Appendix 10--ISO Process Support

A10.1 Processes

DEFINITION MODULE Processes;

  (* This module allows concurrent algorithms to be expressed using processes.  A process is a unit of a program that has the potential to run in parallel with other processes. *)

IMPORT SYSTEM;

TYPE    
  ProcessId;                      (* Used to identify processes *)
  Parameter     = SYSTEM.ADDRESS; (* Used to pass data between processes *)
  Body          = PROC;   (* Used as the type of a process body *)
  Urgency       = INTEGER;     (* Used by the internal scheduler *)
  Sources       = CARDINAL;    (* Used to identify event sources *)
  ProcessesExceptions =        (* Exceptions raised by this module *)
    (passiveProgram, processError);

(* The following procedures create processes and switch control between them. *)

PROCEDURE Create (procBody: Body; extraSpace: CARDINAL; procUrg: Urgency; procParams: Parameter; VAR procId: ProcessId);
  (* Creates a new process with procBody as its body, and with urgency and parameters given by procUrg and procParams.  At least as much workspace (in units of SYSTEM.LOC) as is specified by extraSpace is allocated to the process.
     An identity for the new process is returned in procId.
     The process is created in the passive state; it will not run until
activated. *)

PROCEDURE Start (procBody: Body; extraSpace: CARDINAL; procUrg: Urgency; procParams: Parameter; VAR procId: ProcessId);
  (* Creates a new process, with parameters as for Create.
     The process is created in the ready state; it is eligible to run
immediately. *)

PROCEDURE StopMe ();
  (* Terminates the calling process.
     The process must not be associated with a source of events. *)

PROCEDURE SuspendMe ();
  (* Causes the calling process to enter the passive state.  The procedure only returns when the calling process is again activated by another process. *)

PROCEDURE Activate (procId: ProcessId);
  (* Causes the process identified by procId to enter the ready state, and thus to become eligible to run again. *)

PROCEDURE SuspendMeAndActivate (procId: ProcessId);
  (* Executes an atomic sequence of SuspendMe() and Activate(procId). *)

PROCEDURE Switch (procId: ProcessId; VAR info: Parameter);
  (* Causes the calling process to enter the passive state; the process
identified by procId becomes the currently executing process. Info is used to pass parameter information from the calling to the activated process.
     On return, info will contain information from the process that chooses to switch back to this one (or will be NIL if Activate or SuspendMeAndActivate are used instead of Switch). *)

PROCEDURE Wait ();
  (* Causes the calling process to enter the waiting state.  The procedure will return when the calling process is activated by another process, or when one of its associated eventSources has generated an event. *)

(* The following procedures allow the association of processes with 
sources of external events. *)

PROCEDURE Attach (eventSource: Sources);
  (* Associates the specified eventSource with the calling process. *)

PROCEDURE Detach (eventSource: Sources);
  (* Dissociates the specified eventSource from the program. *)

PROCEDURE IsAttached (eventSource: Sources): BOOLEAN;
  (* Returns TRUE if and only if the specified eventSource is currently
associated with one of the processes of the program. *)

PROCEDURE Handler (eventSource: Sources): ProcessId;
  (* Returns the identity of the process, if any, that is associated with the
specified eventSource. *)

(* The following procedures allow processes to obtain their identity,
parameters, and urgency. *)

PROCEDURE Me (): ProcessId;
  (* Returns the identity of the calling process (as assigned when the process was first created). *)

PROCEDURE MyParam (): Parameter;
  (* Returns the value specified as procParams when the calling process was created. *)

PROCEDURE UrgencyOf (procId: ProcessId): Urgency;
  (* Returns the urgency established when the process identified by procId was first created. *)

(* The following procedure provides facilities for exception handlers. *)

PROCEDURE ProcessesException (): ProcessesExceptions;
  (* If the current coroutine is in the exceptional execution state because of the raising of a language exception, returns the corresponding enumeration value, and otherwise raises an exception. *)

PROCEDURE IsProcessesException (): BOOLEAN;
  (* Returns TRUE if the current coroutine is in the exceptional execution state because of the raising of an exception in a routine from this module; otherwise returns FALSE. *)

END Processes.

A10.2 Semaphores

DEFINITION MODULE Semaphores;

  (* Provides mutual exclusion facilities for use by processes. *)

TYPE
  SEMAPHORE;

PROCEDURE Create (VAR s: SEMAPHORE; initialCount: CARDINAL );
  (* Creates and returns s as the identity of a new semaphore that has its associated count initialized to initialCount, and has no processes yet waiting on it. *)

PROCEDURE Destroy (VAR s: SEMAPHORE);
  (* Recovers the resources used to implement the semaphore s, provided that no process is waiting for s to become free. *)

PROCEDURE Claim (s: SEMAPHORE);
  (* If the count associated with the semaphore s is non-zero, decrements this count and allows the calling process to continue; otherwise suspends the calling process until s is released. *)

PROCEDURE Release (s: SEMAPHORE);
  (* If there are any processes waiting on the semaphore s, allows one of them to enter the ready state; otherwise increments the count associated with s. *)

PROCEDURE CondClaim (s: SEMAPHORE): BOOLEAN;
  (* Returns TRUE if the call Claim(s) would cause the calling process to be suspended; in this case the count associated with s is not changed. Otherwise returns TRUE and the associated count is decremented. *)

END Semaphores.