Condition.md

Condition

Condition variable module. Provides procedures for thread synchronization using condition variables paired with mutexes. Requires --m2plus and links with pthreads.

Exported Types

TYPE Condition;  (* opaque condition variable handle *)

Exported Procedures

PROCEDURE Create(): Condition;
PROCEDURE Wait(c: Condition; m: Mutex);
PROCEDURE Signal(c: Condition);
PROCEDURE Broadcast(c: Condition);
PROCEDURE Destroy(c: Condition);

Notes

• Wait atomically releases the mutex m and blocks until the condition is signaled, then reacquires m before returning.
• Signal wakes one thread waiting on the condition.
• Broadcast wakes all threads waiting on the condition.
• Always check the wait predicate in a loop, as spurious wakeups are possible.

Example

MODULE CondDemo;
FROM Mutex IMPORT Mutex, Create, Lock, Unlock;
FROM Condition IMPORT Condition, Wait, Signal;

VAR
  mu: Mutex;
  cv: Condition;
  ready: BOOLEAN;

PROCEDURE Producer;
BEGIN
  Lock(mu);
  ready := TRUE;
  Signal(cv);
  Unlock(mu);
END Producer;

PROCEDURE Consumer;
BEGIN
  Lock(mu);
  WHILE NOT ready DO Wait(cv, mu) END;
  Unlock(mu);
END Consumer;

END CondDemo.