Proper use of condition variable

examples/threading/cv-broadcast.scm

@@ -1,33 +1,49 @@
-;;;; An example of using condition variable broadcast to wake other threads.
+;;;; An example of using a condition variable to wake other threads.
 (import (scheme base)
         (scheme read)
         (scheme write)
         (srfi 18))
 
+(define *done* #f)
 (define cv (make-condition-variable))
 (define m (make-mutex))
 
+(define (trace msg)
+  (display msg)
+  (newline))
+
 ;; Thread - Sleep, then wake other threads up via broadcast
 (thread-start!
   (make-thread
     (lambda ()
-      (write "started thread")
       (thread-sleep! 3000)
+      (set! *done* #t)
       (condition-variable-broadcast! cv)
-      (write "thread done"))))
+      (trace "broadcast thread done"))))
 
 ;; Thread - wait for broadcast
 (thread-start!
   (make-thread
     (lambda ()
-      (write "started waiting thread")
-      (mutex-lock! m)
-      (write "register waiting thread cv")
-      (mutex-unlock! m cv)
-      (write "waiting thread done"))))
+      (let loop ()
+        (mutex-lock! m)
+        (cond
+          (*done*
+            (trace "waiting thread done")
+            (mutex-unlock! m))
+          (else
+            (trace "waiting thread - waiting for cv")
+            (mutex-unlock! m cv)
+            (loop)))))))
 
 ;; Main thread - wait for broadcast
-(mutex-lock! m)
-(mutex-unlock! m cv) ;; Wait on cv
-(write "main thread done")
-(thread-sleep! 500)
+(let loop ()
+  (mutex-lock! m)
+  (cond
+    (*done*
+      (mutex-unlock! m)
+      (thread-sleep! 500)
+      (trace "main thread done"))
+    (else
+      (mutex-unlock! m cv) ;; Wait on cv
+      (loop))))