rust / expert
Snippet
Constructing an Explicit RawWaker and VTable for Low-Level Executors
Creating custom asynchronous runtime executors requires a direct interface with Rust's Waker machinery via RawWaker and RawWakerVTable. By manually managing reference counts on an Arc heap allocation, we can convert thread-safe notification signals directly into standard Wakers to resume suspended Futures.
snippet.rs
rust
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use std::sync::Arc;use std::task::{RawWaker, RawWakerVTable, Waker};pub struct Signal;impl Signal {pub fn wake(self: Arc<Self>) {// Implementation of notification logic goes here}}const VTABLE: RawWakerVTable = RawWakerVTable::new(clone_raw,wake_raw,wake_by_ref_raw,drop_raw,);fn clone_raw(data: *const ()) -> RawWaker {unsafe { Arc::increment_strong_count(data as *const Signal); }RawWaker::new(data, &VTABLE)}fn wake_raw(data: *const ()) {let arc = unsafe { Arc::from_raw(data as *const Signal) };arc.wake();}fn wake_by_ref_raw(data: *const ()) {let arc = unsafe { Arc::from_raw(data as *const Signal) };let cloned = arc.clone();let _ = Arc::into_raw(arc);cloned.wake();}fn drop_raw(data: *const ()) {unsafe { Arc::from_raw(data as *const Signal); }}pub fn create_waker(signal: Arc<Signal>) -> Waker {let raw = RawWaker::new(Arc::into_raw(signal) as *const (), &VTABLE);unsafe { Waker::from_raw(raw) }}
Breakdown
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const VTABLE: RawWakerVTable = RawWakerVTable::new(
Defines the static function pointer table handling clone, wake, wake_by_ref, and drop behavior for the raw pointer.
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unsafe { Arc::increment_strong_count(data as *const Signal); }
Manually increments the Arc reference count of our Signal since cloning a raw pointer does not invoke standard Arc clone logic.
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unsafe { Waker::from_raw(raw) }
Constructs a safe, standard Waker instance out of the raw waker and its associated virtual table.