#include <stdio.h>

int main(void) {
    int nums[] = {10, 20, 30, 40};
    size_t len = sizeof nums / sizeof nums[0];
    for (size_t i = 0; i < len; i++) {
        printf("%zu: %d\n", i, nums[i]);
    }
    return 0;
}

fixed length, length via sizeof