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1 ------------------------------------------------------------------------------
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2 -- --
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3 -- GNAT RUN-TIME COMPONENTS --
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4 -- --
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5 -- S Y S T E M . T A S K _ L O C K --
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6 -- --
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7 -- S p e c --
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8 -- --
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9 -- Copyright (C) 1998-2017, AdaCore --
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10 -- --
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11 -- GNAT is free software; you can redistribute it and/or modify it under --
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12 -- terms of the GNU General Public License as published by the Free Soft- --
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13 -- ware Foundation; either version 3, or (at your option) any later ver- --
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14 -- sion. GNAT is distributed in the hope that it will be useful, but WITH- --
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15 -- OUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY --
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16 -- or FITNESS FOR A PARTICULAR PURPOSE. --
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17 -- --
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18 -- As a special exception under Section 7 of GPL version 3, you are granted --
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19 -- additional permissions described in the GCC Runtime Library Exception, --
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20 -- version 3.1, as published by the Free Software Foundation. --
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21 -- --
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22 -- You should have received a copy of the GNU General Public License and --
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23 -- a copy of the GCC Runtime Library Exception along with this program; --
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24 -- see the files COPYING3 and COPYING.RUNTIME respectively. If not, see --
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25 -- <http://www.gnu.org/licenses/>. --
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26 -- --
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27 -- GNAT was originally developed by the GNAT team at New York University. --
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28 -- Extensive contributions were provided by Ada Core Technologies Inc. --
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29 -- --
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30 ------------------------------------------------------------------------------
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31
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32 -- Simple task lock and unlock routines
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33
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34 -- A small package containing a task lock and unlock routines for creating
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35 -- a critical region. The lock involved is a global lock, shared by all
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36 -- tasks, and by all calls to these routines, so these routines should be
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37 -- used with care to avoid unnecessary reduction of concurrency.
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38
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39 -- These routines may be used in a non-tasking program, and in that case
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40 -- they have no effect (they do NOT cause the tasking runtime to be loaded).
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41
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42 -- Note: this package is in the System hierarchy so that it can be directly
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43 -- be used by other predefined packages. User access to this package is via
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44 -- a renaming of this package in GNAT.Task_Lock (file g-tasloc.ads).
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45
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46 package System.Task_Lock is
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47 pragma Preelaborate;
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48
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49 procedure Lock;
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50 pragma Inline (Lock);
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51 -- Acquires the global lock, starts the execution of a critical region
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52 -- which no other task can enter until the locking task calls Unlock
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53
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54 procedure Unlock;
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55 pragma Inline (Unlock);
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56 -- Releases the global lock, allowing another task to successfully
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57 -- complete a Lock operation. Terminates the critical region.
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58 --
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59 -- The recommended protocol for using these two procedures is as
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60 -- follows:
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61 --
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62 -- Locked_Processing : begin
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63 -- Lock;
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64 -- ...
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65 -- TSL.Unlock;
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66 --
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67 -- exception
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68 -- when others =>
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69 -- Unlock;
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70 -- raise;
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71 -- end Locked_Processing;
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72 --
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73 -- This ensures that the lock is not left set if an exception is raised
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74 -- explicitly or implicitly during the critical locked region.
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75 --
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76 -- Note on multiple calls to Lock: It is permissible to call Lock
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77 -- more than once with no intervening Unlock from a single task,
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78 -- and the lock will not be released until the corresponding number
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79 -- of Unlock operations has been performed. For example:
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80 --
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81 -- System.Task_Lock.Lock; -- acquires lock
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82 -- System.Task_Lock.Lock; -- no effect
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83 -- System.Task_Lock.Lock; -- no effect
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84 -- System.Task_Lock.Unlock; -- no effect
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85 -- System.Task_Lock.Unlock; -- no effect
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86 -- System.Task_Lock.Unlock; -- releases lock
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87 --
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88 -- However, as previously noted, the Task_Lock facility should only
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89 -- be used for very local locks where the probability of conflict is
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90 -- low, so usually this kind of nesting is not a good idea in any case.
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91 -- In more complex locking situations, it is more appropriate to define
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92 -- an appropriate protected type to provide the required locking.
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93 --
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94 -- It is an error to call Unlock when there has been no prior call to
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95 -- Lock. The effect of such an erroneous call is undefined, and may
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96 -- result in deadlock, or other malfunction of the run-time system.
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97
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98 end System.Task_Lock;
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