pointer Variable int x [5] = { 1, 2, 3, 4, 5 }; int *p = x; &x x &p p x is a constant symbol cannot be changed p[0] p[1] p[2] p[3] p[4] x[0] x[1] x[2] x[3] x[4] *(x+0) *(x+1) *(x+2) *(x+3) *(x+4) *(p+0) *(p+1) *(p+2) *(p+3) *(p+4) p is a variable can point to other es 22

23 Pointer Type Cast 23

24 Integer Pointer Types long a; &a a int b; &b b short c; &c c char d; &d d 24

25 Integer Pointer Types int *p; &p p short *q; &q q char *r; &r r 25

26 Byte Address long a; &a a MSByte LSByte Increasing Little Endian Big Endian Increasing 26

27 Pointer Type Cast long a; &a starting int *p; p p = (int *) &a short *q; q char *r; r 27

28 Integer Pointer Types long a; &a int *p; short *q; q p = (short *) &a char *r; r 28

29 Integer Pointer Types long a; &a int *p; short *q; char *r; r p = (char *) &a 29

30 Re-interpretation of memory int a; &a int *p; p = (int *) &a short *q; p = (short *) &a char *r; p = (char *) &a 30

31 const pointers 31

32 const type, const pointer type (1) const int * p; int * const q ; const int * const r ; 32

33 const type, const pointer type (2) const int * p; int * const q ; read only integer integer wr integer p read only q q wr 33

34 const type, const pointer type (3) const int * const r ; read only r r wr read only integer integer wr 34

35 Pointer Types and Associated Data 8 bits increasing char val short val int val pc ps pi char *pc; short *ps; int *pi; 35

36 Pointer Types 8 bits increasing pc ps pi char *pc; short *ps; int *pi; 36

37 Little Endian Example 8 bits 8 bits &a increasing &c &b c b a int a; short b; char c; the order of definition &b &c a b c increasing &a 37

38 int *, short *, char * type variables int * pi; short * ps; char * pc; pi ps pc Not a sized representation 38

39 Pointer Variable Assignment 8 bits &c &b &a c b a pi ps pc char * pc; short * ps; int * pi; int a; short b; char c; pi = &a; ps = &b; pc = &c; 39

40 Pointer Type Casting 8 bits &a a *pc &a a *ps &a *pi a pc ps pi char *pc; pc = (char *) &a short *ps; ps = (short *) &a int *pi; pi = (int *) &a 40

41 Accessing bytes of a variable 8 bits pc+3 *(pc+3) pc+3 a pc+2 pc+1 *(pc+2) *(pc+1) pc+2 pc+1 &a pc &a *(pc+0) pc pc pc char *pc; pc = (char *) &a char *pc; pc = (char *) &a 41

42 32-bit and 64-bit Address 32-bit machine : : 4 bytes 64-bit machine : : 8 bytes 8 bits 8 bits pi ps pc 64-bit machine : 8 bytes 32-bit machine : 4 bytes 42

43 64-bit machine : 8-byte pi ps pc char *pc; short *ps; int *pi; 43

44 64-bit machine : 8-byte & buses 8 bits char *pc; pc 8 bits short *ps; ps 8 bits int *pi; pi 44

45 32-bit machine : 4-byte pi ps pc char *pc; short *ps; int *pi; 45

46 64-bit machine : 8-byte and buses 8 bits pc char *pc; 8 bits ps short *ps; 8 bits pi int *pi; 46

47 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 47