gcc/ada/s-shasto.ads

   1 ------------------------------------------------------------------------------
   2 --                                                                          --
   3 --                         GNAT COMPILER COMPONENTS                         --
   4 --                                                                          --
   5 --                 S Y S T E M . S H A R E D _ S T O R A G E                --
   6 --                                                                          --
   7 --                                 S p e c                                  --
   8 --                                                                          --
   9 --          Copyright (C) 1998-2014, Free Software Foundation, Inc.         --
  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 --  This package manages the shared/persistent storage required for
  33 --  full implementation of variables in Shared_Passive packages, more
  34 --  precisely variables whose enclosing dynamic scope is a shared
  35 --  passive package. This implementation is specific to GNAT and GLADE
  36 --  provides a more general implementation not dedicated to file
  37 --  storage.
  38
  39 --  --------------------------
  40 --  -- Shared Storage Model --
  41 --  --------------------------
  42
  43 --  The basic model used is that each partition that references the
  44 --  Shared_Passive package has a local copy of the package data that
  45 --  is initialized in accordance with the declarations of the package
  46 --  in the normal manner. The routines in System.Shared_Storage are
  47 --  then used to ensure that the values in these separate copies are
  48 --  properly synchronized with the state of the overall system.
  49
  50 --  In the GNAT implementation, this synchronization is ensured by
  51 --  maintaining a set of files, in a designated directory. The
  52 --  directory is designated by setting the environment variable
  53 --  SHARED_MEMORY_DIRECTORY. This variable must be set for all
  54 --  partitions. If the environment variable is not defined, then the
  55 --  current directory is used.
  56
  57 --  There is one storage for each variable. The name is the fully
  58 --  qualified name of the variable with all letters forced to lower
  59 --  case. For example, the variable Var in the shared passive package
  60 --  Pkg results in the storage name pkg.var.
  61
  62 --  If the storage does not exist, it indicates that no partition has
  63 --  assigned a new value, so that the initial value is the correct
  64 --  one. This is the critical component of the model. It means that
  65 --  there is no system-wide synchronization required for initializing
  66 --  the package, since the shared storages need not (and do not)
  67 --  reflect the initial state. There is therefore no issue of
  68 --  synchronizing initialization and read/write access.
  69
  70 --  -----------------------
  71 --  -- Read/Write Access --
  72 --  -----------------------
  73
  74 --  The approach is as follows:
  75
  76 --    For each shared variable, var, an instantiation of the below generic
  77 --    package is created which provides Read and Write supporting procedures.
  78
  79 --    The routine Read in package System.Shared_Storage.Shared_Var_Procs
  80 --    ensures to assign variable V to the last written value among processes
  81 --    referencing it. A call to this procedure is generated by the expander
  82 --    before each read access to the shared variable.
  83
  84 --    The routine Write in package System.Shared_Storage.Shared_Var_Proc
  85 --    set a new value to the shared variable and, according to the used
  86 --    implementation, propagate this value among processes referencing it.
  87 --    A call to this procedure is generated by the expander after each
  88 --    assignment of the shared variable.
  89
  90 --  Note: a special circuit allows the use of stream attributes Read and
  91 --  Write for limited types (using the corresponding attribute for the
  92 --  full type), but there are limitations on the data that can be placed
  93 --  in shared passive partitions. See sem_smem.ads/adb for details.
  94
  95 --  ----------------------------------------------------------------
  96 --  -- Handling of Protected Objects in Shared Passive Partitions --
  97 --  ----------------------------------------------------------------
  98
  99 --  In the context of GNAT, during the execution of a protected
 100 --  subprogram call, access is locked out using a locking mechanism
 101 --  per protected object, as provided by the GNAT.Lock_Files
 102 --  capability in the specific case of GNAT. This package contains the
 103 --  lock and unlock calls, and the expander generates a call to the
 104 --  lock routine before the protected call and a call to the unlock
 105 --  routine after the protected call.
 106
 107 --  Within the code of the protected subprogram, the access to the
 108 --  protected object itself uses the local copy, without any special
 109 --  synchronization.  Since global access is locked out, no other task
 110 --  or partition can attempt to read or write this data as long as the
 111 --  lock is held.
 112
 113 --  The data in the local copy does however need synchronizing with
 114 --  the global values in the shared storage. This is achieved as
 115 --  follows:
 116
 117 --    The protected object generates a read and assignment routine as
 118 --    described for other shared passive variables. The code for the
 119 --    'Read and 'Write attributes (not normally allowed, but allowed
 120 --    in this special case) simply reads or writes the values of the
 121 --    components in the protected record.
 122
 123 --    The lock call is followed by a call to the shared read routine to
 124 --    synchronize the local copy to contain the proper global value.
 125
 126 --    The unlock call in the procedure case only is preceded by a call
 127 --    to the shared assign routine to synchronize the global shared
 128 --    storages with the (possibly modified) local copy.
 129
 130 --    These calls to the read and assign routines, as well as the lock
 131 --    and unlock routines, are inserted by the expander (see exp_smem.adb).
 132
 133 package System.Shared_Storage is
 134
 135    procedure Shared_Var_Lock (Var : String);
 136    --  This procedure claims the shared storage lock. It is used for
 137    --  protected types in shared passive packages. A call to this
 138    --  locking routine is generated as the first operation in the code
 139    --  for the body of a protected subprogram, and it busy waits if
 140    --  the lock is busy.
 141
 142    procedure Shared_Var_Unlock (Var : String);
 143    --  This procedure releases the shared storage lock obtained by a
 144    --  prior call to the Shared_Var_Lock procedure, and is to be
 145    --  generated as the last operation in the body of a protected
 146    --  subprogram.
 147
 148    --  This generic package is instantiated for each shared passive
 149    --  variable. It provides supporting procedures called upon each
 150    --  read or write access by the expanded code.
 151
 152    generic
 153
 154       type Typ is limited private;
 155       --  Shared passive variable type
 156
 157       V : in out Typ;
 158       --  Shared passive variable
 159
 160       Full_Name : String;
 161       --  Shared passive variable storage name
 162
 163    package Shared_Var_Procs is
 164
 165       procedure Read;
 166       --  Shared passive variable access routine. Each reference to the
 167       --  shared variable, V, is preceded by a call to the corresponding
 168       --  Read procedure, which either leaves the initial value unchanged
 169       --  if the storage does not exist, or reads the current value from
 170       --  the shared storage.
 171
 172       procedure Write;
 173       --  Shared passive variable assignment routine. Each assignment to
 174       --  the shared variable, V, is followed by a call to the corresponding
 175       --  Write procedure, which writes the new value to the shared storage.
 176
 177    end Shared_Var_Procs;
 178
 179 end System.Shared_Storage;