Dynamic Memory Allocation PDF

Dynamic Memory Allocation An array is a collection of a fixed number of values. Once the size of an array is declared, you cannot change it. Sometimes the size of the array you declared may be insufficient. To solve this issue, you can allocate memory manually during run-time. This is kno...

Dynamic Memory Allocation An array is a collection of a fixed number of values. Once the size of an array is declared, you cannot change it. Sometimes the size of the array you declared may be insufficient. To solve this issue, you can allocate memory manually during run-time. This is known as dynamic memory allocation in C programming. To allocate memory dynamically, library functions are malloc(), calloc(), realloc() and free() are used. These functions are defined in the header file. malloc() The name "malloc" stands for memory allocation. The malloc() function reserves a block of memory of the specified number of bytes. And, it returns a pointer which can be casted into pointers of any form. Syntax of malloc() ptr = (castType*) malloc(size); Example ptr = (float*) malloc(100 * sizeof(float)); The above statement allocates 400 bytes of memory. It's because the size of float is 4 bytes. And, the pointer ptr holds the address of the first byte in the allocated memory. The expression results in a NULL pointer if the memory cannot be allocated. free() Dynamically allocated memory created with either calloc() or malloc() doesn't get freed on their own. You must explicitly use free() to release the space. Syntax of free() free(ptr); This statement frees the space allocated in the memory pointed by ptr. // Program to calculate the sum of n numbers entered by the user-(use malloc() and free() #include #include int main() { int n, i, *ptr, sum = 0; printf("Enter number of elements: "); scanf("%d", &n); ptr = (int*) malloc(n * sizeof(int)); // if memory cannot be allocated if(ptr == NULL) { printf("Error! memory not allocated."); exit(0); }  printf("Enter elements: "); for(i = 0; i < n; i++) { scanf("%d", ptr + i); sum += *(ptr + i); } printf("Sum = %d", sum); // deallocating the memory free(ptr); return 0; } Output Enter number of elements: 3 Enter elements: 100 20 36 Sum = 156 calloc() calloc() is a method in C which is also used to allocate memory blocks, but it is generally used to allocate a sequence of memory blocks (contiguous memory) like an array of elements. It is also present in header file. Syntax of calloc() ptr = (castType*)calloc(n, size); Example: ptr = (float*) calloc(25, sizeof(float)); The above statement allocates contiguous space in memory for 25 elements of type float. // Program to calculate the sum of n numbers entered by the user (use calloc() and free() #include #include int main() { int n, i, *ptr, sum = 0; printf("Enter number of elements: "); scanf("%d", &n); ptr = (int*) calloc(n, sizeof(int)); if(ptr == NULL) { printf("Error! memory not allocated."); exit(0); }  printf("Enter elements: "); for(i = 0; i < n; ++i) { scanf("%d", ptr + i); sum += *(ptr + i); } printf("Sum = %d", sum); free(ptr); return 0; } Output Enter number of elements: 3 Enter elements: 100 20 36 Sum = 156 realloc() If the dynamically allocated memory is insufficient or more than required, you can change the size of previously allocated memory using the realloc() function. Syntax of realloc() ptr = realloc(ptr, x); Here, ptr is reallocated with a new size x. realloc()-Example #include #include int main() { int *ptr, i , n1, n2; printf("Enter size: "); scanf("%d", &n1); ptr = (int*) malloc(n1 * sizeof(int)); printf("Addresses of previously allocated memory:\n"); for(i = 0; i < n1; ++i) printf("%d\n",ptr + i); printf("\nEnter the new size: "); scanf("%d", &n2); // rellocating the memory ptr = realloc(ptr, n2 * sizeof(int)); printf("Addresses of newly allocated memory:\n"); for(i = 0; i < n2; ++i) printf("%d\n", ptr + i); free(ptr); return 0; } Output Enter size: 2 Addresses of previously allocated memory: 26855472 26855476 Enter the new size: 4 Addresses of newly allocated memory: 26855472 26855476 26855480 26855484