Cookbook for C arrays and array passing to method

array.c

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

/*
 * inspect
 *
 * Demonstrates what int **parr, *parr and **parr mean
 * when a pointer is passed by address.
 *
 * Parameters:
 *   parr - pointer to a pointer to int (int **)
 *
 * In a typical caller, parr will point to the caller's
 * pointer variable (e.g. &arr), so:
 *   parr   : address of caller's pointer variable
 *   *parr  : the caller's pointer value (array pointer)
 *   **parr : the first int in the array (arr[0])
 */
void inspect(int **parr)
{
    printf("=== inspect() ===\n");

    /* address of the local parameter variable 'parr' itself */
    printf("&parr   (address of parameter 'parr'):       %p\n", &parr);

    /* value stored in 'parr': address of the caller's pointer variable */
    printf("parr    (address of caller's pointer):       %p\n", parr);

    /* value stored in the caller's pointer variable (e.g. 'arr'): heap address */
    printf("*parr   (pointer to first int in array):     %p\n", *parr);

    /* the actual first int in the array: (*parr)[0] */
    printf("**parr  (first int value in the array):      %d\n", **parr);

    printf("=================\n\n");
}

/*
 * main
 *
 * Sets up a simple example and calls inspect() to show the relationships:
 *
 *   int *arr  : pointer to first element of an int array
 *   &arr      : address of that pointer, type int **
 *
 * Passing &arr into inspect() lets inspect work with:
 *   parr   == &arr
 *   *parr  == arr
 *   **parr == arr[0]
 */
int main(void)
{
    int *arr = malloc(3 * sizeof(int));
    if (!arr) {
        perror("malloc");
        return 1;
    }

    arr[0] = 10;
    arr[1] = 20;
    arr[2] = 30;

    printf("=== main() ===\n");
    printf("&arr   (address of pointer variable 'arr'):  %p\n", (void *)&arr);
    printf("arr    (pointer to first int in array):      %p\n", (void *)arr);
    printf("arr[0] (first int value in the array):       %d\n", arr[0]);
    printf("================\n\n");

    /* Pass address of 'arr' so inspect can see & manipulate the pointer itself */
    inspect(&arr);

    free(arr);
    return 0;
}