C Pointers Systems Programming Concepts Pointers and Addresses

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C Pointers Systems Programming Concepts

C Pointers Systems Programming Concepts

Pointers and Addresses § Pointers § Using Pointers in Call by Reference § Swap

Pointers and Addresses § Pointers § Using Pointers in Call by Reference § Swap – A Pointer Example § Pointers and Arrays § Operator Precedence Example § Systems Programming Pointers 2

Variables § Variable names correspond to memory locations in memory. Every variable has a

Variables § Variable names correspond to memory locations in memory. Every variable has a type, a name and a value. int i; i i = 4; 4 32212242 (the address of i ) &i Systems Programming Pointers 3

Print an Address int main () { int i; i = 4; printf(‘i =

Print an Address int main () { int i; i = 4; printf(‘i = %d, address of i = %un”, i, &i); return 0; } $. /ptr 1 i = 4, address of i = 3220392980 Systems Programming Pointers 4

Pointers § What is a pointer? – a variable that contains a memory address

Pointers § What is a pointer? – a variable that contains a memory address as its value. – Pointers contain the address of a variable that has a specific value (an indirect reference). § Pointers in C are typed – a pointer to a variable of type int – a pointer to a variable of type char – a pointer to a defined type or an object. 2007 Pearson Ed -All rights reserved. Systems Programming Pointers 5

Fig. 7. 1 Directly and Indirectly Referencing a Variable 2007 Pearson Ed -All rights

Fig. 7. 1 Directly and Indirectly Referencing a Variable 2007 Pearson Ed -All rights reserved. Systems Programming Pointers 6

Pointers /* Welcome to the world of Pointers! Pointers are a powerful tool */

Pointers /* Welcome to the world of Pointers! Pointers are a powerful tool */ int main () { int i; int *ptr; /* pointer declaration */ } i = 4; ptr = &i; printf(" i = %dn address of i = %un address of pointer = %un", i, ptr, &ptr); return 0; . /ptr 2 i=4 address of i = 3219352564 address of pointer = 3219352560 Systems Programming Pointers 7

Pointers /* Do you think in Hex ? */ int main () { int

Pointers /* Do you think in Hex ? */ int main () { int i; int *ptr; ptr bfe 07240 bfe 07244 4 i i = 4; ptr = &i; printf(" i = %dn address of i = %pn address of pointer = %pn", i, ptr, &ptr); return 0; } . /ptr 3 i=4 address of i = 0 xbfe 07244 address of pointer = 0 xbfe 07240 Systems Programming Pointers 8

Pointers /* Never trust a Compiler. */ int j, i; /* think globally! */

Pointers /* Never trust a Compiler. */ int j, i; /* think globally! */ int *ptr 1, *ptr 2; void printit () { printf(" i = %2 d, ptr 1 = %pn", i, ptr 1); printf(" j = %2 d, ptr 2 = %pn", j, ptr 2); } int main () { i = 4; j = 8; The unary (or indirection) operator returns the value of the object to ptr 1 = &i; which its operand points. ptr 2 = &j; printit (); *ptr 2 = *ptr 2 + 1; ptr 1 = ptr 1 - 2; /* You cannot know this */ printit (); i = 6; *ptr 1 = *ptr 1 + 10; printit (); return 0; } Systems Programming ptr 1 ptr 2 j i 8049654 804964 c 8 19 9 4 6 . /ptr 4 i = 4, ptr 1 = 0 x 8049654 j = 8, ptr 2 = 0 x 804964 c i = 4, ptr 1 = 0 x 804964 c j = 9, ptr 2 = 0 x 804964 c i = 6, ptr 1 = 0 x 804964 c j = 19, ptr 2 = 0 x 804964 c Pointers 9

7. 4 Passing Arguments to Functions by Reference § § § All arguments in

7. 4 Passing Arguments to Functions by Reference § § § All arguments in C are passed by value!! Call by reference is done with pointer arguments. – Pass address of argument using & (address operator). – Allows you to change the value of the variable in the caller. – Arrays are not passed with & because the array name is already a pointer. * indirection operator – Used as alias/nickname for variable inside of function void double( int *number ) { *number = 2 * ( *number ); 2007 Pearson Ed -All rights reserved. } – *number used as nickname for the variable passed. Systems Programming Pointers 10

Using Pointers in Call by Reference 2007 Pearson Ed -All rights reserved. Function prototype

Using Pointers in Call by Reference 2007 Pearson Ed -All rights reserved. Function prototype takes a pointer argument Function cube. By. Reference is passed an address, which can be the value of a pointer variable. In this program, *n. Ptr is number, so this statement modifies the value of number itself. Systems Programming Pointers 11

Swap: A Pointer Example /* A simple memory swap using pointers */ void swap

Swap: A Pointer Example /* A simple memory swap using pointers */ void swap (int *i, int *j) { int temp; } temp = *i; *i = *j; *j = temp; Systems Programming Pointers 12

