1------------------------------------------------------------------------------
2--                                                                          --
3--                         GNAT RUN-TIME COMPONENTS                         --
4--                                                                          --
5--                     S Y S T E M . T A S K _ L O C K                      --
6--                                                                          --
7--                                 S p e c                                  --
8--                                                                          --
9--                     Copyright (C) 1998-2018, AdaCore                     --
10--                                                                          --
11-- GNAT is free software;  you can  redistribute it  and/or modify it under --
12-- terms of the  GNU General Public License as published  by the Free Soft- --
13-- ware  Foundation;  either version 3,  or (at your option) any later ver- --
14-- sion.  GNAT is distributed in the hope that it will be useful, but WITH- --
15-- OUT ANY WARRANTY;  without even the  implied warranty of MERCHANTABILITY --
16-- or FITNESS FOR A PARTICULAR PURPOSE.                                     --
17--                                                                          --
18-- As a special exception under Section 7 of GPL version 3, you are granted --
19-- additional permissions described in the GCC Runtime Library Exception,   --
20-- version 3.1, as published by the Free Software Foundation.               --
21--                                                                          --
22-- You should have received a copy of the GNU General Public License and    --
23-- a copy of the GCC Runtime Library Exception along with this program;     --
24-- see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see    --
25-- <http://www.gnu.org/licenses/>.                                          --
26--                                                                          --
27-- GNAT was originally developed  by the GNAT team at  New York University. --
28-- Extensive contributions were provided by Ada Core Technologies Inc.      --
29--                                                                          --
30------------------------------------------------------------------------------
31
32--  Simple task lock and unlock routines
33
34--  A small package containing a task lock and unlock routines for creating
35--  a critical region. The lock involved is a global lock, shared by all
36--  tasks, and by all calls to these routines, so these routines should be
37--  used with care to avoid unnecessary reduction of concurrency.
38
39--  These routines may be used in a non-tasking program, and in that case
40--  they have no effect (they do NOT cause the tasking runtime to be loaded).
41
42--  Note: this package is in the System hierarchy so that it can be directly
43--  be used by other predefined packages. User access to this package is via
44--  a renaming of this package in GNAT.Task_Lock (file g-tasloc.ads).
45
46package System.Task_Lock is
47   pragma Preelaborate;
48
49   procedure Lock;
50   pragma Inline (Lock);
51   --  Acquires the global lock, starts the execution of a critical region
52   --  which no other task can enter until the locking task calls Unlock
53
54   procedure Unlock;
55   pragma Inline (Unlock);
56   --  Releases the global lock, allowing another task to successfully
57   --  complete a Lock operation. Terminates the critical region.
58   --
59   --  The recommended protocol for using these two procedures is as
60   --  follows:
61   --
62   --    Locked_Processing : begin
63   --       Lock;
64   --       ...
65   --       TSL.Unlock;
66   --
67   --    exception
68   --       when others =>
69   --          Unlock;
70   --          raise;
71   --    end Locked_Processing;
72   --
73   --  This ensures that the lock is not left set if an exception is raised
74   --  explicitly or implicitly during the critical locked region.
75   --
76   --  Note on multiple calls to Lock: It is permissible to call Lock
77   --  more than once with no intervening Unlock from a single task,
78   --  and the lock will not be released until the corresponding number
79   --  of Unlock operations has been performed. For example:
80   --
81   --    System.Task_Lock.Lock;     -- acquires lock
82   --    System.Task_Lock.Lock;     -- no effect
83   --    System.Task_Lock.Lock;     -- no effect
84   --    System.Task_Lock.Unlock;   -- no effect
85   --    System.Task_Lock.Unlock;   -- no effect
86   --    System.Task_Lock.Unlock;   -- releases lock
87   --
88   --  However, as previously noted, the Task_Lock facility should only
89   --  be used for very local locks where the probability of conflict is
90   --  low, so usually this kind of nesting is not a good idea in any case.
91   --  In more complex locking situations, it is more appropriate to define
92   --  an appropriate protected type to provide the required locking.
93   --
94   --  It is an error to call Unlock when there has been no prior call to
95   --  Lock. The effect of such an erroneous call is undefined, and may
96   --  result in deadlock, or other malfunction of the run-time system.
97
98end System.Task_Lock;
99