unsafe_debug.rs

#[derive(PartialEq, Eq, Clone, Debug)]
pub struct ListNode {
    pub val: i32,
    pub next: Option<Box<ListNode>>,
}

impl ListNode {
    #[inline]
    fn new(val: i32) -> Self {
        ListNode { next: None, val }
    }
}

#[inline(never)]
fn get_linklist() -> Box<ListNode> {
    let mut head = Box::new(ListNode::new(0));
    let mut node_1 = Box::new(ListNode::new(1));
    let mut node_2 = Box::new(ListNode::new(2));
    node_1.next = Some(node_2);
    head.next = Some(node_1);

    head
}

#[inline(never)]
fn test() {
    println!("ListNode      size: {:?}", std::mem::size_of::<ListNode>());
    println!("&ListNode     size: {:?}", std::mem::size_of::<&ListNode>());
    println!("Box<ListNode> size: {:?}\r\n", std::mem::size_of::<Box<ListNode>>());

    let head = get_linklist();
    let mut slow = head.as_ref();
    let mut fast = head.as_ref();
    let prev = slow;

    slow = slow.next.as_ref().unwrap();
    fast = fast.next.as_ref().unwrap();
    fast = fast.next.as_ref().unwrap();

    // head -> node_1 -> node_2
    // prev -> slow   -> fast
    println!("Address and memory bytes:");
    println!("prev =====> {:p}", prev);
    debug_memory(prev);
    println!("slow =====> {:p}", slow);
    debug_memory(slow);
    println!("fast =====> {:p}", fast);
    debug_memory(fast);

    unsafe {
        let n = &mut *(prev as *const ListNode as *mut ListNode);
        n.val = 15;
        n.next = None;
    }
    println!("\r\n----After changed----\r\n");

    println!("Address and memory bytes:");
    println!("prev =====> {:p}", prev);
    debug_memory(prev);
    println!("slow =====> {:p}", slow);
    debug_memory(slow);
    println!("fast =====> {:p}", fast);
    debug_memory(fast);

    // segmentation fault here
    println!("{:?}", slow.next);
    println!("Never got here");
}

#[inline(never)]
fn main() {
    test()
}

#[inline(never)]
fn debug_memory(node: &ListNode) {
    let view = node as *const _ as *const u8;
    for i in 0..(std::mem::size_of::<ListNode>() as isize) {
        print!("{:02x} ", unsafe { *view.offset(i) });
    }
    println!();
}