Function core::ptr::read_volatile

1.9.0 · source ·
pub unsafe fn read_volatile<T>(src: *const T) -> T
Expand description

Performs a volatile read of the value from src without moving it. This leaves the memory in src unchanged.

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.


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 read_volatile) are noops and may be ignored.


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

  • src must be valid for reads.

  • src must be properly aligned.

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

Like read, read_volatile 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. However, storing non-Copy types in volatile memory is almost certainly incorrect.

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 read_volatile and any write operation to the same location is undefined behavior.


Basic usage:

let x = 12;
let y = &x as *const i32;

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