執行緒
Rust 執行緒的運作方式與其他語言類似:
use std::thread; use std::time::Duration; fn main() { thread::spawn(|| { for i in 1..10 { println!("Count in thread: {i}!"); thread::sleep(Duration::from_millis(5)); } }); for i in 1..5 { println!("Main thread: {i}"); thread::sleep(Duration::from_millis(5)); } }
- 執行緒都是 daemon 執行緒,主執行緒不會等待這類執行緒完成運作。
- 執行緒恐慌均為各自獨立,並非彼此相關。
- 如果恐慌附帶酬載,可使用
downcast_ref
解除封裝。
- 如果恐慌附帶酬載,可使用
-
Rust thread APIs look not too different from e.g. C++ ones.
-
執行範例。
- 5ms timing is loose enough that main and spawned threads stay mostly in lockstep.
- Notice that the program ends before the spawned thread reaches 10!
- This is because main ends the program and spawned threads do not make it persist.
- Compare to pthreads/C++ std::thread/boost::thread if desired.
-
How do we wait around for the spawned thread to complete?
-
thread::spawn
returns aJoinHandle
. Look at the docs.JoinHandle
has a.join()
method that blocks.
-
Use
let handle = thread::spawn(...)
and laterhandle.join()
to wait for the thread to finish and have the program count all the way to 10. -
Now what if we want to return a value?
-
Look at docs again:
thread::spawn
’s closure returnsT
JoinHandle
.join()
returnsthread::Result<T>
-
Use the
Result
return value fromhandle.join()
to get access to the returned value. -
Ok, what about the other case?
- Trigger a panic in the thread. Note that this doesn’t panic
main
. - Access the panic payload. This is a good time to talk about
Any
.
- Trigger a panic in the thread. Note that this doesn’t panic
-
Now we can return values from threads! What about taking inputs?
- Capture something by reference in the thread closure.
- An error message indicates we must move it.
- Move it in, see we can compute and then return a derived value.
-
If we want to borrow?
- Main kills child threads when it returns, but another function would just return and leave them running.
- That would be stack use-after-return, which violates memory safety!
- How do we avoid this? see next slide.