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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) 1998-2007, 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 2,  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.  See the GNU General Public License --

-- for  more details.  You should have  received  a copy of the GNU General --

-- Public License  distributed with GNAT;  see file COPYING.  If not, write --

-- to  the  Free Software Foundation,  51  Franklin  Street,  Fifth  Floor, --

-- Boston, MA 02110-1301, USA.                                              --

--                                                                          --

-- As a special exception,  if other files  instantiate  generics from this --

-- unit, or you link  this unit with other files  to produce an executable, --

-- this  unit  does not  by itself cause  the resulting  executable  to  be --

-- covered  by the  GNU  General  Public  License.  This exception does not --

-- however invalidate  any other reasons why  the executable file  might be --

-- covered by the  GNU Public License.                                      --

--                                                                          --

-- GNAT was originally developed  by the GNAT team at  New York University. --

-- Extensive contributions were provided by Ada Core Technologies Inc.      --

--                                                                          --

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--  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;