malloc , calloc and realloc functions in c

malloc(), calloc(), and realloc() are functions in C programming language that are used for dynamic memory allocation.

1. malloc(): This function is used to allocate a block of memory dynamically. The size of the memory block is specified in bytes as the argument. It returns a pointer to the first byte of the allocated space if the allocation is successful, or NULL if the allocation fails. The syntax is as follows:

void* malloc(size_t size);

2. calloc(): This function is similar to malloc(), but it has two arguments: the number of elements and the size of each element. It also initializes the allocated memory to zero. The syntax is as follows:

void* calloc(size_t num, size_t size);

3. realloc(): This function is used to change the size of the memory block that was previously allocated using malloc() or calloc(). It takes two arguments: a pointer to the memory previously allocated and the new size in bytes. The syntax is as follows:

void* realloc(void* ptr, size_t size);

Here is an example of how these functions can be used:

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

int main() {
    int *ptr;
    ptr = (int*) malloc(5 * sizeof(int)); // Allocating memory for 5 integers using malloc()

    if(ptr == NULL) {
        printf("Memory not allocated.\n");
        exit(0);
    }
    else {
        printf("Memory successfully allocated using malloc.\n");

        // Get the elements of the array
        for(int i = 0; i < 5; ++i) {
            ptr[i] = i + 1;
        }

        // Print the elements of the array
        printf("The elements of the array are: ");
        for(int i = 0; i < 5; ++i) {
            printf("%d, ", ptr[i]);
        }

        // Reallocate memory
        ptr = realloc(ptr, 10 * sizeof(int));
        printf("\nMemory successfully reallocated using realloc.\n");

        // Get the new elements of the array
        for(int i = 5; i < 10; ++i) {
            ptr[i] = i + 1;
        }

        // Print the elements of the array
        printf("The elements of the array are: ");
        for(int i = 0; i < 10; ++i) {
            printf("%d, ", ptr[i]);
        }

        free(ptr);
    }

    return 0;
}

In this example, we first allocate memory for 5 integers using malloc(), then we reallocate the memory to hold 10 integers using realloc(). Finally, we free the memory using free().