Pointers (1A) Young Won Lim 12/4/17

Transcription

1 Pointers (1A)

2 Coyright (c) Young W. Lim. Permission is granted to coy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version ublished by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A coy 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 roduced by using OenOffice.

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 and their es int a; a int * ; & int **; & 4

5 Variables with initializations int a; a int * = ; & = int ** = &; & = & 5

6 Pointed es :, int a; a int * = ; int ** = &; & 6

7 Dereferenced Variables : * int a; * * a int * = ; int ** = &; & 7

8 Dereferenced Variables : *, ** int a; * ** ** a int * = ; * * int ** = &; & 8

9 Two more ways to access a : *, ** a * * ** & * & & & a * a ** a * 9

10 Two more ways to access a : *, ** a & * a ** a * & 1) a 2) * 3) ** 10

11 Variable Scoes int func1 (int a, int b) { int i, int j; } i and j s variable scoe Only to stack frame is active and its variable can be accessed Communications are erformed only through the arameter variables cannot access Each other int main () { int x, int y; x and y s variable scoe func1 s Stack Frame int i, int j; int a, int b } func1 ( 10, 20 ); main s Stack Frame ( 10, 20 ) int x, int y; 11

12 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=&x and b=&y *a and *b int main () { int x, int y; x and y s variable scoe func1 s Stack Frame int i, int j; int* a, int* b a b } func1 ( &x, &y ); ( &x, &y ) main s Stack Frame int x, int y; x y *a *b Functions 1 Overview 12

13 Pass by integer reference void swa(int *, int *) { int tm; a } tm = *; * = *; * = tm; &b b int a, b; swa(, &b ); 13

14 Swaing ointers & a & &b b & a & &b b 14

15 Pass by integer ointer reference void swa(int **m, int **n) { int* tm; & } tm = *m; *m = *n; *n = tm; & int a, b; int *, *; swa( &, & ); 15

16 Integer Pointer Tyes long a; a int *; & short *; & char *r; &r r 16

17 Byte Address long a; a MSByte LSByte Increasing Little Endian Big Endian Increasing 17

18 Pointer Tye Cast long a; starting int *; = (int *) short *; char *r; r 18

19 Integer Pointer Tyes long a; int *; short *; = (short *) char *r; r 19

20 Integer Pointer Tyes long a; int *; short *; char *r; r = (char *) 20

21 Re-interretation of memory int a; int *; = (int *) short *; = (short *) char *r; = (char *) 21

22 Variables int a; a can hold an integer a a = 100; a holds an integer 100 a

23 Pointer Variables int * ; holds an int * ; holds an of a int tye & ointer to int int * ; * holds an integer int * 23

24 Pointer to Pointer Variable int ** ; holds an int ** ; ointer to ointer to int holds an of a ointer to int tye & int * *; ointer to int * holds an of a int tye variable * int **; ** holds an integer * ** int 24

25 Pointer Variables int a; int * = & a; & 0x3CE = 0x3AB int ** = & ; & 0x3AB 0x3A0 0x3A0 a 200 & 0x3CE 0x3AB * 0x3A0 **

26 Pointer Variables & & 0x3CE = 0x3AB * & 0x3AB 0x3A0 **a a 0x3A0 a 200 & 0x3CE 0x3AB * 0x3A0 **

27 Pointer Variables int * ; & 0x3CE = 0x3AB & 0x3CE = 0x3AB 0x3AB * 0x3A0 0x3AB * 200 * 0x3A0 ** 200 & * 0x3CE 0x3AB 200 & 0x3CE 0x3AB * 0x3A0 **

28 Pointer to Pointer Variable int ** ; & 0x3CE = 0x3AB holds an 0x3AB * 0x3A0 int ** ; ointer to ointer to int holds an of a ointer to int tye * 0x3A0 ** 200 int * *; ointer to int * holds an of a int tye variable int **; ** holds an integer & 0x3CE 0x3AB * 0x3A0 ** 200 int 28

29 Interretation of Pointer (1) (int) (int **) (int *) (int *) * (int **) * (int *) (int) * * ** 29

30 Interretation of Pointer (2) The of a variable : Address of oerator & content of az ointer : Dereferencing oerator * & & * * * * 30

31 Integer Pointer Examles (1) int i; int tye (int) i int * int ** i; i; int * tye (int *) i i (int) *i i holds an integers i holds an of a int tye int ** tye (int **) i i holds an of a ointer to int tye int * tye i (int *) *i *i (int) **i 31

32 Integer Pointer Examles (2) int i; int * i; int ** i; i holds an integers (int) (int **) i X X (int *) i holds an of Int tye (float *) (int) i holds an of Pointer to Int tye (float) 32

33 Integer Pointer Examles (3) int i = 200; tyes int ** i &i i = &i int * i = &i; int ** i = &i; int * i &i i = &i i holds an integers int i &i i =200 i holds an of a int tye i holds an of a ointer to Int tye *i = i *i = i *i = i = &i **i = *i = i 33

34 Variable Declarations int a ; a =100 The variable a holds an integer int * ; & 200 The ointer variable holds an, where an integer is stored int * * ; & 30 The ointer variable holds an, where another is stored, where an integer is stored 34

35 Access Data Via Pointer Variables (1) int a ; a =100 Direct Access value a integer int * ; & *=200 Indirect Access Dereference Oerator * the content of the ointed location & value *(&) * * integer 35

36 Access Data Via Pointer Variables (2) int * * ; & * * **=30 Double Indirect Access & value Dereference Oerator * the content of the ointed location *(&) * * Dereference Oerator * the content of the ointed location * ** * ** integer 36

37 Access Data Via Pointer Variables (3) int a ; a =100 Direct Access value a integer int * ; & *=200 Indirect Access Dereference Oerator * the content of the ointed location value & * integer int * * ; & * * **=30 Double Indirect Access Dereference Oerator * the content of the ointed location value & * * ** integer 37

38 Access Data Via Pointer Variables (4) * int a a =100 *() = a * * int * & *=200 *(&) = *() = * * * * int * * & * * **=30 *(&) = *() = * *(*) = ** 38

39 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 Exress, I. K. Chun 39