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) 1998-2007, 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 2,  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.  See the GNU General Public License --
  17 -- for  more details.  You should have  received  a copy of the GNU General --
  18 -- Public License  distributed with GNAT;  see file COPYING.  If not, write --
  19 -- to  the  Free Software Foundation,  51  Franklin  Street,  Fifth  Floor, --
  20 -- Boston, MA 02110-1301, USA.                                              --
  21 --                                                                          --
  22 -- As a special exception,  if other files  instantiate  generics from this --
  23 -- unit, or you link  this unit with other files  to produce an executable, --
  24 -- this  unit  does not  by itself cause  the resulting  executable  to  be --
  25 -- covered  by the  GNU  General  Public  License.  This exception does not --
  26 -- however invalidate  any other reasons why  the executable file  might be --
  27 -- covered by the  GNU Public License.                                      --
  28 --                                                                          --
  29 -- GNAT was originally developed  by the GNAT team at  New York University. --
  30 -- Extensive contributions were provided by Ada Core Technologies Inc.      --
  31 --                                                                          --
  32 ------------------------------------------------------------------------------
  33
  34 --  Simple task lock and unlock routines
  35
  36 --  A small package containing a task lock and unlock routines for creating
  37 --  a critical region. The lock involved is a global lock, shared by all
  38 --  tasks, and by all calls to these routines, so these routines should be
  39 --  used with care to avoid unnecessary reduction of concurrency.
  40
  41 --  These routines may be used in a non-tasking program, and in that case
  42 --  they have no effect (they do NOT cause the tasking runtime to be loaded).
  43
  44 --  Note: this package is in the System hierarchy so that it can be directly
  45 --  be used by other predefined packages. User access to this package is via
  46 --  a renaming of this package in GNAT.Task_Lock (file g-tasloc.ads).
  47
  48 package System.Task_Lock is
  49    pragma Elaborate_Body;
  50
  51    procedure Lock;
  52    pragma Inline (Lock);
  53    --  Acquires the global lock, starts the execution of a critical region
  54    --  which no other task can enter until the locking task calls Unlock
  55
  56    procedure Unlock;
  57    pragma Inline (Unlock);
  58    --  Releases the global lock, allowing another task to successfully
  59    --  complete a Lock operation. Terminates the critical region.
  60    --
  61    --  The recommended protocol for using these two procedures is as
  62    --  follows:
  63    --
  64    --    Locked_Processing : begin
  65    --       Lock;
  66    --       ...
  67    --       TSL.Unlock;
  68    --
  69    --    exception
  70    --       when others =>
  71    --          Unlock;
  72    --          raise;
  73    --    end Locked_Processing;
  74    --
  75    --  This ensures that the lock is not left set if an exception is raised
  76    --  explicitly or implicitly during the critical locked region.
  77    --
  78    --  Note on multiple calls to Lock: It is permissible to call Lock
  79    --  more than once with no intervening Unlock from a single task,
  80    --  and the lock will not be released until the corresponding number
  81    --  of Unlock operations has been performed. For example:
  82    --
  83    --    System.Task_Lock.Lock;     -- acquires lock
  84    --    System.Task_Lock.Lock;     -- no effect
  85    --    System.Task_Lock.Lock;     -- no effect
  86    --    System.Task_Lock.Unlock;   -- no effect
  87    --    System.Task_Lock.Unlock;   -- no effect
  88    --    System.Task_Lock.Unlock;   -- releases lock
  89    --
  90    --  However, as previously noted, the Task_Lock facility should only
  91    --  be used for very local locks where the probability of conflict is
  92    --  low, so usually this kind of nesting is not a good idea in any case.
  93    --  In more complex locking situations, it is more appropriate to define
  94    --  an appropriate protected type to provide the required locking.
  95    --
  96    --  It is an error to call Unlock when there has been no prior call to
  97    --  Lock. The effect of such an erroneous call is undefined, and may
  98    --  result in deadlock, or other malfunction of the run-time system.
  99
 100 end System.Task_Lock;