free

Why do we need to deallocate the dynamic memory?

When we try to create the dynamic memory, the memory will be created in the heap section.

Memory leak

Let's assume total heap size as K.

If we allocate N byte of memory dynamically, it will consume N bytes of memory in heap section.

When the particular piece of code executed M number of time, then M * N bytes of memory will be consumed by our program.

At some point in time (M * N > K), the whole heap memory will be consumed by the program it will result in the system crash due to low available memory.

Pictorial Explanation

So, it is programmers responsibility to deallocate the dynamic memory which is no longer needed.

How to deallocate the dynamic memory?

Syntax of free

free(ptr);

Example

char *ptr;
ptr = malloc(N);
//do something
free(ptr);

Pictorial Explanation

Sample program

Sum of N numbers. Get N from the user and use dynamic memory to store the inputs.

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

int main()
{
    int *ptr,n,i,sum = 0;

    //get number of elements
    scanf("%d",&n);

    //allocate dynamic memory
    ptr = malloc(n * sizeof(int));

    //if success
    if(ptr != NULL)
    {
        //get input from the user
        for(i = 0; i < n; i++)
              scanf("%d", ptr + i);

        //add all elements
        for(i = 0; i < n; i++)
              sum += *(ptr + i);

        //print the result
        printf("sum = %d\n",sum);

        //deallocate the memory
        free(ptr);
    }

    return 0;
}

Double free is undefined

If we free the same pointer two or more time, then the behavior is undefined.

Example

char *ptr;
ptr = malloc(10);
free(ptr);
free(ptr);