Rust - Concurrency

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In Concurrent programming, different parts of a program execute independently. On the other hand, in parallel programming, different parts of a program execute at the same time. Both the models are equally important as more computers take advantage of their multiple processors.

Threads

We can use threads to run codes simultaneously. In current operating systems, an executed program’s code is run in a process, and the operating system manages multiple processes at once. Within your program, you can also have independent parts that run simultaneously. The features that run these independent parts are called threads.

Creating a Thread

The thread::spawn function is used to create a new thread. The spawn function takes a closure as parameter. The closure defines code that should be executed by the thread. The following example prints some text from a main thread and other text from a new thread.

//import the necessary modules
use std::thread;
use std::time::Duration;

fn main() {
   //create a new thread
   thread::spawn(|| {
      for i in 1..10 {
         println!("hi number {} from the spawned thread!", i);
         thread::sleep(Duration::from_millis(1));
      }
   });
   //code executed by the main thread
   for i in 1..5 {
      println!("hi number {} from the main thread!", i);
      thread::sleep(Duration::from_millis(1));
   }
}

Output

hi number 1 from the main thread!
hi number 1 from the spawned thread!
hi number 2 from the main thread!
hi number 2 from the spawned thread!
hi number 3 from the main thread!
hi number 3 from the spawned thread!
hi number 4 from the spawned thread!
hi number 4 from the main thread!

The main thread prints values from 1 to 4.

NOTE − The new thread will be stopped when the main thread ends. The output from this program might be a little different every time.

The thread::sleep function forces a thread to stop its execution for a short duration, allowing a different thread to run. The threads will probably take turns, but that is not guaranteed – it depends on how the operating system schedules the threads. In this run, the main thread is printed first, even though the print statement from the spawned thread appears first in the code. Moreover, even if the spawned thread is programmed to print values till 9, it only got to 5 before the main thread shut down.

Join Handles

A spawned thread may not get a chance to run or run completely. This is because the main thread completes quickly. The function spawn<F, T>(f: F) -> JoinHandlelt;T> returns a JoinHandle. The join() method on JoinHandle waits for the associated thread to finish.

use std::thread;
use std::time::Duration;

fn main() {
   let handle = thread::spawn(|| {
      for i in 1..10 {
         println!("hi number {} from the spawned thread!", i);
         thread::sleep(Duration::from_millis(1));
      }
   });
   for i in 1..5 {
      println!("hi number {} from the main thread!", i);
      thread::sleep(Duration::from_millis(1));
   }
   handle.join().unwrap();
}

Output

hi number 1 from the main thread!
hi number 1 from the spawned thread!
hi number 2 from the spawned thread!
hi number 2 from the main thread!
hi number 3 from the spawned thread!
hi number 3 from the main thread!
hi number 4 from the main thread!
hi number 4 from the spawned thread!
hi number 5 from the spawned thread!
hi number 6 from the spawned thread!
hi number 7 from the spawned thread!
hi number 8 from the spawned thread!
hi number 9 from the spawned thread!

The main thread and spawned thread continue switching.

NOTE − The main thread waits for spawned thread to complete because of the call to the join() method.

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