[Sbcl-devel] How to use condition variables

[Helmut E. <e96...@st...>]

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Hi,

I'd like to implement a simple mechanism to pass message between
threads, but I don't quite understand how condition variables are
supposed to be used.  My code looks much like the example below.
There is a global mutex and a global waitqueue.  I create one thread
that reads all messages out from the message queue and prints them to
stdout and the main thread sends messages to the other thread in a
endless loop.  This seems to work for a while (the counter goes
sometimes up to one million), but then SBCL just hangs and even
C-c-ing has no effect.  Is there a bug in my code? in SBCL?  Should I
write this differently?  

It's also bit strange that "New thread: xxx" is printed two times.
Are streams not thread safe by default?  All this happens with SBCL
0.8.7.36, Linux 2.4.12, glibc 2.2.2.


(defpackage :sb-ipc-test
  (:use :cl))

(in-package :sb-ipc-test)

(defvar *mutex* (sb-thread:make-mutex))
(defvar *waitqueue* (sb-thread:make-waitqueue))
(defvar *message-queue* (list))

(defun receive ()
  (sb-thread:with-mutex (*mutex*)
    (loop
     (cond (*message-queue* (return (pop *message-queue*)))
	   (t (sb-thread:condition-wait *waitqueue* *mutex*))))))

(defun send (message)
  (sb-thread:with-mutex (*mutex*)
    (setf *message-queue* (nconc *message-queue* (list message)))
    (sb-thread:condition-notify *waitqueue*)))
  
(defun receive-loop ()
  (let ((i 0))
    (loop
     (force-output)
     (let ((message (receive)))
       (format t "received: ~S " message)
       (ecase message
	 (:print (format t "~D~%" (incf i)))
	 (:stop (terpri) (return)))))))

(defun test ()
  (let ((thread (sb-thread:make-thread #'receive-loop)))
    (format t "New thread: ~S~%" thread)
    (loop (send :print))))

(test)