Dynamic Allocation and Linked Lists

Dynamic memory allocation in C C uses the functions malloc() and free() to implement dynamic allocation. malloc is defined in 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)

Null Pointers When a memory allocation function is called, theres always a possibility that it wont 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){ }

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

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 );

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);

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

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.

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

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.

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.

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 nodes next and the start becomes the new node.

Deleting a node P Q I want to delete Q R

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