std::ptr::write_unaligned - Rust
Description: API documentation for the Rust `write_unaligned` fn in crate `std`.
Keywords: rust, rustlang, rust-lang, write_unaligned

# 1.17.0[−][src]Function std::ptr::write_unaligned

    pub unsafe fn write_unaligned<T>(dst: *mut T, src: T)

Overwrites a memory location with the given value without reading or dropping the old value.

Unlike `write`, the pointer may be unaligned.

`write_unaligned` does not drop the contents of `dst`. This is safe, but it could leak allocations or resources, so care should be taken not to overwrite an object that should be dropped.

Additionally, it does not drop `src`. Semantically, `src` is moved into the location pointed to by `dst`.

This is appropriate for initializing uninitialized memory, or overwriting memory that has previously been read with `read_unaligned`.

# Safety

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

- `dst` must be valid for writes.

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

## On `packed` structs

It is currently impossible to create raw pointers to unaligned fields of a packed struct.

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.

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

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

    let v = 0x01020304;
    let mut packed: Packed = unsafe { std::mem::zeroed() };

    let v = unsafe {
        // Here we attempt to take the address of a 32-bit integer which is not aligned.
        let unaligned =
            // A temporary unaligned reference is created here which results in
            // undefined behavior regardless of whether the reference is used or not.
            &mut packed.unaligned
            // Casting to a raw pointer doesn't help; the mistake already happened.
            as *mut u32;

        std::ptr::write_unaligned(unaligned, v);

        v
    };

Run

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

# Examples

Write an usize value to a byte buffer:

    use std::mem;

    fn write_usize(x: &mut [u8], val: usize) {
        assert!(x.len() >= mem::size_of::<usize>());

        let ptr = x.as_mut_ptr() as *mut usize;

        unsafe { ptr.write_unaligned(val) }
    }

Run