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------------------------------------------------------------------------------
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-- --
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-- GNAT RUN-TIME COMPONENTS --
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-- --
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-- G N A T . 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) 1998-2005, 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 2, 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. See the GNU General Public License --
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-- for more details. You should have received a copy of the GNU General --
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-- Public License distributed with GNAT; see file COPYING. If not, write --
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-- to the Free Software Foundation, 51 Franklin Street, Fifth Floor, --
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-- Boston, MA 02110-1301, USA. --
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-- --
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-- As a special exception, if other files instantiate generics from this --
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-- unit, or you link this unit with other files to produce an executable, --
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-- this unit does not by itself cause the resulting executable to be --
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-- covered by the GNU General Public License. This exception does not --
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-- however invalidate any other reasons why the executable file might be --
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-- covered by the GNU Public License. --
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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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package GNAT.Task_Lock is
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pragma Elaborate_Body;
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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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-- GNAT.Task_Lock.Lock; -- acquires lock
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-- GNAT.Task_Lock.Lock; -- no effect
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-- GNAT.Task_Lock.Lock; -- no effect
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-- GNAT.Task_Lock.Unlock; -- no effect
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-- GNAT.Task_Lock.Unlock; -- no effect
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-- GNAT.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 GNAT.Task_Lock;
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