Arrays and pointers are closely related but not the same thing: an array is storage; a pointer refers to storage. The link is decay — an array name converts to a pointer to its first element.

a[i] ≡ *(a + i) ≡ *(i + a) ≡ i[a]

#include <stdio.h>

int main(void) {
    int a[4] = {1, 2, 3, 4};
    int *p = a;                      /* decay: p = &a[0] */

    printf("%d %d %d %d\n", a[2], *(a + 2), p[2], 2[a]);   /* all the same */
    printf("sizeof a = %zu, sizeof p = %zu\n", sizeof a, sizeof p);

    p++;                             /* a pointer can move... */
    printf("*p = %d\n", *p);
    /* a++;  ...an array cannot: error, array type is not assignable */
    return 0;
}

Output

3 3 3 3
sizeof a = 16, sizeof p = 8
*p = 2

	Array `int a[4]`	Pointer `int *p`
What it is	4 ints of storage	8 bytes holding an address
`sizeof`	16	8
Assignable (`a = …`)	No	Yes
`&x`	`int (*)[4]` — pointer to the whole array	`int **`
Where the data is	Inside the array itself	Somewhere else

&a vs a: same address, different types

#include <stdio.h>

int main(void) {
    int a[4] = {0};
    int *first = a;              /* int *      */
    int (*whole)[4] = &a;        /* int (*)[4] */
    printf("same address: %d\n", (void *)first == (void *)whole);
    printf("first + 1 moves %td bytes\n", (char *)(first + 1) - (char *)first);
    printf("whole + 1 moves %td bytes\n", (char *)(whole + 1) - (char *)whole);
    return 0;
}

Output

same address: 1
first + 1 moves 4 bytes
whole + 1 moves 16 bytes

Arrays of pointers vs pointer to array

#include <stdio.h>

int main(void) {
    int x = 1, y = 2, z = 3;
    int *arr_of_ptrs[3] = {&x, &y, &z};    /* 3 pointers */
    int grid[2][3] = {{1, 2, 3}, {4, 5, 6}};
    int (*row)[3] = grid;                  /* pointer to an array of 3 ints */

    printf("%d %d\n", *arr_of_ptrs[1], *arr_of_ptrs[2]);
    row++;                                 /* moves one whole row */
    printf("%d %d\n", (*row)[0], row[0][2]);
    return 0;
}

Output

2 3
4 6

sizeof inside a function

C — clang flags the misleading sizeof (-Wsizeof-array-argument)

#include <stdio.h>

void f(int arr[10]) {                    /* the 10 is ignored: arr is int * */
    printf("in f: %zu\n", sizeof arr);
}

int main(void) {
    int data[10];
    printf("in main: %zu\n", sizeof data);
    f(data);
    return 0;
}

Are arrays and pointers the same in C?

What does int (*p)[5] declare, versus int *p[5]?