Threads overview in Rust - Mini Project: Build & Apply

Threads overview

📖 Scenario: You are building a simple Rust program that uses threads to run tasks at the same time. This is like having multiple friends working on different parts of a project together.

🎯 Goal: Create a Rust program that starts a new thread to print a message, while the main thread prints its own message. You will learn how to create and run threads and wait for them to finish.

📋 What You'll Learn

Create a variable called handle that starts a new thread using std::thread::spawn

Inside the new thread, print the message "Hello from the spawned thread!"

In the main thread, print the message "Hello from the main thread!"

Use handle.join().unwrap() to wait for the spawned thread to finish

💡 Why This Matters

🌍 Real World

Threads let programs do many things at once, like downloading files while showing a progress bar.

💼 Career

Understanding threads is important for building fast and responsive software in many jobs, including systems programming and web servers.

1

Create a new thread

Write a line of Rust code to create a variable called handle that starts a new thread using std::thread::spawn. Inside the thread, print the message "Hello from the spawned thread!".

Rust

# Create a variable called handle that starts a new thread
# Your code here

Hint

Use thread::spawn with a closure || { ... } to create the thread.

2

Print from the main thread

Add a line of Rust code to print the message "Hello from the main thread!" in the main thread after creating the handle variable.

Rust

use std::thread;

let handle = thread::spawn(|| {
    println!("Hello from the spawned thread!");
});

# Print "Hello from the main thread!" here
# Your code here

Hint

Use println!("Hello from the main thread!"); to print the message.

3

Wait for the spawned thread to finish

Add a line of Rust code to wait for the spawned thread to finish by calling handle.join().unwrap() after printing the main thread message.

Rust

use std::thread;

let handle = thread::spawn(|| {
    println!("Hello from the spawned thread!");
});

println!("Hello from the main thread!");

# Wait for the spawned thread to finish
# Your code here

Hint

Use handle.join().unwrap(); to wait for the thread to finish and handle any errors.

4

Run the program and see the output

Run the complete Rust program and print the output. The output should show messages from both the spawned thread and the main thread.

Rust

use std::thread;

fn main() {
    let handle = thread::spawn(|| {
        println!("Hello from the spawned thread!");
    });

    println!("Hello from the main thread!");

    handle.join().unwrap();

    # Run the program and observe the output
    # Your code here
}

Hint

The order of the messages may vary because threads run at the same time.

Practice

1. What is the main purpose of using threads in Rust?

easy

A. To make the program run slower

B. To avoid using variables

C. To write code without functions

D. To run multiple parts of a program at the same time

Solution

1. Step 1: Understand what threads do

   Threads allow a program to perform multiple tasks simultaneously, improving efficiency.

2. Step 2: Match purpose with options

   Only To run multiple parts of a program at the same time correctly states that threads run parts of a program at the same time.

3. Final Answer:

   To run multiple parts of a program at the same time -> Option D

4. Quick Check:

   Threads = run tasks simultaneously [OK]

Hint: Threads let programs do many things at once [OK]

Common Mistakes:

• Thinking threads slow down the program
• Confusing threads with functions
• Believing threads remove variables

2. Which of the following is the correct way to start a new thread in Rust?

easy

A. thread::start(|| println!("Hello"));

B. thread::spawn(|| println!("Hello"));

C. thread::run(|| println!("Hello"));

D. thread::create(|| println!("Hello"));

Solution

1. Step 1: Recall Rust thread syntax

   Rust uses thread::spawn to create a new thread.

2. Step 2: Compare options with correct syntax

   Only thread::spawn(|| println!("Hello")); uses thread::spawn correctly.

3. Final Answer:

   thread::spawn(|| println!("Hello")); -> Option B

4. Quick Check:

   Start thread = thread::spawn [OK]

Hint: Use thread::spawn to start threads in Rust [OK]

Common Mistakes:

• Using thread::start instead of spawn
• Using thread::run or thread::create which don't exist
• Missing the closure syntax ||

3. What will the following Rust code print?

use std::thread;

fn main() {
    let handle = thread::spawn(|| {
        println!("Inside thread");
    });
    handle.join().unwrap();
    println!("Thread finished");
}

medium

A. Inside thread
Thread finished

B. Thread finished

C. Inside thread

D. Thread finished
Inside thread

Solution

1. Step 1: Understand thread::spawn and join

   The thread prints "Inside thread". The main thread waits for it to finish using join().

2. Step 2: Determine print order

   Because join() waits, "Inside thread" prints first, then "Thread finished" prints.

3. Final Answer:

   Inside thread Thread finished -> Option A

4. Quick Check:

   join() waits, so inside thread prints before main [OK]

Hint: join() waits for thread, so thread prints before main continues [OK]

Common Mistakes:

• Assuming main prints first without waiting
• Ignoring join() effect
• Expecting only one print line

4. Identify the error in this Rust code that uses threads:

use std::thread;

fn main() {
    let handle = thread::spawn(|| {
        println!("Hello from thread");
    });
    println!("Main thread");
}

medium

A. Missing call to join(), so main may end before thread finishes

B. thread::spawn is used incorrectly

C. println! macro is used wrong

D. No error, code runs perfectly

Solution

1. Step 1: Check thread usage

   The code starts a thread but does not wait for it to finish with join().

2. Step 2: Understand consequences

   Without join(), the main thread may finish and exit before the spawned thread prints.

3. Final Answer:

   Missing call to join(), so main may end before thread finishes -> Option A

4. Quick Check:

   Always join threads to ensure they complete [OK]

Hint: Always join threads to wait for their completion [OK]

Common Mistakes:

• Forgetting to call join()
• Thinking thread::spawn syntax is wrong
• Assuming println! is incorrect

5. You want to run two threads that each print a message, then print "Done" after both finish. Which code correctly does this?

hard

A. thread::spawn(|| println!("Thread 1")).join().unwrap();
thread::spawn(|| println!("Thread 2"));
println!("Done");

B. let t1 = thread::spawn(|| println!("Thread 1"));
let t2 = thread::spawn(|| println!("Thread 2"));
println!("Done");

C. let t1 = thread::spawn(|| println!("Thread 1"));
let t2 = thread::spawn(|| println!("Thread 2"));
t1.join().unwrap();
t2.join().unwrap();
println!("Done");

D. thread::spawn(|| println!("Thread 1"));
thread::spawn(|| println!("Thread 2"));
thread::spawn(|| println!("Done"));

Solution

1. Step 1: Understand thread creation and waiting

   To ensure "Done" prints after both threads finish, we must join both threads before printing.

2. Step 2: Analyze options for correct join usage

   let t1 = thread::spawn(|| println!("Thread 1")); let t2 = thread::spawn(|| println!("Thread 2")); t1.join().unwrap(); t2.join().unwrap(); println!("Done"); creates two threads, joins both, then prints "Done". This guarantees order.

3. Final Answer:

   let t1 = thread::spawn(|| println!("Thread 1")); let t2 = thread::spawn(|| println!("Thread 2")); t1.join().unwrap(); t2.join().unwrap(); println!("Done"); -> Option C

4. Quick Check:

   Join all threads before final print to ensure order [OK]

Hint: Join all threads before final action to ensure completion [OK]

Common Mistakes:

• Not joining threads before printing "Done"
• Joining threads immediately after spawn (serializing)
• Spawning extra thread for "Done" message