Threads overview in Rust - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a thread in Rust?

A thread is a way to run code concurrently, allowing multiple tasks to happen at the same time within a program.

beginner

How do you create a new thread in Rust?

Use std::thread::spawn with a closure containing the code to run in the new thread.

beginner

What does the join method do on a thread handle?

It waits for the thread to finish running and returns the result or error if any.

intermediate

Why is Rust's ownership system important for threads?

It helps prevent data races by ensuring safe access to shared data between threads.

intermediate

What is a data race and how does Rust prevent it?

A data race happens when two threads access the same data at the same time and at least one writes. Rust prevents this by enforcing rules at compile time using ownership and borrowing.

Which Rust function is used to start a new thread?

A std::thread::spawn

B std::thread::start

C std::thread::run

D std::thread::create

The correct function to create a new thread in Rust is std::thread::spawn.

What does calling join() on a thread do?

A Creates a new thread

B Starts the thread

C Kills the thread immediately

D Waits for the thread to finish

join() waits for the thread to finish before continuing.

Why does Rust prevent data races at compile time?

A Because Rust uses garbage collection

B Because of ownership and borrowing rules

C Because Rust does not allow threads

D Because Rust runs only single-threaded

Rust uses ownership and borrowing rules to ensure safe access to data across threads.

What is a thread handle in Rust?

A An object to control and communicate with the thread

B A pointer to thread memory

C The thread's ID number

D A function to run the thread

A thread handle lets you control the thread, like waiting for it to finish with join().

Which of these is NOT a benefit of using threads?

A Running tasks at the same time

B Making programs faster on multi-core CPUs

C Automatically fixing bugs

D Improving responsiveness

Threads help with concurrency and speed but do not automatically fix bugs.

Explain how to create and use threads in Rust, including how to wait for them to finish.

Think about how you start a thread and then wait for it to finish.

Describe why Rust's ownership system helps prevent data races when using threads.

Consider how Rust controls who can access data and when.

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