Swap: A Pointer Example int main ( ) {. /swap mem 1: 81 mem

Swap: A Pointer Example int main ( ) {. /swap mem 1: 81 mem 2: 12 int i; ray 1[0] = 0 ray 1[1] = 10 ray 1[2] = 20 ray 1[3] = 30 int mem 1; int mem 2; int ray 1[4]; mem 1 = 12; mem 2 = 81; swap (&mem 1, &mem 2); /* swap two integers */ printf("mem 1: %4 d mem 2: %4 dn", mem 1, mem 2); for (i = 0; i < 4; i++) { ray 1[i] = 10*i; printf("ray 1[%d] =%4 d ", i, ray 1[i]); } printf("n"); Systems Programming Pointers 13

Swap: A Pointer Example swap (&mem 1, &ray 1[3]); swap (&mem 2, &ray 1[2]);

Swap: A Pointer Example swap (&mem 1, &ray 1[3]); swap (&mem 2, &ray 1[2]); printf("mem 1: %4 d mem 2: %4 dn", mem 1, mem 2); for (i = 0; i < 4; i++) printf("ray 1[%d] =%4 d ", i, ray 1[i]); } printf("n"); return 0; mem 1: 30 mem 2: 20 ray 1[0] = 0 ray 1[1] = 10 ray 1[2] = 12 ray 1[3] = 81 Systems Programming Pointers 14

Pointers and Arrays r 1 1 1 0 0 0 r[0] r[1] r[2] r[3]

Pointers and Arrays r 1 1 1 0 0 0 r[0] r[1] r[2] r[3] r[4] r[5] ptr int main () { int i, r[6] ={1, 1, 1}; int *ptr; ptr = r; *ptr = 83; *(ptr +2) = 33; for (i=0; i < 6; i++) printf (" r[%d] = %dn", i, r[i]); Systems Programming Pointers 15

Pointers and Arrays r 83 1 33 0 0 0 r[0] r[1] r[2] r[3]

Pointers and Arrays r 83 1 33 0 0 0 r[0] r[1] r[2] r[3] r[4] r[5] ptr } r[4] = *ptr; ptr++; *ptr = 6; *(ptr +2) = 7; for (i=0; i < 6; i++) printf (" r[%d] = %dn", i, r[i]); return 0; Systems Programming Pointers 16

Pointers and Arrays r 83 1 33 0 83 0 r[0] r[1] r[2] r[3]

Pointers and Arrays r 83 1 33 0 83 0 r[0] r[1] r[2] r[3] r[4] r[5] ptr } r[4] = *ptr; ptr++; *ptr = 6; *(ptr +2) = 7; for (i=0; i < 6; i++) printf (" r[%d] = %dn", i, r[i]); return 0; Systems Programming Pointers 17

Pointers and Arrays r 83 6 33 7 83 0 r[0] r[1] r[2] r[3]

Pointers and Arrays r 83 6 33 7 83 0 r[0] r[1] r[2] r[3] r[4] r[5] ptr } r[4] = *ptr; ptr++; *ptr = 6; *(ptr +2) = 7; for (i=0; i < 6; i++) printf (" r[%d] = %dn", i, r[i]); return 0; Systems Programming Pointers 18

Operator Precedence Example /* An example of operator precedence trouble */ int main ()

Operator Precedence Example /* An example of operator precedence trouble */ int main () { $. /prec float x, y, z; 3220728372 3220728368 3220728364 float *ptr 1, *ptr 2, *ptr 3; x =2. 0; y = 8. 0; z = 4. 0; 2. 000000 8. 000000 4. 000000 3220728372 3220728368 2. 000000 8. 000000 ptr 1 = &x; ptr 2 = &y; ptr 3 = &z; printf (" %u %u %un", ptr 1, ptr 2, ptr 3); *ptr 3++; printf (" %f printf (" %u printf (" %f %f %u %f %fn", %un", %fn", x, y, z); ptr 1, ptr 2, ptr 3); *ptr 1, *ptr 2, *ptr 3); Systems Programming Pointers 19

Precedence Example (*ptr 1)++; printf (" %f %f %fn", *ptr 1, *ptr 2, *ptr

Precedence Example (*ptr 1)++; printf (" %f %f %fn", *ptr 1, *ptr 2, *ptr 3); } --*ptr 2; printf (" %f %f %fn", *ptr 1, *ptr 2, *ptr 3); printf (" %f %f %fn", x, y, z); return 0; 3. 000000 8. 000000 3. 000000 7. 000000 4. 000000 Systems Programming Pointers 20

Review of Pointer Basics § § This section demonstrated the relationship between pointers and

Review of Pointer Basics § § This section demonstrated the relationship between pointers and addresses and introduced the respective operators & and *. Showed the use of pointers in simple examples. Introduced call by reference with pointers. Detailed the relationship between pointers and arrays. Systems Programming Pointers 21

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