# Macro addr_of_mut

1.51.0 · Source

    pub macro addr_of_mut($place:expr) {
        ...
    }

Expand description

Creates a `mut` raw pointer to a place, without creating an intermediate reference.

`addr_of_mut!(expr)` is equivalent to `&raw mut expr`. The macro is soft-deprecated; use `&raw mut` instead.

Creating a reference with `&`/`&mut` is only allowed if the pointer is properly aligned and points to initialized data. For cases where those requirements do not hold, raw pointers should be used instead. However, `&mut expr as *mut _` creates a reference before casting it to a raw pointer, and that reference is subject to the same rules as all other references. This macro can create a raw pointer without creating a reference first.

## §Safety

The `expr` in `addr_of_mut!(expr)` is evaluated as a place expression, but never loads from the place or requires the place to be dereferenceable. This means that `addr_of_mut!((*ptr).field)` still requires the projection to `field` to be in-bounds, using the same rules as `offset`. However, `addr_of_mut!(*ptr)` is defined behavior even if `ptr` is null, dangling, or misaligned.

Note that `Deref`/`Index` coercions (and their mutable counterparts) are applied inside `addr_of_mut!` like everywhere else, in which case a reference is created to call `Deref::deref` or `Index::index`, respectively. The statements above only apply when no such coercions are applied.

## §Examples

Correct usage: Creating a pointer to unaligned data

    use std::ptr;

    #[repr(packed)]
    struct Packed {
        f1: u8,
        f2: u16,
    }

    let mut packed = Packed { f1: 1, f2: 2 };
    // `&mut packed.f2` would create an unaligned reference, and thus be Undefined Behavior!
    let raw_f2 = ptr::addr_of_mut!(packed.f2);
    unsafe { raw_f2.write_unaligned(42); }
    assert_eq!({packed.f2}, 42); // `{...}` forces copying the field instead of creating a reference.

Correct usage: Creating a pointer to uninitialized data

    use std::{ptr, mem::MaybeUninit};

    struct Demo {
        field: bool,
    }

    let mut uninit = MaybeUninit::<Demo>::uninit();
    // `&uninit.as_mut().field` would create a reference to an uninitialized `bool`,
    // and thus be Undefined Behavior!
    let f1_ptr = unsafe { ptr::addr_of_mut!((*uninit.as_mut_ptr()).field) };
    unsafe { f1_ptr.write(true); }
    let init = unsafe { uninit.assume_init() };

Incorrect usage: Out-of-bounds fields projection

    use std::ptr;

    #[repr(C)]
    struct MyStruct {
        field1: i32,
        field2: i32,
    }

    let ptr: *mut MyStruct = ptr::null_mut();
    let fieldptr = unsafe { ptr::addr_of_mut!((*ptr).field2) }; // Undefined Behavior ⚠️

The field projection `.field2` would offset the pointer by 4 bytes, but the pointer is not in-bounds of an allocation for 4 bytes, so this offset is Undefined Behavior. See the `offset` docs for a full list of requirements for inbounds pointer arithmetic; the same requirements apply to field projections, even inside `addr_of_mut!`. (In particular, it makes no difference whether the pointer is null or dangling.)