File : s-tasloc.ads
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) 1998-2013, 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 -- --
19 -- --
20 -- --
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
46 package 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
98 end System.Task_Lock;