Sizeof array through function in C [duplicate]

Question

This question already has an answer here:

• How to find the 'sizeof' (a pointer pointing to an array)? 13 answers

I'm not sure why I cannot use sizeof(array) when passing the array through my function only outputs a value of 1, instead of 1000000. Before passing the array to the function, I printed out the sizeof(array) to get 4000000 and when I try to printout the sizeof(array) in the function, I only get 4. I can iterate through the array in both the function and the main to display all values, I just cannot display sizeof within the function. Did I pass the array through incorrectly?

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

int
searchMe(int numbers[], int target)
{
    printf("%d\n", sizeof(numbers));
    int position = -1;

    int i;
    int k = sizeof(numbers) / sizeof(numbers[0]);   

    for (i = 0; i < k && position == -1; i++)
    {
        if (numbers[i] == target)
        {
            position = i;
        }
    }
    return position;
}

int
main(void)
{
    int numbers[1000000];
    int i;
    int search;
    for (i = 0; i < 1000000; i++) {
        numbers[i] = i;
    }

    printf("Enter a number: ");
    scanf("%d", &search);

    printf("%d\n", sizeof(numbers));
    int position = searchMe(numbers, search);

    printf("Position of %d is at %d\n", search, position);
    return 0;
}

Answer 1

int searchMe(int numbers[], int target)

is equivalent to

int searchMe(int *numbers, int target)

There is a special C rules for function parameters that says parameters of array type are adjusted to a pointer type.

It means in your program that sizeof numbers actually yields the size of the int * pointer type and not of the array.

To get the size you have to add a third parameter to your function and explicitly pass the size of the array when you call the function.

Answer 2

Because arrays are passed as reference. Meaning only pointer to array is passed. Sizeof inside the function would only print the size of pointer, not the size of array.

Besides just making sure you know, size of returns number of bytes, so if you have an integer array (where int is of size 4 for e.g. sizeof would give elements * 4 as output.