Sharing a read-only thring between threads

help
1

As I was trying to work through an example sharing a read-only string among threads.
While it seems the "right" way would be to wrap the string in an Arc::new i was confused by the result of the following code.

I thought this would create 3 copies of the original string for the 3 threads. However if I understand the output correctly I get one copy only.

If I understand correctly using :p we get the address of the variable on the stack and the as_ptr call returns the location of the content of the string in the heap.

So why are the 3 printed from the threads showing the same values?

fn main() {
    let answer = String::from("Hello World!");

    println!("Before:     {answer} {:p} {:?}", &answer, answer.as_ptr());
    for _ in 1..=3 {
        let answer = answer.clone();
        let handle = std::thread::spawn(move || {
            println!(
                "{:?} {} {:p} {:?}",
                std::thread::current().id(),
                answer,
                &answer,
                answer.as_ptr()
            );
        });
        handle.join().unwrap();
    }
    println!("After:      {answer} {:p} {:?}", &answer, answer.as_ptr());
}

Before:     Hello World! 0x7ffec81bf918 0x5c8c20f18b80
ThreadId(2) Hello World! 0x7b2f151ffb70 0x5c8c20f18ba0
ThreadId(3) Hello World! 0x7b2f151ffb70 0x5c8c20f18ba0
ThreadId(4) Hello World! 0x7b2f151ffb70 0x5c8c20f18ba0
After:      Hello World! 0x7ffec81bf918 0x5c8c20f18b80
2

Your threads run serially and free the String at the end, allowing the allocator to reuse the same memory.

Add answer.leak() to the thread closure to see a difference.

2 Likes
3

Oh I think I mistakenly put the join inside the for loop and if I understand correctly that makes these threads run serially, right?

4

yes

5

thanks

6

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