Dynamic Allocation and Linked Lists

1. Dynamic Allocation and Linked Lists

2. Dynamic memory allocation in C • C uses the functions malloc() and free() to implement dynamic allocation. • malloc is defined in <stdlib.h> header files. • malloc returns a void pointer. • the pointer returned from malloc requires a type cast to make it usable. (void *: just a memory address)

3. Null Pointers • When a memory allocation function is called, there’s always a possibility that it won’t be able to locate a block of memory large enough to satisfy the requres. • A null pointer will be returned. • Test the return value: p = malloc(10000); if(p == NULL){ }

4. Example struct rec { char LastName[41]; char FirstName[41]; }; struct rec *r; r = (struct rec *) malloc(sizeof(struct rec));

5. Freeing Memory • Memory allocated with malloc must be released after you are done with them. • this memory is released by calling the function free(). • free expects as an argument the pointer returned by malloc. • free( r );

6. Allocate Memory for Arrays int *p; // one dimension int **q; // tow dimension int i; p = (int *) malloc( sizeof(int) * n); q = (int **) malloc( sizeof(int*) * n); for(i=0; i<n; ++i) { q[i] = (int *) malloc( sizeof(int) * n); } //…… free(p); free(q);

7. The “Dangling Pointer” Problem char *p = malloc(4); …. free(p); …. strcpy(p, “abc”); // WRONG

8. Linked List • A linked list is a basic data structure. • For easier understanding divide the linked list into two parts. • Nodes make up linked lists. • Nodes are structures made up of data and a pointer to another node. • Usually the pointer is called next.

9. Nodes struct node { struct rec r; struct node *next; };

10. LIST • The list will contain a pointer to the start of the list. • For our purposes all data will be added to the start of the list. • Initialize the start pointer to NULL.

11. Creating a New Node • Allocate space for a new node. • Set the next pointer to the value of NULL • Set the data value for the node.

12. Adding Nodes to the List • If the start node is null then the start node becomes the new node. • If start is not null then start becomes the new node’s next and the start becomes the new node.

17. Deleting a node Q R P I want to delete Q

18. Deleting a node Q R P P->next = Q->next; Q->next = NULL; Free(Q); I want to delete Q