Threads

Rust threads work similarly to threads in other languages:

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));
    }
}

Threads are all daemon threads, the main thread does not wait for them.
Thread panics are independent of each other.
- Panics can carry a payload, which can be unpacked with downcast_ref.

Rust thread APIs look not too different from e.g. C++ ones.
Run the example.
- 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 a JoinHandle. Look at the docs.
- JoinHandle has a .join() method that blocks.
Use let handle = thread::spawn(...) and later handle.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 returns T
- JoinHandle .join() returns thread::Result<T>
Use the Result return value from handle.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.
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.

Comprehensive Rust 🦀

Threads