As you know, an array is a collection of a fixed number of values. Once the size of an array is declared, you cannot change it.
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;
}
Output
Enter number of elements: 3
Enter elements: 100
20
36
Sum = 156
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;
}
Output
Enter number of elements: 3
Enter elements: 100
20
36
Sum = 156
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:\n");
for(i = 0; i < n1; ++i)
printf("%pc\n",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:\n");
for(i = 0; i < n2; ++i)
printf("%pc\n", ptr + i);
free(ptr);
return 0;
}
Output
Enter size: 2 Addresses of previously allocated memory: 26855472 26855476 Enter the new size: 4 Addresses of newly allocated memory: 26855472 26855476 26855480 26855484