Module prim_pointer

Raw, unsafe pointers, *const T, and *mut T.

See also the std::ptr module.

Working with raw pointers in Rust is uncommon, typically limited to a few patterns. Raw pointers can be out-of-bounds, unaligned, or null. However, when loading from or storing to a raw pointer, it must be valid for the given access and aligned. When using a field expression, tuple index expression, or array/slice index expression on a raw pointer, it follows the rules of in-bounds pointer arithmetic.

Storing through a raw pointer using *ptr = data calls drop on the old value, so write must be used if the type has drop glue and memory is not already initialized - otherwise drop would be called on the uninitialized memory.

Use the null and null_mut functions to create null pointers, and the is_null method of the *const T and *mut T types to check for null. The *const T and *mut T types also define the offset method, for pointer math.

Common ways to create raw pointers

1. Coerce a reference (&T) or mutable reference (&mut T).

let my_num: i32 = 10;
let my_num_ptr: *const i32 = &my_num;
let mut my_speed: i32 = 88;
let my_speed_ptr: *mut i32 = &mut my_speed;

To get a pointer to a boxed value, dereference the box:

let my_num: Box<i32> = Box::new(10);
let my_num_ptr: *const i32 = &*my_num;
let mut my_speed: Box<i32> = Box::new(88);
let my_speed_ptr: *mut i32 = &mut *my_speed;

This does not take ownership of the original allocation and requires no resource management later, but you must not use the pointer after its lifetime.

2. Consume a box (Box<T>).

The into_raw function consumes a box and returns the raw pointer. It doesn’t destroy T or deallocate any memory.

let my_speed: Box<i32> = Box::new(88);
let my_speed: *mut i32 = Box::into_raw(my_speed);

// By taking ownership of the original `Box<T>` though
// we are obligated to put it together later to be destroyed.
unsafe {
    drop(Box::from_raw(my_speed));
}

Note that here the call to drop is for clarity - it indicates that we are done with the given value and it should be destroyed.

3. Create it using &raw

Instead of coercing a reference to a raw pointer, you can use the raw borrow operators &raw const (for *const T) and &raw mut (for *mut T). These operators allow you to create raw pointers to fields to which you cannot create a reference (without causing undefined behavior), such as an unaligned field. This might be necessary if packed structs or uninitialized memory is involved.

#[derive(Debug, Default, Copy, Clone)]
#[repr(C, packed)]
struct S {
    aligned: u8,
    unaligned: u32,
}
let s = S::default();
let p = &raw const s.unaligned; // not allowed with coercion

4. Get it from C.

#[allow(unused_extern_crates)]
extern crate libc;

unsafe {
    let my_num: *mut i32 = libc::malloc(size_of::<i32>()) as *mut i32;
    if my_num.is_null() {
        panic!("failed to allocate memory");
    }
    libc::free(my_num as *mut core::ffi::c_void);
}

Usually you wouldn’t literally use malloc and free from Rust, but C APIs hand out a lot of pointers generally, so are a common source of raw pointers in Rust.