ReturnFromThreads.md

Returning Data from Threads

See the code thread_return in code/02_threads.

The thread handle will return any value returned by the thread. It's generic, so it can be of any type (that supports sync+send; we'll cover that later). Each thread has its own stack, and can make normal variables inside the thread---and they won't be affected by other threads.

Let's build an example:

fn do_math(i: u32) -> u32 {
    let mut n = i+1;
    for _ in 0 .. 10 {
        n *= 2;
    }
    n
}

fn main() {
    let mut thread_handles = Vec::new();
    for i in 0..10 {
        thread_handles.push(std::thread::spawn(move || {
            do_math(i)
        }));
    }

    for handle in thread_handles {
        println!("Thread returned: {}", handle.join().unwrap());
    }
}

This returns:

Thread returned: 1024
Thread returned: 2048
Thread returned: 3072
Thread returned: 4096
Thread returned: 5120
Thread returned: 6144
Thread returned: 7168
Thread returned: 8192
Thread returned: 9216
Thread returned: 10240

Notice that each thread is doing its own math, and returning its own value. The join function waits for the thread to finish, and returns the value from the thread.