Memory Allocation in C

Tom Kelliher, CS 320

Jan. 31, 2005

Administrivia

Announcements

Assignment

Project 0 handout.

From Last Time

Exercise, structures, pointers.

Outline

1. Introduction.

2. Simple Examples: array, doubly-linked list.

Coming Up

Discussion of Chapter 1.

Introduction

1. malloc() --- memory allocation.

   void *malloc(size_t size);
   

2. free() --- memory de-allocation.

   void free(void *ptr)
   

Small Examples

1. A dynamically sized int array:

   void createAndDestroy(int size)
   {
      int *data;
      int i;
   
      data = (int *) malloc(size * sizeof(int));
   
      if (data == NULL)
         return;
   
      for (i = 0; i < size; i++)
         data[i] = 0;
   
      free((void *) data);
   }
   

2. Dynamically allocating and accessing structures:

   typedef struct qnode {
     struct qnode *next;
     struct qnode *prev;
     int key;
     /* Additional or alternate fields here. */
   } qnode;
   
   qnode* head, temp, insert, delete;
   
   /* Don't forget to check malloc's return value! */
   
   head = (qnode*) malloc(sizeof(qnode));
   head->next = head->prev = NULL;
   head->key = 5;
   
   temp = (qnode*) malloc(sizeof(qnode));
   temp->key = 99;
   head->next = head->prev = temp;
   temp->next = temp->prev = head
   
   temp = (qnode*) malloc(sizeof(qnode));
   temp->key = 73;
   
   insert = head;  /* New item inserted AFTER item pointed to by insert. */
   
   temp->next = insert->next;
   temp->prev = insert;
   insert->next->prev = temp;
   insert->next = temp;
   
   delete = head->prev;
   
   /* The following is incorrect, from a memory allocation point-of-view.
    * Why?  How can it be fixed?
    */
   
   free((void*) delete);
   delete->prev->next = delete->next;
   delete->next->prev = delete->prev;