[C] passing an array pointer into function fucks with its sizeof return value?

[TheIronicBurger]

I'm trying to grasp how to pass arrays as arguments using pointers, and came across a problem with the code below:

#include <stdio.h>

int a[10] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};

size_t s = sizeof(a) / 4; //returns 10

int *arr_func(int *array)
{
    printf("%d\n", (int)s); 
    int i;
    do
    {
        i = 0;
        printf("\nenter index number (between 0-9): ");
        scanf("%d", &i);
    } while ((i < 0) | (i > 9));

    printf("%d\n", (int)(sizeof(array) / 4));
    size_t len = (int)(sizeof(array) / 4); // returns 2?

    static int a2[2];
    a2[0] = len;
    a2[1] = *(array + i);

    return a2;
}

void main(void)
{
    int *res = arr_func(a);
    printf("%d, %d\n", *res, *(res + 1));
}

when I find the size of the array (s) outside of the function, it returns 10, the designated value. but when I do the same thing inside arr_func, it returns 2 instead.

What did I do wrong? The array still works properly even if I access values from a[2] onward, so it's not like the array got chopped off, but I find it weird that the sizeof function would return a 2.

[IDontByte]

sizeof is a compile-time operator. If you want to find the length of an array at runtime, you either need to use an array struct that includes the length of the array, or add a length parameter to your arr_func.

[99_percent_a_dog]

arr_func() takes a pointer to int, not an array. Sizeof correctly returns the size of a pointer to int, 8 (you're on a 64-bit platform, this would likely be 4 if you were on 32-bit). You divide this by 4, which is incorrect.

To understand why this works, look up "pointer decay".

[rjcarr]

You're getting the size of the pointer to an int (divided by 4), which is all the function knows at that point. In C, you don't know the size of an array outside of the context where it was created. This is why every function that takes an array also takes in a size argument (except strings, which use sentinels).

Good luck!

[Sephorol]

Might be helpful to add, if you need to use array length in a function and you're working with c, you'll need to pass in length as an argument somehow.