Dynamic Memory Allocation

Dynamic memory allocation is a process of assigning or allocating memory to a variable during the execution of a program. Dynamic memory allocation provides storage according to the user's need and also helps to reduce the wastage of memory.

Different ways to allocate memory dynamically:

Malloc() – this function allocates a block of memory and returns a pointer to the first byte of the allocated space.
Calloc() – this function allocates memory for n number of elements and returns a pointer to the memory
Realloc() – this function allocates new memory for previously allocated memory.
Free() – this function free up space acquired by some pointer.

Malloc () function

Malloc allocates a block of memory and returns a pointer to the first byte of allocated memory. It returns NULL if requested memory cannot be allocated.

Syntax:

Ptr =  (data type *)malloc(size);

Example-

A = (int *)malloc(10 *sizeof(int));

In this, a memory block of 10 * 4(size of integer) will be allocated, and the pointer 'a' will point to the first byte of allocated memory.

Code example:

#include <stdio.h>
#include <stdlib.h>
int main()
{
    int i, n, *p;
    printf("Enter number of elements: ");
    scanf("%d", &n);
    p=(int*)malloc(n * sizeof(int));  //memory allocated using malloc
    if(p == NULL)   {
             printf("memory cannot be allocated\n");
     }
    else {
              printf("Enter elements in array:\n");
     	  for(i=0; i<n; i++)  {
               scanf("%d",&*(p+i));
          }
    printf("Elements of array are\n");
    for(i=0; i<n; i++) {
        printf("%d ",*(p+i));
      }
    return 0;
	}
}

OUTPUT

Calloc () function

The callocfunction allocates more than one memory block, each of the same size. It is the same as the malloc function, but the only difference is it takes two arguments: the number of elements and size.

Syntax-

ptr = (data type *)calloc(elements, size);

Example-

x =( int *)calloc(10, sizeof(int));

In this, 10 blocks of memory are allocated of the size of integer 'x'

pointer will point at the first block of memory.

Code example:

#include <stdio.h>
#include <stdlib.h>
int main() {
    int count, i, *ptr;
    printf("Enter number of elements: ");
    scanf("%d", &count);


    ptr = (int*) calloc(count, sizeof(int));
    if(ptr == NULL)
    {
        printf("Error! memory not allocated.");
        exit(0);
    }
     printf("Enter elements: ");
    for(i = 0; i < count; i++)   {
        scanf("%d", ptr + i);
	}
     return 0;
}

OUTPUT

Free () function

Free function frees up the space/ memory allocated previously by calloc or malloc.

Syntax-

 Free(ptr);

Example-

Free(x);

Code example:

#include <stdio.h>
#include <stdlib.h>
int main() {
	int* ptr, n;
	printf("Enter number of Elements: ");
	scanf("%d", &n);
	ptr = (int*)malloc(n * sizeof(int));
	if (ptr == NULL) {
		printf("Memory not allocated \n");
		exit(0);
	}
	free(ptr);
printf("Successfully freed memory using free function.");
return 0;
}

OUTPUT

Realloc () function

The realloc function is used to alter the size of memory allocated previously. It is helpful if a user needs more memory during the execution of a program.

Syntax-

realloc(pointer, size);

Example-

x =( int *)calloc(10, sizeof(int));
x= (int *)realloc(x, 100);

In this, the realloc function increases the memory for 'x' pointer to 100 bytes.

Code example:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
  int main () {
  char *str;
  str = (char *) malloc(15);
  strcpy(str, "burger");
  printf("String = %s,  Address = %u\n", str, str);
  /* Reallocating memory */
  str = (char *) realloc(str, 25);
  strcat(str, ".com");
  printf("\nString = %s,  Address = %u\n", str, str);
  free(str);
   return(0);
}

OUTPUT