Function core::ptr::read_unaligned

1.17.0 (const: unstable) · source · []
pub unsafe fn read_unaligned<T>(src: *const T) -> T
Expand description

Reads the value from src without moving it. This leaves the memory in src unchanged.

Unlike read, read_unaligned works with unaligned pointers.


Behavior is undefined if any of the following conditions are violated:

  • src must be valid for reads.

  • src must point to a properly initialized value of type T.

Like read, read_unaligned creates a bitwise copy of T, regardless of whether T is Copy. If T is not Copy, using both the returned value and the value at *src can violate memory safety.

Note that even if T has size 0, the pointer must be non-null.

On packed structs

Attempting to create a raw pointer to an unaligned struct field with an expression such as &packed.unaligned as *const FieldType creates an intermediate unaligned reference before converting that to a raw pointer. That this reference is temporary and immediately cast is inconsequential as the compiler always expects references to be properly aligned. As a result, using &packed.unaligned as *const FieldType causes immediate undefined behavior in your program.

Instead you must use the ptr::addr_of! macro to create the pointer. You may use that returned pointer together with this function.

An example of what not to do and how this relates to read_unaligned is:

#[repr(packed, C)]
struct Packed {
    _padding: u8,
    unaligned: u32,

let packed = Packed {
    _padding: 0x00,
    unaligned: 0x01020304,

// Take the address of a 32-bit integer which is not aligned.
// In contrast to `&packed.unaligned as *const _`, this has no undefined behavior.
let unaligned = std::ptr::addr_of!(packed.unaligned);

let v = unsafe { std::ptr::read_unaligned(unaligned) };
assert_eq!(v, 0x01020304);

Accessing unaligned fields directly with e.g. packed.unaligned is safe however.


Read a usize value from a byte buffer:

use std::mem;

fn read_usize(x: &[u8]) -> usize {
    assert!(x.len() >= mem::size_of::<usize>());

    let ptr = x.as_ptr() as *const usize;

    unsafe { ptr.read_unaligned() }