DEFINITION MODULE Processes;
(*
        (pseudo-) concurrent programming with SEND/WAIT


From the book 'Programming in Modula-2' by Prof. N. Wirth.
*)

EXPORT QUALIFIED  SIGNAL, SEND, WAIT,
                  StartProcess, Awaited, Init;


TYPE SIGNAL;
(* SIGNAL's are the means of synchronization between processes. *)


PROCEDURE StartProcess (P: PROC; n: CARDINAL);
(*- Start up a new process.
in:     P       top-level procedure that will execute in this process.
        n       number of bytes of workspace to be allocated to it.

Allocates (from Storage) a workspace of n bytes, and creates a process
executing procedure P in that workspace.  Control is given to the new process.

Caution: The caller must ensure that the workspace size is sufficient for P.
Errors:  StartProcess may fail due to insufficient memory. 
*)

PROCEDURE SEND (VAR s: SIGNAL);
(*- Send a signal
in:     s       the signal to be sent. [Must have been Init'd]
out:    s       the signal with one less process waiting for it.

If no process is waiting for s, SEND has precisely no effect. Otherwise,
some process which is waiting for s is given control and allowed to
continue from WAIT.
*)

PROCEDURE WAIT (VAR s: SIGNAL);
(*- Wait for some other process to send a signal.
in:     s       the signal to wait for. [Must have been Init'd]

The current process waits for the signal s. At some later time, a
SEND(s) by some other process can cause this process to return from WAIT.

Errors:  If all other processes are waiting, WAIT terminates the program.
*)

PROCEDURE Awaited (s:SIGNAL): BOOLEAN;
(*- Test whether any process is waiting for a signal.
in:     s       the signal of interest. [Must have been Init'd]
out:            TRUE if and only if at least one process is waiting for s.
*)

PROCEDURE Init (VAR s: SIGNAL);
(*- Initialize a SIGNAL object.
in:     s       the signal to be initialized
out:    s       the initialized signal (ready to be used as above)

An object of type SIGNAL must be initialized with this procedure before
it can be used with any of the other operations.  After Init(S), Awaited(S)
is FALSE.
*)

END Processes.