Update the same trays

When main passes trays, the function receives an address that leads to the same array elements. It does not receive a second array. Because this parameter is not const, assignments to trays[i] change the caller's elements. The separate count limits the loop; for a read-only function, use const int[] instead.

#include <stdio.h>

void add_one(int trays[], size_t count) {
    for (size_t i = 0; i < count; i++) {
        trays[i]++;
    }
}

int main(void) {
    int trays[] = {3, 4};
    add_one(trays, 2);
    printf("Trays: %d, %d\n", trays[0], trays[1]);
    return 0;
}

Before Run, predict 4 and 5. The function does not need to return the array: trays[i] accesses caller-owned elements. This differs from passing one plain int, which only copies that integer value. Try changing the initial values and check that the function still adds one to each.