1 files changed, 41 insertions, 1 deletions
diff --git a/solutions/20_threads/threads2.rs b/solutions/20_threads/threads2.rs
index 4e18198..bc268d6 100644
--- a/solutions/20_threads/threads2.rs
+++ b/solutions/20_threads/threads2.rs
@@ -1 +1,41 @@
-// Solutions will be available before the stable release. Thank you for testing the beta version 🥰
+// Building on the last exercise, we want all of the threads to complete their
+// work. But this time, the spawned threads need to be in charge of updating a
+// shared value: `JobStatus.jobs_done`
+
+use std::{
+    sync::{Arc, Mutex},
+    thread,
+    time::Duration,
+};
+
+struct JobStatus {
+    jobs_done: u32,
+}
+
+fn main() {
+    // `Arc` isn't enough if you want a **mutable** shared state.
+    // We need to wrap the value with a `Mutex`.
+    let status = Arc::new(Mutex::new(JobStatus { jobs_done: 0 }));
+    //                    ^^^^^^^^^^^                          ^
+
+    let mut handles = Vec::new();
+    for _ in 0..10 {
+        let status_shared = Arc::clone(&status);
+        let handle = thread::spawn(move || {
+            thread::sleep(Duration::from_millis(250));
+
+            // Lock before you update a shared value.
+            status_shared.lock().unwrap().jobs_done += 1;
+            //           ^^^^^^^^^^^^^^^^
+        });
+        handles.push(handle);
+    }
+
+    // Waiting for all jobs to complete.
+    for handle in handles {
+        handle.join().unwrap();
+    }
+
+    println!("Jobs done: {}", status.lock().unwrap().jobs_done);
+    //                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+}