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------------------------------------------------------------------------------
-- --
-- GNAT RUN-TIME COMPONENTS --
-- --
-- S Y S T E M . T A S K _ L O C K --
-- --
-- S p e c --
-- --
-- Copyright (C) 1998-2010, AdaCore --
-- --
-- GNAT is free software; you can redistribute it and/or modify it under --
-- terms of the GNU General Public License as published by the Free Soft- --
-- ware Foundation; either version 3, or (at your option) any later ver- --
-- sion. GNAT is distributed in the hope that it will be useful, but WITH- --
-- OUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY --
-- or FITNESS FOR A PARTICULAR PURPOSE. --
-- --
-- As a special exception under Section 7 of GPL version 3, you are granted --
-- additional permissions described in the GCC Runtime Library Exception, --
-- version 3.1, as published by the Free Software Foundation. --
-- --
-- You should have received a copy of the GNU General Public License and --
-- a copy of the GCC Runtime Library Exception along with this program; --
-- see the files COPYING3 and COPYING.RUNTIME respectively. If not, see --
-- <http://www.gnu.org/licenses/>. --
-- --
-- GNAT was originally developed by the GNAT team at New York University. --
-- Extensive contributions were provided by Ada Core Technologies Inc. --
-- --
------------------------------------------------------------------------------
-- Simple task lock and unlock routines
-- A small package containing a task lock and unlock routines for creating
-- a critical region. The lock involved is a global lock, shared by all
-- tasks, and by all calls to these routines, so these routines should be
-- used with care to avoid unnecessary reduction of concurrency.
-- These routines may be used in a non-tasking program, and in that case
-- they have no effect (they do NOT cause the tasking runtime to be loaded).
-- Note: this package is in the System hierarchy so that it can be directly
-- be used by other predefined packages. User access to this package is via
-- a renaming of this package in GNAT.Task_Lock (file g-tasloc.ads).
package System.Task_Lock is
pragma Elaborate_Body;
procedure Lock;
pragma Inline (Lock);
-- Acquires the global lock, starts the execution of a critical region
-- which no other task can enter until the locking task calls Unlock
procedure Unlock;
pragma Inline (Unlock);
-- Releases the global lock, allowing another task to successfully
-- complete a Lock operation. Terminates the critical region.
--
-- The recommended protocol for using these two procedures is as
-- follows:
--
-- Locked_Processing : begin
-- Lock;
-- ...
-- TSL.Unlock;
--
-- exception
-- when others =>
-- Unlock;
-- raise;
-- end Locked_Processing;
--
-- This ensures that the lock is not left set if an exception is raised
-- explicitly or implicitly during the critical locked region.
--
-- Note on multiple calls to Lock: It is permissible to call Lock
-- more than once with no intervening Unlock from a single task,
-- and the lock will not be released until the corresponding number
-- of Unlock operations has been performed. For example:
--
-- System.Task_Lock.Lock; -- acquires lock
-- System.Task_Lock.Lock; -- no effect
-- System.Task_Lock.Lock; -- no effect
-- System.Task_Lock.Unlock; -- no effect
-- System.Task_Lock.Unlock; -- no effect
-- System.Task_Lock.Unlock; -- releases lock
--
-- However, as previously noted, the Task_Lock facility should only
-- be used for very local locks where the probability of conflict is
-- low, so usually this kind of nesting is not a good idea in any case.
-- In more complex locking situations, it is more appropriate to define
-- an appropriate protected type to provide the required locking.
--
-- It is an error to call Unlock when there has been no prior call to
-- Lock. The effect of such an erroneous call is undefined, and may
-- result in deadlock, or other malfunction of the run-time system.
end System.Task_Lock;
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