CS 102 Lecture Notes: Dynamic Memory Allocation

Dynamic Memory Allocation

Dynamic Memory Allocation means to grab storage at run time. Until this point, we had to request memory at compile time. To use the dynamic memory allocation functions below, you must   #include<stdlib.h>     For an animated pointer demo, go to   fun with Binky   (thanks to Chris Johnson).

malloc(), calloc(), realloc(), and free()

malloc(), calloc(), and realloc() are 3 C functions that fulfill requests for storage at run time.
free() returns memory to the O/S.
• malloc() - used for general storage requests in the heap, usually structures.
  
• calloc() - gives contiguous storage allocation in the heap, used with arrays.
  
• realloc() - used to relocate an array area in the heap that is too small, without losing data.
  
• free() - reclaims in the heap the storage requested by one of the above functions.

malloc()

syntax (form)

        pointer_variable = (cast)malloc(#bytes);

We must cast the returned pointer since malloc() returns a pointer of type void.

Example 1

   #include<stdlib.h>

   int *p1;
   p1 = (int *)malloc(sizeof(int));      // in C++ we have   p1 = new int;
   *p1 = 5;

    Initially,          

     _______________
p1: |____ ???? _____|
               4A00

After   malloc() and *p1 = 5;

    _______________        _________________
p1:|____01F8_____| ---> |_______5_______|
              4A00                               01F8

    ______________           ________________
p2:|____????_____|       x:   |_____????______|
            1764                                 2A44

After p2 = &x; and *p2 = 5;

    _______________           ________________
p2:|____2A44_____| ---> x:|_______5_______|
               1764                                    2A44

How do examples 1 and 2 differ?

Example

int *ptr;     /*define integer pointer*/

/* get space for integer in heap, assign address of integer to ptr. */

ptr = (int *)malloc(sizeof(int));


                  ______________
                  |            | Memory for system stack,
                  |            | C code, etc.
      _____       |____________|
     |     | ---> |            |
     |_____|      |____________| heap
      ptr         |            |
                  |____________|

*ptr = 17;     /* store 17 in heap */

______________________________

Since a pointer holds an address, as does an array name, you can dynamically allocate an entire array with a single pointer.

/* Allocate an array of 100 floats */
float *fptr; int i;
fptr = (float *)malloc(100 * sizeof(float));

/* Initialize the array to 0.0 */
for(i = 0; i < 100; i++)
  fptr[i] = 0.0;

/* Store 23.8 in 21st element  */
fptr[20] = 23.8;

/* Store 23.8 in 21st element  */
*(fptr+20) = 23.8;

/* Return space for 100 floats to heap */
free(fptr);

scanf("%d", &number);
p = (int *)malloc(number * sizeof(int));

   -and-

scanf("%d", &number);
for(i = 0; i < number; i++)
  p = (int *)malloc(sizeof(int));

The NULL Pointer

Example

      int *p = NULL, *q;
      if(p == q)  puts("Pointers are equal");

NULL pointers are used to denote the end of a list or indicate an empty list.

Structure Example

#include<stdio.h>
#include<stdlib.h>

typedef struct data   {
  int x;   char list[21];   
} DATA_TYPE;

DATA_TYPE *sub(); /* prototype; returns pointer to structure */

main()  
{
  DATA_TYPE *p;
  p = sub();
  printf("value still there %d \n", p->x);
}

Note: We could use (*p).x but p->x is more intuitive.

        _______
    p: |___?___|      /* in main() */
        _______        ___________ 
    q: |_______| ---->|_55_|_list_|  /* in sub() */


in main() after return(q);
        _______        ___________
    p: |_______| ---->|_55_|_list_|  and q is gone!!!

                          letter  link                            
struct node {             |_???_|__???__|
  char letter;            
  struct node *link;
};

struct node *head,       /* pointer to head of list */
            *p;          /* traveling pointer       */

p = (struct node *)malloc(sizeof(struct node));
head = p;
p->letter = 'Z';
p->link = NULL;
p = p->link;

/*Create a linked list with N nodes*/
p = (struct node *)malloc(sizeof(struct node));
head = p;    /* one node, 2 pointers */
scanf("%c", p->letter);

for(i = 1; i < N; i++)  {
  p->link = (struct node *)malloc(sizeof(struct node));
  p = p->link;     /* Common as i++ */
  scanf("%c", p->letter);
}
p->link = NULL;  

       _________________
      |_data__|__link___|