If a shared variable is written concurrently by two threads, or written by one and read concurrently by another, the effect is to set the variable to an implementation-dependent value of its type. For example, if one thread writes a[0] while another concurrently writes a[1], one of the writes might be lost. Thus, portable programs must use the Thread interface to provide mutual exclusion for shared variables.
INTERFACE Thread; TYPE T <: REFANY; Mutex = MUTEX; Condition <: ROOT; Closure = OBJECT METHODS apply(): REFANY END;A Thread.T is a handle on a thread. A Mutex is locked by some thread, or unlocked. A Condition is a set of waiting threads. A newly-allocated Mutex is unlocked; a newly-allocated Condition is empty. It is a checked runtime error to pass the NIL Mutex, Condition, or T to any procedure in this interface.
PROCEDURE Fork(cl: Closure): T;A handle on a newly-created thread executing cl.apply().
PROCEDURE Join(t: T): REFANY;Wait until t has terminated and return its result. It is a checked error to call this more than once for any t.
PROCEDURE Wait(m: Mutex; c: Condition);The calling thread must have m locked. Atomically unlocks m and waits on c. Then relocks m and returns.
PROCEDURE Acquire(m: Mutex);Wait until m is unlocked and then lock it.
PROCEDURE Release(m: Mutex);The calling thread must have m locked. Unlocks m.
PROCEDURE Broadcast(c: Condition);All threads waiting on c become eligible to run.
PROCEDURE Signal(c: Condition);One or more threads waiting on c become eligible to run.
PROCEDURE Self(): T;Return the handle of the calling thread.
EXCEPTION Alerted;Used to approximate asynchronous interrupts.
PROCEDURE Alert(t: T);Mark t as an alerted thread.
PROCEDURE TestAlert(): BOOLEAN;TRUE if the calling thread has been marked alerted.
PROCEDURE AlertWait(m: Mutex; c: Condition) RAISES {Alerted};
Like Wait, but if the thread is marked alerted at the time of
call or sometime during the wait, lock m and raise Alerted.
PROCEDURE AlertJoin(t: T): REFANY RAISES {Alerted};
Like Join, but if the calling thread is marked alerted at
the time of call or sometime during the wait, raise Alerted.
CONST AtomicSize = ...;An implementation-dependent integer constant: the number of bits in a memory-coherent block. If two components of a record or array fall in different blocks, they can be accessed concurrently by different threads without locking.
END Thread.
The CM Modula-3 version of Thread is here.