Lecture 13 Static vs Dynamic Memory Allocation malloc(), calloc(), free() Reading Assignments: Chapter 9, Section 9

Static Memory Allocation If the size of array is known before compilation, we do Memory a[1] a[2] a[0] int a[3]; How about if the number of elements we want is unknown?

Dynamic Memory Allocation int *a = NULL; int numbers; printf(“How many elements you want?\n”); scanf( “%d”, &numbers); a /* memory allocation */ a = (int *) malloc (numbers*sizeof(int)); a Memory

Dynamic Memory Allocation malloc calloc Take one parameter: void *malloc(object_size); Take two parameters: void *calloc(n, object_size); Return a pointer to the starting address of the allocated memory Or Return NULL Storage is not initialized Storage is initialized to zero

Dynamic memory allocation In order to acquire storage during program execution, C provides calloc() and malloc() (available in stdlib.h) for dynamic memory allocation To allocate a contiguous memory space for n object of a type, use calloc(n, object_size) calloc() returns a pointer to the starting address of the allocated memory if enough memory can be found; otherwise, a NULL value is returned; the storage gained is initialized to zero

Dynamic memory allocation (cont’) malloc() returns a pointer to the starting address of the allocated memory if enough memory can be found; otherwise, a NULL value is returned; its format is malloc(object_size) The function prototypes of calloc() and malloc() are void *malloc(size_t) void *calloc(int, size_t) The storage set aside by malloc() is NOT initialized

Dynamic memory allocation (cont’) As the pointers returned by malloc() and malloc() is of type void *, they can be assigned to other pointers without casting Space allocated by calloc() and malloc() is not released on function exit (but it is released on program exit) and has to be released by calling a system library function free() (available in stdlib.h) The prototype for free() is void free(void *ptr);

Dynamic memory allocation (cont’) Since the size of a data object of a particular data type is machine-dependent, sizeof() is often used when malloc() or calloc() is called, e.g., int *a; … /* get memory space for 5 integers */ if ((a = (int *) calloc(5, sizeof(int)))==NULL) { printf(“calloc() failed\n”); exit(1); } ... free(a); /* free the space */

Dynamic Memory Allocation int *a = NULL; int i, numbers; printf(“How many elements you want?\n”); scanf( “%d”, &numbers); /* allocate memory space */ if ((a=(int *) malloc(numbers*sizeof(int))) == NULL){ printf(“Not Enough Memory Space!\n”); exit(-1); /* end the program */ } for (i = 0; i < numbers; i++) a[i] = i*i; /* free memory space */ free(a);