FirstThread.md

Create Your First Thread

This uses the first_thread code, in code/02_threads.

Create a new project - with a workspace

Looking back at the workspaces class from last week, it's a great idea to have a workspace. Let's create one:

cargo new LiveWeek2

Now edit Cargo.toml to include a workspace:

[workspace]
members = []

Now change directory to the LiveWeek2 directory and create a new project named FirstThread:

cd LiveWeek2
cargo new FirstThread

And add the project to the workspace:

[workspace]
members = [
    "FirstThread"
]

Your First Thread

In main.rs, replace the contents with the following:

fn hello_thread() {
    println!("Hello from thread!");
}

fn main() {
    println!("Hello from main thread!");

    let thread_handle = std::thread::spawn(hello_thread);
    thread_handle.join().unwrap();
}

Now run the program:

Hello from main thread!
Hello from thread!

So what's going on here? Let's break it down:

1. The program starts in the main thread.
2. The main thread prints a message.
3. We create a thread using std::thread::spawn and tell it to run the function hello_thread.
4. The return value is a "thread handle". You can use these to "join" threads---wait for them to finish.
5. We call join on the thread handle, which waits for the thread to finish.

What happens if we don't join the thread?

Run the program a few times. Sometimes the secondary thread finishes, sometimes it doesn't. Threads don't outlive the main program, so if the main program exits before the thread finishes, the thread is killed.