#include <stdio.h>
#include <stdbool.h>

struct Reading { int id; int depth; };
static bool set_depth(struct Reading *reading, int depth) {
    if (reading == NULL || depth < 0 || depth > 500) { return false; }
    reading->depth = depth;
    return true;
}

int main(void) {
    struct Reading reading = {7, 18};
    if (!set_depth(&reading, 25)) { return 1; }
    printf("Station %d: %d mm\n", reading.id, reading.depth);
    return 0;
}