Chapter 4 (week 5) Section 4.10 Pointer and Dynamic Memory Allocation. CS 50 Hathairat Rattanasook

Transcription

1 Chapter 4 (week 5) Section 4.10 Pointer and Dynamic Memory Allocation. CS 50 Hathairat Rattanasook

2 Pointer and Dynamic Memory Allocation Pointers are variables. They need to be initialized before they are used. We will discuss a second way of initializing a pointer variable. A dynamic allocation is allocating memory of a variable during an execution time. CS 50 - Hathairat Rattanasook 1

3 The Dynamic allocation Memory is allocated during program execution by calling the function malloc() from ANSI C library. The function malloc() takes one parameter, the number of bytes of memory to allocate. The function malloc() returns the address of the newly allocated memory or NULL if the memory allocation fails. The malloc() is provided by the standard library. You need to include (#include <stdlib.h>) the library before using it. CS 50 - Hathairat Rattanasook 2

4 The Dynamic allocation example #include<stdio.h> double *dptr; dptr = (double *)malloc(sizeof(double)); *dptr = ; printf("dptr is %lf", *dptr); printf("\ndpre contains %p", dptr); printf("\ndpre has memory %p", &dptr); CS 50 - Hathairat Rattanasook 3

5 The terminal screen output The result may vary CS 50 - Hathairat Rattanasook 4

6 Pointer and Dynamic Memory Allocation dptr double *dptr; double *dptr2; dptr = (double *)malloc(sizeof(double)); *dptr = ; dptr2 = dptr; printf("dptr is %lf", *dptr); printf("\ndptr2 is %lf", *dptr2); printf("\ndptr contains %p", dptr); printf("\ndptr has memory at %p", &dptr); Memory address : 01E92FB8 CS 50 - Hathairat Rattanasook 5

7 Pointer and Dynamic Memory Allocation dptr double *dptr; double *dptr2; dptr = (double *)malloc(sizeof(double)); *dptr = ; dptr2 = dptr; printf("dptr is %lf", *dptr); printf("\ndptr2 is %lf", *dptr2); printf("\ndptr contains %p", dptr); printf("\ndptr has memory at %p", &dptr); Memory address : 0019FF48 dptr2 Memory address : 01E92FF44 CS 50 - Hathairat Rattanasook 6

8 Pointer and Dynamic Memory Allocation dptr double *dptr; double *dptr2; dptr = (double *)malloc(sizeof(double)); 01F02FB8 Memory address : 0019FF48 dptr2 *dptr = ; dptr2 = dptr; printf("dptr is %lf", *dptr); printf("\ndptr2 is %lf", *dptr2); printf("\ndptr contains %p", dptr); printf("\ndptr has memory at %p", &dptr); Memory address : 01E92FF44 Memory address : 01F02FB8 CS 50 - Hathairat Rattanasook 7

9 Pointer and Dynamic Memory Allocation dptr double *dptr; double *dptr2; dptr = (double *)malloc(sizeof(double)); *dptr = ; dptr2 = dptr; printf("dptr is %lf", *dptr); printf("\ndptr2 is %lf", *dptr2); printf("\ndptr contains %p", dptr); printf("\ndptr has memory at %p", &dptr); 01F02FB8 Memory address : 0019FF48 dptr2 Memory address : 01E92FF Memory address : 01F02FB8 CS 50 - Hathairat Rattanasook 8

10 Pointer and Dynamic Memory Allocation dptr double *dptr; double *dptr2; dptr = (double *)malloc(sizeof(double)); *dptr = ; dptr2 = dptr; printf("dptr is %lf", *dptr); printf("\ndptr2 is %lf", *dptr2); printf("\ndptr contains %p", dptr); printf("\ndptr has memory at %p", &dptr); 01F02FB8 Memory address : 0019FF48 dptr2 01F02FB8 Memory address : 01E92FF Memory address : 01F02FB8 CS 50 - Hathairat Rattanasook 9

11 Pointer and Dynamic Memory Allocation double *dptr; double *dptr2; dptr = (double *)malloc(sizeof(double)); *dptr = ; dptr2 = dptr; printf("dptr is %lf", *dptr); printf("\ndptr2 is %lf", *dptr2); printf("\ndptr contains %p", dptr); printf("\ndptr has memory at %p", &dptr); Output: dptr is CS 50 - Hathairat Rattanasook 10

12 Pointer and Dynamic Memory Allocation double *dptr; double *dptr2; dptr = (double *)malloc(sizeof(double)); *dptr = ; dptr2 = dptr; printf("dptr is %lf", *dptr); printf("dptr2 is %lf", *dptr2); printf("\ndptr contains %p", dptr); printf("\ndptr has memory at %p", &dptr); Output: dptr is dptr2 is CS 50 - Hathairat Rattanasook 11

13 Pointer and Dynamic Memory Allocation double *dptr; double *dptr2; dptr = (double *)malloc(sizeof(double)); *dptr = ; dptr2 = dptr; printf("dptr is %lf", *dptr); printf("dptr2 is %lf", *dptr2); printf("\ndptr contains %p", dptr); printf("\ndptr has memory at %p", &dptr); Output: dptr is dptr2 is dptr contains 01F02FB8 CS 50 - Hathairat Rattanasook 12

14 Pointer and Dynamic Memory Allocation double *dptr; double *dptr2; dptr = (double *)malloc(sizeof(double)); *dptr = ; dptr2 = dptr; printf("dptr is %lf", *dptr); printf("dptr2 is %lf", *dptr2); printf("\ndptr contains %p", dptr); printf("\ndptr has memory at %p", &dptr); Output: dptr is dptr2 is dptr contains 01F02FB8 dptr has memory at 0019FF48 CS 50 - Hathairat Rattanasook 13

15 De-allocation De-allocation frees the memory when it is no longer needed. Failure to do this de-allocation is called a memory leak, and can cause serious problems when large programs are running. free() function is used to de-allocate memory. CS 50 - Hathairat Rattanasook 14

16 De-allocation example free((void *)dptr); CS 50 - Hathairat Rattanasook 15

17 Creating an array by using malloc() The code below creates an array of size 4 with the name intarray. You can tread the inarray just like an array that you learned about last week CS 50 - Hathairat Rattanasook 16

18 Diagram the memory configuration int * intarray; intarray = (int *)malloc(5 * sizeof(int)); Memory allocation by malloc() M intarray CS 50 - Hathairat Rattanasook 17

19 An array by using malloc() The code shows how to access each element of an array. It also shows how to free the memory allocation #include<stdio.h> int * intarray; int i = 0; intarray = (int *)malloc(5 * sizeof(int)); for (; i < 5; i++) intarray[i] = i * 10; for (i = 0; i < 5; i++) printf("array element is %d\n", intarray[i]); free((void *)intarray); CS 50 - Hathairat Rattanasook 18

20 Output of the program CS 50 - Hathairat Rattanasook 19