Send and Sync
- Rust course
- Advanced
- Lesson 17/81
- 4 min
Learn the Send and Sync marker traits and why they matter.
Send and Sync
Rust's thread safety rests on two marker traits; they have no methods, and the compiler infers them automatically:
Send: ownership of the value can be moved to another thread.Sync: the value can be accessed through &T from other threads (T: Sync⇔&T: Send).
// Send + Sync: i32, String, Vec<T>, Arc<T> (T: Send + Sync), Mutex<T> (T: Send)
// Send but not Sync: Cell<T>, RefCell<T>
// neither: Rc<T>, raw pointersThe signature of thread::spawn requires F: Send + 'static; that's why data races are prevented at compile time.
Compile-time checking
You can check at compile time whether a type is Send/Sync with empty-bodied generic functions:
fn send_mi<T: Send>() {}
fn sync_mi<T: Sync>() {}
send_mi::<Arc<Mutex<i32>>>(); // passes
sync_mi::<Arc<Mutex<i32>>>(); // passes
// send_mi::<Rc<i32>>(); // error: `Rc<i32>` cannot be sent between threads safelyLibrary authors use such tests to ensure their public types' thread safety isn't accidentally broken.
Practice Thread safety check
Write the send_mi and sync_mi functions above. Call both for Arc<Mutex<i32>> and send_mi for String; if it compiles, print denetim geçti.
Starter code
use std::sync::{Arc, Mutex};
fn main() {
// buraya yaz
}Sample solution
use std::sync::{Arc, Mutex};
fn send_mi<T: Send>() {}
fn sync_mi<T: Sync>() {}
fn main() {
send_mi::<Arc<Mutex<i32>>>();
sync_mi::<Arc<Mutex<i32>>>();
send_mi::<String>();
println!("denetim geçti");
}Expected output
denetim geçti
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Why isn't <code>Rc<T></code> Send?
- Because it uses the heap
- Because it's Clone
- It is Send
- Its counter isn't atomic; if two threads change it at once, the count breaks Correct answer
Explanation: Arc does the same job with an atomic counter; it's slightly slower but is Send + Sync.
2 Movability bound
fn send_mi<T: ____>() {}fn send_mi<T: Send>() {}Explanation: The Send bound checks that the type can be moved to another thread.
3 Type and property
| Term | Meaning |
|---|---|
Arc<Mutex<i32>> |
Send + Sync |
Rc<i32> |
Neither |
RefCell<i32> |
Send, not Sync |
*const i32 |
Raw pointer: neither |