------------------------------------------------------------------------------ -- -- -- GNAT RUN-TIME COMPONENTS -- -- -- -- S Y S T E M . T A S K _ L O C K -- -- -- -- S p e c -- -- -- -- Copyright (C) 1998-2013, 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 -- -- . -- -- -- -- 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 Preelaborate; 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;