Function std::ptr::write_volatile

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

Performs a volatile write of a memory location with the given value without reading or dropping the old value.

Volatile operations are intended to act on I/O memory, and are guaranteed to not be elided or reordered by the compiler across other volatile operations.

Memory written with write_volatile should almost always be read from using read_volatile.

write_volatile does not drop the contents of dst. This is safe, but it could leak allocations or resources, so care must 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.

Notes

Rust does not currently have a rigorously and formally defined memory model, so the precise semantics of what "volatile" means here is subject to change over time. That being said, the semantics will almost always end up pretty similar to C11's definition of volatile.

The compiler shouldn't change the relative order or number of volatile memory operations. However, volatile memory operations on zero-sized types (e.g. if a zero-sized type is passed to write_volatile) are no-ops and may be ignored.

Safety

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

• dst must be valid.

• dst must be properly aligned.

Examples

Basic usage:

let mut x = 0;
let y = &mut x as *mut i32;
let z = 12;

unsafe {
    std::ptr::write_volatile(y, z);
    assert_eq!(std::ptr::read_volatile(y), 12);
}