Function 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.

write_volatile 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.

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 noops and may be ignored.

Safety

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

• dst must be valid for writes.

• dst must be properly aligned.

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

Just like in C, whether an operation is volatile has no bearing whatsoever on questions involving concurrent access from multiple threads. Volatile accesses behave exactly like non-atomic accesses in that regard. In particular, a race between a write_volatile and any other operation (reading or writing) on the same location is undefined behavior.

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);
}