99 lines
5.1 KiB
Ada
99 lines
5.1 KiB
Ada
------------------------------------------------------------------------------
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-- --
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-- GNAT RUN-TIME COMPONENTS --
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-- --
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-- S Y S T E M . T A S K _ L O C K --
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-- --
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-- S p e c --
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-- --
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-- Copyright (C) 1998-2013, AdaCore --
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-- --
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-- GNAT is free software; you can redistribute it and/or modify it under --
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-- terms of the GNU General Public License as published by the Free Soft- --
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-- ware Foundation; either version 3, or (at your option) any later ver- --
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-- sion. GNAT is distributed in the hope that it will be useful, but WITH- --
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-- OUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY --
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-- or FITNESS FOR A PARTICULAR PURPOSE. --
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-- --
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-- As a special exception under Section 7 of GPL version 3, you are granted --
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-- additional permissions described in the GCC Runtime Library Exception, --
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-- version 3.1, as published by the Free Software Foundation. --
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-- --
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-- You should have received a copy of the GNU General Public License and --
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-- a copy of the GCC Runtime Library Exception along with this program; --
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-- see the files COPYING3 and COPYING.RUNTIME respectively. If not, see --
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-- <http://www.gnu.org/licenses/>. --
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-- --
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-- GNAT was originally developed by the GNAT team at New York University. --
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-- Extensive contributions were provided by Ada Core Technologies Inc. --
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-- --
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------------------------------------------------------------------------------
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-- Simple task lock and unlock routines
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-- A small package containing a task lock and unlock routines for creating
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-- a critical region. The lock involved is a global lock, shared by all
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-- tasks, and by all calls to these routines, so these routines should be
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-- used with care to avoid unnecessary reduction of concurrency.
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-- These routines may be used in a non-tasking program, and in that case
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-- they have no effect (they do NOT cause the tasking runtime to be loaded).
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-- Note: this package is in the System hierarchy so that it can be directly
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-- be used by other predefined packages. User access to this package is via
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-- a renaming of this package in GNAT.Task_Lock (file g-tasloc.ads).
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package System.Task_Lock is
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pragma Preelaborate;
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procedure Lock;
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pragma Inline (Lock);
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-- Acquires the global lock, starts the execution of a critical region
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-- which no other task can enter until the locking task calls Unlock
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procedure Unlock;
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pragma Inline (Unlock);
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-- Releases the global lock, allowing another task to successfully
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-- complete a Lock operation. Terminates the critical region.
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--
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-- The recommended protocol for using these two procedures is as
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-- follows:
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--
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-- Locked_Processing : begin
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-- Lock;
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-- ...
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-- TSL.Unlock;
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--
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-- exception
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-- when others =>
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-- Unlock;
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-- raise;
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-- end Locked_Processing;
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--
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-- This ensures that the lock is not left set if an exception is raised
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-- explicitly or implicitly during the critical locked region.
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--
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-- Note on multiple calls to Lock: It is permissible to call Lock
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-- more than once with no intervening Unlock from a single task,
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-- and the lock will not be released until the corresponding number
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-- of Unlock operations has been performed. For example:
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--
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-- System.Task_Lock.Lock; -- acquires lock
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-- System.Task_Lock.Lock; -- no effect
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-- System.Task_Lock.Lock; -- no effect
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-- System.Task_Lock.Unlock; -- no effect
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-- System.Task_Lock.Unlock; -- no effect
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-- System.Task_Lock.Unlock; -- releases lock
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--
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-- However, as previously noted, the Task_Lock facility should only
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-- be used for very local locks where the probability of conflict is
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-- low, so usually this kind of nesting is not a good idea in any case.
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-- In more complex locking situations, it is more appropriate to define
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-- an appropriate protected type to provide the required locking.
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--
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-- It is an error to call Unlock when there has been no prior call to
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-- Lock. The effect of such an erroneous call is undefined, and may
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-- result in deadlock, or other malfunction of the run-time system.
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end System.Task_Lock;
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