Dynamic Memory Allocation

November 10, 2020

Dynamic Memory Allocation

Dynamic Memory Allocation refers to allocating memory manually during run-time. It allows to solve memory issues such as increasing size, decreasing size or releasing space when no space is required.

There are four types of functions for dynamic memory allocation: -

SL.No.	Function	Description
1.	malloc()	allocates requested size of bytes and returns a pointer first byte of allocated space.
2.	calloc()	allocates space for an array elements initializes to 0 and then returns a pointer to memory.
3.	realloc()	changes the size of previously allocated space.
4.	free()	deallocate the previously allocated space.

malloc()

It stands for memory allocation. This function reserves a block of memory of specified size and return a pointer of type void which can be casted into pointer of any form.

Syntax:

ptr = (data type *) malloc(byte size);

/*
Program to find sum of n elements entered by user using malloc() function.
*/
#include<stdio.h>
#include<stdlib.h>
#include<process.h>
int main(){
  int *ptr, n, i, sum=0;
  printf("Enter number of elements:");
  scanf("%d",&n);
  ptr = (int *) malloc(n * sizeof(int));
  if(ptr==NULL){
    printf("Error!\nMemory not Allocated.");
    exit(0);
  }
  printf("Enter elements values:");
  for(i=0;i<n;i++){
    scanf("%d",ptr+i);
    sum += *(ptr+i);
  }
  printf("Sum = %d",sum);
  return 0;
}

calloc()

It stands for contiguous allocation. This function allocates multiple blocks of memory each of same size and sets all bytes to 0.

Syntax:

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

realloc()

It stands for re-allocation. This function is used to enlarge or shrink the size of the previously allocated memory.

Syntax:

ptr = realloc(ptr,new-size);

Example:

#include<stdio.h>
#include<stdlib.h>
int main(){
	int *ptr, i,n1,n2;
	printf("Enter size of array:");
	scanf("%d",&n1);
	ptr = (int *)malloc(n1*sizeof(int));
	printf("Address of previously allocated memory\n");
	for(i=0;i<n1;++i){
		printf("%u\n",ptr+i);
	}
	printf("\nEnter new size of array:");
	scanf("%d",&n2);
	ptr = realloc(ptr,n2);
	for(i=0;i<n2;++i){
		printf("%u\n",ptr+i);
	}
	return 0;
}

Output:

Enter size of array:3
Address of previously allocated memory
7541696
7541700
7541704

Enter new size of array:5
7541696
7541700
7541704
7541708
7541712

free()

This function is used to deallocated memory or release space.

Syntax:

free(ptr);