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1 Announcements assign due tonight No late submissions Labs start this week Very helpful for assign1

2 Goals for Today Pointer operators Allocating memory in the heap malloc and free Arrays and pointer arithmetic

3 Pointers In C++, from 16B/X Linked lists, trees, graphs Shared data Also in C Arrays, strings Pass by reference

4 Pointer Operators int i = 42, j = 17; int *p = &i; int *q = &j; & -- address-of & on int gives int * Address Memory q p j i

5 Pointer Operators int i = 42, j = 17; int *p = &i; int *q = &j; printf( %d\n, *p); // 42 & -- address-of & on int gives int * * -- dereference * on int * gives int Address Memory q p j i

6 Exercise int i = 42, j = 17; int *p = &i; int *q = &j; // What do these lines do? p = q; // (1) *p = *q; // (2) *p = q; // (3) p = *q; // (4) Address Memory q p j i

7 Exercise (1a) Address Memory int i = 42, j = 17; int *p = &i; int *q = &j; q 9 p // What do these lines do? p = q; // (1) j 9 42 i

8 Exercise (1b) Address Memory int i = 42, j = 17; int *p = &i; int *q = &j; q 94 p // What do these lines do? p = q; // (1) j 9 42 i p and q are aliases

9 Exercise (2a) Address Memory int i = 42, j = 17; int *p = &i; int *q = &j; q 9 p // What do these lines do? *p = *q; // (2) j 9 42 i

10 Exercise (2b) Address Memory int i = 42, j = 17; int *p = &i; int *q = &j; q 9 p // What do these lines do? *p = *q; // (2) j 9 17 i

11 Exercise (3a) Address Memory int i = 42, j = 17; int *p = &i; int *q = &j; q 9 p // What do these lines do? *p = q; // (3) j 9 42 i

12 Exercise (3b) Address Memory int i = 42, j = 17; int *p = &i; int *q = &j; q 9 p // What do these lines do? *p = q; // (3) j 9 94 i warning: assignment makes integer from pointer without a cast

13 Exercise (4a) Address Memory int i = 42, j = 17; int *p = &i; int *q = &j; q 9 p // What do these lines do? p = *q; // (4) j 9 42 i

14 Exercise (4b) Address Memory int i = 42, j = 17; int *p = &i; int *q = &j; q 17 p // What do these lines do? p = *q; // (4) j 9 42 i warning: assignment makes pointer from integer without a cast

15 Exercise (4c) int i = 42, j = 17; int *p = &i; int *q = &j; // What do these lines do? p = *q; // (4) printf( %d\n, *p); // seg fault Address Memory q p j i

16 Exercise int i = 42, j = 17; int *p = &i; int *q = &j; p = q; // (1) *p = *q; // (2) *p = q; // (3) p = *q; // (4) Address Memory q p j i

17 Uninitialized Pointers int i; int *p; // How bad are these? printf( %d\n, i); printf( %d\n, *p); Address Memory???? 94 9???? p i

18 Uninitialized Pointers int i; int *p; // How bad are these? printf( %d\n, i); // unpredictable, no crash printf( %d\n, *p); Address Memory???? 94 9???? p i

19 Uninitialized Pointers int i; int *p; // How bad are these? printf( %d\n, i); // unpredictable, no crash printf( %d\n, *p); // probably crash Address Memory???? 94 9???? p i

20 NULL int *p = NULL; printf( %d\n, *p); // seg fault if (p == NULL) { // ok } Address 9 Memory p

21 Pointee Types int i = 5; double d = 2.5; int *ip = &i; double *dp = &d; dp = ip; warning: assignment from incompatible pointer type

22 Allocating Memory So far: the stack Lasts until function returns Automatically cleaned up Now: the heap Recall new from C++

23 malloc malloc(size) Argument is number of bytes needed Returns pointer to space i nheap Memory stays around until freed

24 malloc malloc(size) Argument is number of bytes needed Returns pointer to space i nheap Memory stays around until freed sizeof(type) Number of bytes a given type needs

