Multiple threads

If you use multiple threads, you can do many things at the same time. Modern computers have more than one core so they can do more than one thing at the same time, and Rust lets you use them. Rust uses threads that are called "OS threads". OS thread means the operating system creates the thread on a different core. (Some other languages use "green threads", which are less powerful)

You create threads with std::thread::spawn and then a closure to tell it what to do. Threads are interesting because they run at the same time, and you can test it to see what happens. Here is a simple example:

fn main() {
    std::thread::spawn(|| {
        println!("I am printing something");
    });
}

If you run this, it will be different every time. Sometimes it will print, and sometimes it won't print (this depends on your computer speed too). That is because sometimes main() finishes before the thread finishes. And when main() finishes, the program is over. This is easier to see in a for loop:

fn main() {
    for _ in 0..10 { // set up ten threads
        std::thread::spawn(|| {
            println!("I am printing something");
        });
    }   // Now the threads start.
}       // How many can finish before main() ends here?

Usually about four threads will print before main ends, but it is always different. If your computer is faster then it might not print any. Also, sometimes the threads will panic:

thread 'thread 'I am printing something
thread '<unnamed><unnamed>thread '' panicked at '<unnamed>I am printing something
' panicked at 'thread '<unnamed>cannot access stdout during shutdown' panicked at '<unnamed>thread 'cannot access stdout during
shutdown

This is the error when the thread tries to do something right when the program is shutting down.

You can give the computer something to do so it won't shut down right away:

fn main() {
    for _ in 0..10 {
        std::thread::spawn(|| {
            println!("I am printing something");
        });
    }
    for _ in 0..1_000_000 { // make the program declare "let x = 9" one million times
                            // It has to finish this before it can exit the main function
        let _x = 9;
    }
}

But that is a silly way to give the threads time to finish. The better way is to bind the threads to a variable. If you add let, then you will create a JoinHandle. You can see this in the signature for spawn:

pub fn spawn<F, T>(f: F) -> JoinHandle<T>
where
    F: FnOnce() -> T,
    F: Send + 'static,
    T: Send + 'static,

(f is the closure - we will learn how to put closures into our functions later)

So now we have a JoinHandle every time.

fn main() {
    for _ in 0..10 {
        let handle = std::thread::spawn(|| {
            println!("I am printing something");
        });

    }
}

handle is now a JoinHandle. What do we do with it? We use a method called .join(). This method means "wait until all the threads are done" (it waits for the threads to join it). So now just write handle.join() and it will wait for each of the threads to finish.

fn main() {
    for _ in 0..10 {
        let handle = std::thread::spawn(|| {
            println!("I am printing something");
        });

        handle.join(); // Wait for the threads to finish
    }
}

Now we will learn about the three types of closures. The three types are:

FnOnce: takes the whole value
FnMut: takes a mutable reference
Fn: takes a regular reference

A closure will try to use Fn if it can. But if it needs to change the value it will use FnMut, and if it needs to take the whole value, it will use FnOnce. FnOnce is a good name because it explains what it does: it takes the value once, and then it can't take it again.

Here is an example:

fn main() {
    let my_string = String::from("I will go into the closure");
    let my_closure = || println!("{}", my_string);
    my_closure();
    my_closure();
}

String is not Copy, so my_closure() is Fn: it takes a reference.

If we change my_string, it will be FnMut.

fn main() {
    let mut my_string = String::from("I will go into the closure");
    let mut my_closure = || {
        my_string.push_str(" now");
        println!("{}", my_string);
    };
    my_closure();
    my_closure();
}

This prints:

I will go into the closure now
I will go into the closure now now

And if you take by value, then it will be FnOnce.

fn main() {
    let my_vec: Vec<i32> = vec![8, 9, 10];
    let my_closure = || {
        my_vec
            .into_iter() // into_iter takes ownership
            .map(|x| x as u8) // turn it into u8
            .map(|x| x * 2) // multiply by 2
            .collect::<Vec<u8>>() // collect into a Vec
    };
    let new_vec = my_closure();
    println!("{:?}", new_vec);
}

We took by value, so we can't run my_closure() more than once. That is where the name comes from.

So now back to threads. Let's try to use a value from outside:

fn main() {
    let mut my_string = String::from("Can I go inside the thread?");

    let handle = std::thread::spawn(|| {
        println!("{}", my_string); // ⚠️
    });

    handle.join();
}

The compiler says that this won't work.

error[E0373]: closure may outlive the current function, but it borrows `my_string`, which is owned by the current function
  --> src\main.rs:28:37
   |
28 |     let handle = std::thread::spawn(|| {
   |                                     ^^ may outlive borrowed value `my_string`
29 |         println!("{}", my_string);
   |                        --------- `my_string` is borrowed here
   |
note: function requires argument type to outlive `'static`
  --> src\main.rs:28:18
   |
28 |       let handle = std::thread::spawn(|| {
   |  __________________^
29 | |         println!("{}", my_string);
30 | |     });
   | |______^
help: to force the closure to take ownership of `my_string` (and any other referenced variables), use the `move` keyword
   |
28 |     let handle = std::thread::spawn(move || {
   |                                     ^^^^^^^

It is a long message, but helpful: it says to use the `move` keyword. The problem is that we can do anything to my_string while the thread is using it, but it doesn't own it. That would be unsafe.

Let's try something else that doesn't work:

fn main() {
    let mut my_string = String::from("Can I go inside the thread?");

    let handle = std::thread::spawn(|| {
        println!("{}", my_string); // now my_string is being used as a reference
    });

    std::mem::drop(my_string);  // ⚠️ We try to drop it here. But the thread still needs it.

    handle.join();
}

So you have to take the value with move. Now it is safe:

fn main() {
    let mut my_string = String::from("Can I go inside the thread?");

    let handle = std::thread::spawn(move|| {
        println!("{}", my_string);
    });

    std::mem::drop(my_string);  // ⚠️ we can't drop, because handle has it. So this won't work

    handle.join();
}

So we delete the std::mem::drop, and now it is okay. handle takes my_string and our code is safe.

fn main() {
    let mut my_string = String::from("Can I go inside the thread?");

    let handle = std::thread::spawn(move|| {
        println!("{}", my_string);
    });

    handle.join();
}

So just remember: if you need a value in a thread from outside the thread, you need to use move.