Sometimes the size of the array you declared may be insufficient. To solve this issue, you can allocate memory manually during run-time. This is known as dynamic memory allocation in C programming.

To allocate memory dynamically, library functions are malloc(), calloc(), realloc() and free() are used. These functions are defined in the <stdlib.h> header file.

C malloc()

The name “malloc” stands for memory allocation.

The malloc() function reserves a block of memory of the specified number of bytes. And, it returns a pointer of void which can be casted into pointers of any form.

Syntax of malloc()

ptr = (castType*) malloc(size);

Example

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

The above statement allocates 400 bytes of memory. It’s because the size of float is 4 bytes. And, the pointer ptr holds the address of the first byte in the allocated memory.

The expression results in a NULL pointer if the memory cannot be allocated.

C calloc()

The name “calloc” stands for contiguous allocation.

The malloc() function allocates memory and leaves the memory uninitialized. Whereas, the calloc() function allocates memory and initializes all bits to zero.

Syntax of calloc()

ptr = (castType*)calloc(n, size);

Example:

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

The above statement allocates contiguous space in memory for 25 elements of type float.

C free()

Dynamically allocated memory created with either calloc() or malloc() doesn’t get freed on their own. You must explicitly use free() to release the space.

Syntax of free()

free(ptr);

This statement frees the space allocated in the memory pointed by ptr.

Example 1: malloc() and free()

// 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;
}

Here, we have dynamically allocated the memory for n number of int.

Example 2: calloc() and free()

// 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;
}

C realloc()

If the dynamically allocated memory is insufficient or more than required, you can change the size of previously allocated memory using the realloc() function.

Syntax of realloc()

ptr = realloc(ptr, x);

Here, ptr is reallocated with a new size x.

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("%un",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("%un", ptr + i);
  
    free(ptr);

    return 0;
}

When you run the program, the output will be:

Enter size: 2
Addresses of previously allocated memory:26855472
26855476

Enter the new size: 4
Addresses of newly allocated memory:26855472
26855476
26855480
26855484