% Raw Pointers

Rust has a number of different smart pointer types in its standard library, but there are two types that are extra-special. Much of Rust’s safety comes from compile-time checks, but raw pointers don’t have such guarantees, and are unsafe to use.

*const T and *mut T are called ‘raw pointers’ in Rust. Sometimes, when writing certain kinds of libraries, you’ll need to get around Rust’s safety guarantees for some reason. In this case, you can use raw pointers to implement your library, while exposing a safe interface for your users. For example, * pointers are allowed to alias, allowing them to be used to write shared-ownership types, and even thread-safe shared memory types (the Rc<T> and Arc<T> types are both implemented entirely in Rust).

Here are some things to remember about raw pointers that are different than other pointer types. They:

  • are not guaranteed to point to valid memory and are not even guaranteed to be non-null (unlike both Box and &);
  • do not have any automatic clean-up, unlike Box, and so require manual resource management;
  • are plain-old-data, that is, they don't move ownership, again unlike Box, hence the Rust compiler cannot protect against bugs like use-after-free;
  • lack any form of lifetimes, unlike &, and so the compiler cannot reason about dangling pointers; and
  • have no guarantees about aliasing or mutability other than mutation not being allowed directly through a *const T.

Basics

Creating a raw pointer is perfectly safe:

let x = 5;
let raw = &x as *const i32;

let mut y = 10;
let raw_mut = &mut y as *mut i32;

However, dereferencing one is not. This won’t work:

let x = 5;
let raw = &x as *const i32;

println!("raw points at {}", *raw);

It gives this error:

error: dereference of raw pointer requires unsafe function or block [E0133]
     println!("raw points at {}", *raw);
                                  ^~~~

When you dereference a raw pointer, you’re taking responsibility that it’s not pointing somewhere that would be incorrect. As such, you need unsafe:

let x = 5;
let raw = &x as *const i32;

let points_at = unsafe { *raw };

println!("raw points at {}", points_at);

For more operations on raw pointers, see their API documentation.

FFI

Raw pointers are useful for FFI: Rust’s *const T and *mut T are similar to C’s const T* and T*, respectively. For more about this use, consult the FFI chapter.

References and raw pointers

At runtime, a raw pointer * and a reference pointing to the same piece of data have an identical representation. In fact, an &T reference will implicitly coerce to an *const T raw pointer in safe code and similarly for the mut variants (both coercions can be performed explicitly with, respectively, value as *const T and value as *mut T).

Going the opposite direction, from *const to a reference &, is not safe. A &T is always valid, and so, at a minimum, the raw pointer *const T has to point to a valid instance of type T. Furthermore, the resulting pointer must satisfy the aliasing and mutability laws of references. The compiler assumes these properties are true for any references, no matter how they are created, and so any conversion from raw pointers is asserting that they hold. The programmer must guarantee this.

The recommended method for the conversion is:

// explicit cast
let i: u32 = 1;
let p_imm: *const u32 = &i as *const u32;

// implicit coercion
let mut m: u32 = 2;
let p_mut: *mut u32 = &mut m;

unsafe {
    let ref_imm: &u32 = &*p_imm;
    let ref_mut: &mut u32 = &mut *p_mut;
}

The &*x dereferencing style is preferred to using a transmute. The latter is far more powerful than necessary, and the more restricted operation is harder to use incorrectly; for example, it requires that x is a pointer (unlike transmute).

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