Simple example of manipulating contents of a pointer from within a function call.

pointer_fun.c

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

int pointer_pointer_function(int **pp)
{
    
    printf("EQUAL WAYS OF ADDRESSING:\n");
    printf("*(*pp + 0): %d\n", (int) *(*pp + 0));
    printf("*(*pp + 1): %d\n", (int) *(*pp + 1));
    printf("*(*pp + 2): %d\n", (int) *(*pp + 2));
    
    printf("(*pp)[0]: %d\n", (int) (*pp)[0]);
    printf("(*pp)[1]: %d\n", (int) (*pp)[1]);
    printf("(*pp)[2]: %d\n", (int) (*pp)[2]);
    
    printf("\nREALLOCATING TO STORE FOUR INTS:\n");
    *pp = realloc(*pp, 4 * sizeof(int));
    
    (*pp)[3] = 576;
    
    return 0;
}

int main (int argc, char const *argv[])
{
    int *p = malloc(3 * sizeof(int));;
    
    p[0] = 5;
    p[1] = 15;
    p[2] = 28;
    
    printf("p[0]: %d\n", p[0]);
    printf("p[1]: %d\n", p[1]);
    printf("p[2]: %d\n", p[2]);

    printf("\nREADING OF UNALLOCATED MEMORY:\n");
    printf("p[3]: %d\n\n", p[3]);
    
    pointer_pointer_function(&p);
    
    printf("p[0]: %d\n", p[0]);
    printf("p[1]: %d\n", p[1]);
    printf("p[2]: %d\n", p[2]);
    printf("p[3]: %d\n\n", p[3]);
    
    return 0;
}