66 lines
2.1 KiB
Plaintext
66 lines
2.1 KiB
Plaintext
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DEFINITION MODULE Processes;
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(*
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(pseudo-) concurrent programming with SEND/WAIT
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From the book 'Programming in Modula-2' by Prof. N. Wirth.
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*)
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EXPORT QUALIFIED SIGNAL, SEND, WAIT,
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StartProcess, Awaited, Init;
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TYPE SIGNAL;
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(* SIGNAL's are the means of synchronization between processes. *)
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PROCEDURE StartProcess (P: PROC; n: CARDINAL);
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(*- Start up a new process.
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in: P top-level procedure that will execute in this process.
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n number of bytes of workspace to be allocated to it.
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Allocates (from Storage) a workspace of n bytes, and creates a process
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executing procedure P in that workspace. Control is given to the new process.
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Caution: The caller must ensure that the workspace size is sufficient for P.
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Errors: StartProcess may fail due to insufficient memory.
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*)
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PROCEDURE SEND (VAR s: SIGNAL);
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(*- Send a signal
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in: s the signal to be sent. [Must have been Init'd]
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out: s the signal with one less process waiting for it.
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If no process is waiting for s, SEND has precisely no effect. Otherwise,
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some process which is waiting for s is given control and allowed to
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continue from WAIT.
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*)
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PROCEDURE WAIT (VAR s: SIGNAL);
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(*- Wait for some other process to send a signal.
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in: s the signal to wait for. [Must have been Init'd]
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The current process waits for the signal s. At some later time, a
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SEND(s) by some other process can cause this process to return from WAIT.
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Errors: If all other processes are waiting, WAIT terminates the program.
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*)
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PROCEDURE Awaited (s:SIGNAL): BOOLEAN;
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(*- Test whether any process is waiting for a signal.
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in: s the signal of interest. [Must have been Init'd]
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out: TRUE if and only if at least one process is waiting for s.
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*)
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PROCEDURE Init (VAR s: SIGNAL);
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(*- Initialize a SIGNAL object.
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in: s the signal to be initialized
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out: s the initialized signal (ready to be used as above)
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An object of type SIGNAL must be initialized with this procedure before
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it can be used with any of the other operations. After Init(S), Awaited(S)
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is FALSE.
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*)
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END Processes.
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