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What is Dynamic Memory Allocation in C

Under stdlib header C provides us 4 four special function which helps in allocating memory dynamically and freeing it.

  • malloc()
  • calloc()
  • realloc()
  • free()
The first three function's are used in allocation of memory while the last one frees the allocated memory.

The Malloc function

Syntax


malloc(number_of_bytes);

The malloc function  allocates contiguous memory location for the required  number_of_bytes, returns a void pointer pointing to the base address of that allocated memory. A malloc returns a void pointer,why?because malloc does not know for what we allocated the memory (malloc does not know whether we are allocating memory for ints , floats or chars ).

Usage example


int *a=(int*) malloc(5*sizeof(int));

The (int*) is done for typecasting, it converts the void pointer returned by malloc function to a integer pointer (int*) .

We are then assigning the returned(and type casted) pointer to a integer point a. So a is now pointing to the base address of allocated memory .

Let's look a complete code that uses a malloc function

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

int main()
{
    int n;
    printf("Enter the value of n\n");
    scanf("%d",&n);
    int *a=(int*) malloc(n*sizeof(int));
    if(a==NULL){
       printf("Memory allocation failed");
       exit(1);
    }
    else
    {
     for(int i = 0 ; i < n ; i++)
    {
        printf("Enter number %d ",i+1);
        scanf("%d",(a+i));
    }
            printf("You entered\n");

    for(int i = 0 ;i < n ; i++)
    {
        printf("%d ",*(a+i));
    }
     free(a);
    }
    
}

We are including a header file stdlib because this is where our functions are defined.

So,basically we want to input N numbers from the user and display them.

At line 9.

    
int *a=(int*) malloc(n*sizeof(int));

We are dynamically allocating n*4 bytes of memory (size of a int is 4 bytes) . If n=5 then the function allocates 20 bytes of memory are returns a void pointer which is type casted to an integer pointer which is later assigned to a integer pointer a.

It is advised to check if the required memory is allocated or not this can be done by using the fact that malloc returns a NULL if memory cannot be allocated.

    if(a==NULL){
        printf("Memory allocation failed");
        exit(1);
    }

Remember that these function allocates contiguous memory.

So if  'a' points to the base address then 'a+1' points to the immediate next location after a.

Also the line 21 is equivalent to

        
scanf("%d",&a[i]);

This is  because pointers and Arrays are interchangeable in C.

Note that.


&a[i] is equivalent to (a+i).
a[i] is equivalent to *(a+i).


Line 29


free(a);

will be discussed later, just remember to free the allocated memory if it is of no use .

The calloc Function

Syntax


calloc(number_of_element,number_of_bytes);

The calloc function too allocates memory and returns a void pointer to that location.Unlike the malloc function it takes two arguments number_of_element and  number_of_bytes.

The above line of code is equivalent to 


malloc(n*sizeof(int));

However there is another difference between malloc and calloc function . It is,the memory allocated by calloc function initializes to zero.

Let us look at a complete example to demonstrate the use of calloc function

1.#include<stdio.h>
2.#include<stdlib.h>
3.
4.int main()
5.{
6.    int n;
7.   printf("Enter the value of n\n");
8.    scanf("%d",&n);
9.    int *a=(int*) calloc(n,sizeof(int));
10.    if(a==NULL){
11.        printf("Memory allocation failed");
12.       exit(1);
13.    }
14.    else
15.   {
16.   for(int i = 0 ; i < n ; i++)
17.    {
18.        printf("Enter number %d ",i+1);
19.       scanf("%d",(a+i));
20.   }
21.          printf("You entered\n");
22.
23.    for(int i = 0 ; i < n ; i++)
24.    {
25.        printf("%d ",*(a+i));
26.    }
27.    free(a);
28.    }
29.}

The functionality is similar to the the former program instead we are just using calloc function here.

The Realloc Function

Syntax


realloc(*ptr,number_of_bytes);

Realloc is used to change (increase or decrease) the size of memory pointed by '*ptr' and changes it to 'number_of_bytes' . The newly initialized memory remains uninitialized.

If ptr is NULL then the call will be equivalent to


malloc(number_of_bytes);

if number_of_bytes is equal to zero it is equivalent to

     
free(ptr);

Note that only previously allocated memory can be reallocated.

A program to demonstrate the use of realloc function

#include<stdio.h>
#include<stdlib.h>
int main()
{
    int n,n1=0;
    char choice;
    printf("Enter n\n");
    scanf("%d",&n);
    int *ptr = (int*) calloc(n,sizeof(int));
    for (int i = 0; i < n; i++)
    {
        printf("Enter no %d ",i+1);
        scanf("%d",(ptr+i));
    }
    printf("Want to add more?Y/N\n");
    scanf(" %c",&choice);
    if(choice == 'Y'|| choice == 'y')
    {
     printf("How many more do you want");
     scanf("%d",&n1);
     ptr = (int*)realloc(ptr,(n+n1)*sizeof(int));
      for(int i = n; i < n+n1 ; i++)
       {
         printf("Enter no %d ",i+1);
         scanf("%d",(ptr+i));
        }
    }
    printf("You entered\n");
    for(int i = 0 ;i <(n+n1);i++)
    {
        printf("%d ",*(ptr+i));
    }
    free(ptr);//memory freed here
}

In the above code we allocated certain bytes of memory using calloc function . However we came to know that the previously allocated memory was not enough for us, so we used the realloc function to reallocate the required size of memory.

The flow of above program goes like this:

We ask the user ,the total number he wants to input and then we allocate the required byte of memory using calloc function  and we again prompt the user asking if he/she wants to add more number if his answer is yes we ask how many more and reallocate the memory using realloc function . 

The free function

Memory is a valuable resource,we don't want to waste it . The free() function is for the same purpose.It frees the memory(deallocate) previously allocated by malloc , calloc and realloc function .

Syntax

    
free(ptr);

The above line of code causes the program to give back the block to the heap.

Although we have freed ptr it is not NULL it still contains some garbage value.

Memory Leak

Memory leak refers to the memory which has been previously allocated but is not freed(released) back after it is of no use . Memory leak may not affect the functionality of the program but may cause the program to crash .

A demonstration of memory leak

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

int main()
{
    int *ptr = (int*) malloc(5*sizeof(int));
    int *A;
    *ptr = A;
}

The memory allocated by malloc function is not in any use but it is not released which causes memory leak.

It is advised to free all the allocated memory after it is of no use.

Why dynamic memory allocation over Arrays?

With arrays we have to declare the size of the the array while writing the program.The declared size sometimes may be inadequate or too abundant for our program . None of them are good for us.There is also no way to increase or decrease the size of array during execution of program.

All the problems mentioned above can be tackled using dynamic memory allocation ,we can manage memory properly using functions mentioned earlier.

Note that when we use array,static memory allocation takes place
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Comments

  1. Bibek Dhakal19 December 2020 at 13:23

    Clearly explained brother;)
    Keep up

    ReplyDelete
  2. Unknown21 January 2021 at 15:19

    it is too difficult to find out the memory leak🙂

    ReplyDelete

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