CODEWITHSUNDEEP
listpointerslinked-listrustunsafe
user1899016
user1899016

Reputation: 11

How to fix a SIGSEGV in this simple doubly linked list implementation?

I get a SIGSEGV error when running this code.

The code compiles, the debugger shows what looks like random addresses in pointers.

use std::ptr;

pub struct List<T> {
    head: *mut Node<T>,
    tail: *mut Node<T>,
}

struct Node<T> {
    data: Option<T>,
    next: *mut Node<T>,
    prev: *mut Node<T>,
}

impl<T> List<T> {
    pub fn new() -> Self {
        Self {
            head: ptr::null_mut(),
            tail: ptr::null_mut(),
        }
    }
    pub fn add_tail(&mut self, data: T) {
        let mut new_node = Box::new(Node {
            data: Some(data),
            next: ptr::null_mut(),
            prev: ptr::null_mut(),
        });
        let new_node_ptr: *mut Node<T> = &mut *new_node;
        if self.tail.is_null() {
            self.head = new_node_ptr;
        } else {
            new_node.next = self.tail;
            unsafe {
                (*self.tail).prev = new_node_ptr;
            }
        }
        self.tail = new_node_ptr;
    }
    pub fn remove_tail(&mut self) -> Option<T> {
        if self.tail.is_null() {
            None
        } else {
            let old_tail_ptr = self.tail;
            unsafe {
                if (*old_tail_ptr).next.is_null() {
                    self.tail = ptr::null_mut();
                    self.head = ptr::null_mut();
                } else {
                    let new_tail_ptr = (*old_tail_ptr).next;
                    (*old_tail_ptr).next = ptr::null_mut();
                    (*new_tail_ptr).prev = ptr::null_mut();
                    self.tail = new_tail_ptr;
                }
                (*old_tail_ptr).data.take()
            }
        }
    }
}

My test adds ten integers 0..9 and them pops them. On second pop, I get

signal: 11, SIGSEGV: invalid memory reference.

Upvotes: 0

Views: 96

Answers (1)

user1899016
user1899016

Reputation: 11

The following will fix the error. In add_tail:

  • replace let new_node_ptr: *mut Node<T> = &mut *new_node; with let new_node_ptr = Box::into_raw(new_node);
  • replace new_node.next = self.tail with (*new_node_ptr).next = self.tail;
  • include (*new_node_ptr).next = self.tail; in the unsafe block

The error was due to obtaining the underlying pointer from the Box incorrectly.

After let new_node_ptr: *mut Node<T> = &mut *new_node; the Box continued to manage the memory pointed to by new_node_ptr. At the end of the block, the Box deallocated the memory automatically leaving new_node_ptr dangling.

To obtain a pointer with manual memory management and to release control of memory from the Box, use Box::into_raw instead of &mut *.

Upvotes: 1

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