gcc/ada/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 -- 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
  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;