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}!");

    for i in 1..5 {
        println!("Main thread: {i}");
  • Threads are all daemon threads, the main thread does not wait for them.
  • Thread panics are independent of each other.
  • 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:

  • 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, but join() returns an error(Err).
  • 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.