DYNAMIC MEMORY ALLOCATION

Dynamic Memory Allocation can be defined as a procedure in which the size of a data structure (like Array) is changed during the runtime.

C provides some functions to achieve these tasks. There are 4 library functions provided by C defined under <stdlib.h> header file to facilitate dynamic memory allocation in C programming. They are:

1. malloc()


2. calloc()


3. free()


4. realloc()

1. malloc() method

“malloc” or “memory allocation” method in C is used to dynamically allocate a single large block of memory with the specified size.

It returns a pointer of type void which can be cast into a pointer of any form. It initializes each block with default garbage value.

Syntax:

ptr = (cast-type*) malloc(byte-size)

ptr = (int*) malloc(100 * sizeof(int));

Since the size of int is 2 bytes, this statement will allocate 200 bytes of memory. And, the pointer ptr holds the address of the first byte in the allocated memory.

2. calloc() method

“calloc” or “contiguous allocation” method in C is used to dynamically allocate the specified number of blocks of memory of the specified type. It initializes each block with a default value ‘0’.

Syntax:

ptr = (cast-type*)calloc(n, element-size);

ptr = (float*) calloc(25, sizeof(float));

This statement allocates contiguous space in memory for 25 elements each with the size of the float.

3. free() method

“free” method in C is used to dynamically de-allocate the memory. The memory allocated using functions malloc() and calloc() is not de-allocated on their own.

Hence the free() method is used, whenever the dynamic memory allocation takes place. It helps to reduce wastage of memory by freeing it.

Syntax:

free(ptr);

4. realloc() method

“realloc” or “re-allocation” method in C is used to dynamically change the memory allocation of a previously allocated memory.

If the memory previously allocated with the help of malloc or calloc is insufficient, realloc can be used to dynamically re-allocate memory.

re-allocation of memory maintains the already present value and new blocks will be initialized with
default garbage value.

Syntax:

ptr = realloc(ptr, newSize);

where ptr is reallocated with new size 'newSize'.

Example 1:

// Program to calculate the sum of n numbers entered by the user

 #include <stdio.h>

#include <stdlib.h>

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;

}

Example 2:

// Program to calculate the sum of n numbers entered by the user

#include <stdio.h>

#include <stdlib.h>

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;

}

Example 3:

realloc()

#include <stdio.h>

#include <stdlib.h>

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: ");

    for(i = 0; i < n1; ++i)

         printf("%u\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: ");

    for(i = 0; i < n2; ++i)

         printf("%u\n", ptr + i);

     free(ptr);

    return 0;

}