25 malloc and free malloc(size) Argument is number of bytes needed Returns pointer to space i nheap Memory stays around until freed sizeof(type) Number of bytes a given type needs free(ptr) Mark memory as no longer in use

26 Using malloc and free int *p = malloc(sizeof(int)); *p = 17; Address 9 Memory 2 p 2 17

27 Using malloc and free int *p = malloc(sizeof(int)); *p = 17; free(p); Address 9 Memory 2 p 2

28 Using malloc and free int *p = malloc(sizeof(int)); *p = 17; free(p); // BUGGY printf( %d\n, *p); *p = 35; free(p); Address 9 Memory 2 p 2

29 Using malloc and free int *p = malloc(sizeof(int)); *p = 17; int *q = p; free(p); free(q); // BUGGY Address 98 9 Memory 2 q 2 p 2

30 Arrays? Address int *p = malloc(4 * sizeof(int)); 9 Memory 2 p 2

31 Arrays? Address int *p = malloc(4 * sizeof(int)); // What if we could do p[] = 2; p[1] = 4; 9 p[2] = 6; p[3] = 8; Memory 2 p 2

32 Arrays! Address int *p = malloc(4 * sizeof(int)); p[] = 2; p[1] = 4; p[2] = 6; p[3] = 8; 9 Memory 2 p

33 Arrays Address int *p = malloc(4 * sizeof(int)); p[] = 2; p[1] = 4; p[2] = 6; p[3] = 8; 9 Memory 2 p

34 Pointer Arithmetic Address int *p = malloc(4 * sizeof(int)); p[] = 2; p[1] = 4; p[2] = 6; p[3] = 8; int *q = p + 2; Memory q p

35 Pointer Arithmetic Address Memory int *p = malloc(4 * sizeof(int)); 28 q p[] = 2; p[1] = 4; 98 p[2] = 6; p[3] = 8; 2 p int *q = p + 2; printf( %d\n, *q); //

36 Pointer Arithmetic Address Memory int *p = malloc(4 * sizeof(int)); 28 q p[] = 2; p[1] = 4; 98 p[2] = 6; p[3] = 8; 2 p int *q = p + 2; printf( %d\n, *q); // 6 // p[2] == *(p + 2)

37 Pointer Arithmetic Address Memory int *p = malloc(4 * sizeof(int)); 28 q p[] = 2; p[1] = 4; 98 p[2] = 6; p[3] = 8; 2 p int *q = p + 2; printf( %d\n, *q); // // p[2] == *(p + 2) // p[] == *(p + ) == *p

38 Pointer Arithmetic Address Memory int *p = malloc(4 * sizeof(int)); 28 q p[] = 2; p[1] = 4; 98 p[2] = 6; p[3] = 8; 2 p int *q = p + 2; printf( %d\n, *q); // // p[2] == *(p + 2) // p[] == *(p + ) == *p // &p[2] == p // &p[] == p 2 2

39 Subarrays Address Memory int *p = malloc(4 * sizeof(int)); 28 q p[] = 2; p[1] = 4; 98 p[2] = 6; p[3] = 8; 2 p int *q = p + 2; printf( %d\n, q[1]); //

40 Stack Arrays int arr[4]; arr[] = 5; arr[1] = 1; arr[2] = 15; arr[3] = 2; // arr == &arr[] Address Memory arr 2

41 Strings Recall char * from Friday Called C string Pointer to character(s) How does printf know when to stop printing? \ (null terminator) at end char *str = hello ; str 1 h e l l o \

42 Summary Pointer operators Allocating memory in the heap malloc and free Arrays and pointer arithmetic Lab: Strings Next time: More pointer uses Pass by reference, stack vs. heap

Review: C Strings. A string in C is just an array of characters. Lecture #4 C Strings, Arrays, & Malloc

Review: C Strings. A string in C is just an array of characters. Lecture #4 C Strings, Arrays, & Malloc CS61C L4 C Pointers (1) inst.eecs.berkeley.edu/~cs61c CS61C : Machine Structures Lecture #4 C Strings, Arrays, & Malloc Albert Chae Instructor 2008-06-26 Review: C Strings A string in C is just an array