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) 1998-2001 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 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,  59 Temple Place - Suite 330,  Boston, --
  20 -- MA 02111-1307, 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 --  This package manages the shared/persistent storage required for
  35 --  full implementation of variables in Shared_Passive packages, more
  36 --  precisely variables whose enclosing dynamic scope is a shared
  37 --  passive package. This implementation is specific to GNAT and GLADE
  38 --  provides a more general implementation not dedicated to file
  39 --  storage.
  40
  41 --  --------------------------
  42 --  -- Shared Storage Model --
  43 --  --------------------------
  44
  45 --  The basic model used is that each partition that references the
  46 --  Shared_Passive package has a local copy of the package data that
  47 --  is initialized in accordance with the declarations of the package
  48 --  in the normal manner. The routines in System.Shared_Storage are
  49 --  then used to ensure that the values in these separate copies are
  50 --  properly synchronized with the state of the overall system.
  51
  52 --  In the GNAT implementation, this synchronization is ensured by
  53 --  maintaining a set of files, in a designated directory. The
  54 --  directory is designated by setting the environment variable
  55 --  SHARED_MEMORY_DIRECTORY. This variable must be set for all
  56 --  partitions. If the environment variable is not defined, then the
  57 --  current directory is used.
  58
  59 --  There is one storage for each variable. The name is the fully
  60 --  qualified name of the variable with all letters forced to lower
  61 --  case. For example, the variable Var in the shared passive package
  62 --  Pkg results in the storage name pkg.var.
  63
  64 --  If the storage does not exist, it indicates that no partition has
  65 --  assigned a new value, so that the initial value is the correct
  66 --  one. This is the critical component of the model. It means that
  67 --  there is no system-wide synchronization required for initializing
  68 --  the package, since the shared storages need not (and do not)
  69 --  reflect the initial state. There is therefore no issue of
  70 --  synchronizing initialization and read/write access.
  71
  72 --  -----------------------
  73 --  -- Read/Write Access --
  74 --  -----------------------
  75
  76 --  The approach is as follows:
  77
  78 --    For each shared variable, var, an access routine varR is created whose
  79 --    body has the following form (this example is for Pkg.Var):
  80
  81 --      procedure varR is
  82 --         S : Ada.Streams.Stream_IO.Stream_Access;
  83 --      begin
  84 --         S := Shared_Var_ROpen ("pkg.var");
  85 --         if S /= null then
  86 --            typ'Read (S);
  87 --            Shared_Var_Close (S);
  88 --         end if;
  89 --      end varR;
  90
  91 --    The routine Shared_Var_ROpen in package System.Shared_Storage
  92 --    either returns null if the storage does not exist, or otherwise a
  93 --    Stream_Access value that references the corresponding shared
  94 --    storage, ready to read the current value.
  95
  96 --    Each reference to the shared variable, var, is preceded by a
  97 --    call to the corresponding varR procedure, which either leaves the
  98 --    initial value unchanged if the storage does not exist, or reads
  99 --    the current value from the shared storage.
 100
 101 --    In addition, for each shared variable, var, an assignment routine
 102 --    is created whose body has the following form (again for Pkg.Var)
 103
 104 --      procedure VarA is
 105 --         S : Ada.Streams.Stream_IO.Stream_Access;
 106 --      begin
 107 --         S := Shared_Var_WOpen ("pkg.var");
 108 --         typ'Write (S, var);
 109 --         Shared_Var_Close (S);
 110 --      end VarA;
 111
 112 --    The routine Shared_Var_WOpen in package System.Shared_Storage
 113 --    returns a Stream_Access value that references the corresponding
 114 --    shared storage, ready to write the new value.
 115
 116 --    Each assignment to the shared variable, var, is followed by a call
 117 --    to the corresponding varA procedure, which writes the new value to
 118 --    the shared storage.
 119
 120 --    Note that there is no general synchronization for these storage
 121 --    read and write operations, since it is assumed that a correctly
 122 --    operating programs will provide appropriate synchronization. In
 123 --    particular, variables can be protected using protected types with
 124 --    no entries.
 125
 126 --    The routine Shared_Var_Close is called to indicate the end of a
 127 --    read/write operations. This can be useful even in the context of
 128 --    the GNAT implementation. For instance, when a read operation and a
 129 --    write operation occur at the same time on the same partition, as
 130 --    the same stream is used simultaneously, both operations can
 131 --    terminate abruptly by raising exception Mode_Error because the
 132 --    stream has been opened in read mode and then in write mode and at
 133 --    least used by the read opartion. To avoid this unexpected
 134 --    behaviour, we introduce a synchronization at the partition level.
 135
 136 --  Note: a special circuit allows the use of stream attributes Read and
 137 --  Write for limited types (using the corresponding attribute for the
 138 --  full type), but there are limitations on the data that can be placed
 139 --  in shared passive partitions. See sem_smem.ads/adb for details.
 140
 141 --  ----------------------------------------------------------------
 142 --  -- Handling of Protected Objects in Shared Passive Partitions --
 143 --  ----------------------------------------------------------------
 144
 145 --  In the context of GNAT, during the execution of a protected
 146 --  subprogram call, access is locked out using a locking mechanism
 147 --  per protected object, as provided by the GNAT.Lock_Files
 148 --  capability in the specific case of GNAT. This package contains the
 149 --  lock and unlock calls, and the expander generates a call to the
 150 --  lock routine before the protected call and a call to the unlock
 151 --  routine after the protected call.
 152
 153 --  Within the code of the protected subprogram, the access to the
 154 --  protected object itself uses the local copy, without any special
 155 --  synchronization.  Since global access is locked out, no other task
 156 --  or partition can attempt to read or write this data as long as the
 157 --  lock is held.
 158
 159 --  The data in the local copy does however need synchronizing with
 160 --  the global values in the shared storage. This is achieved as
 161 --  follows:
 162
 163 --    The protected object generates a read and assignment routine as
 164 --    described for other shared passive variables. The code for the
 165 --    'Read and 'Write attributes (not normally allowed, but allowed
 166 --    in this special case) simply reads or writes the values of the
 167 --    components in the protected record.
 168
 169 --    The lock call is followed by a call to the shared read routine to
 170 --    synchronize the local copy to contain the proper global value.
 171
 172 --    The unlock call in the procedure case only is preceded by a call
 173 --    to the shared assign routine to synchronize the global shared
 174 --    storages with the (possibly modified) local copy.
 175
 176 --    These calls to the read and assign routines, as well as the lock
 177 --    and unlock routines, are inserted by the expander (see exp_smem.adb).
 178
 179 with Ada.Streams.Stream_IO;
 180
 181 package System.Shared_Storage is
 182
 183    package SIO renames Ada.Streams.Stream_IO;
 184
 185    function Shared_Var_ROpen (Var : String) return SIO.Stream_Access;
 186    --  As described above, this routine returns null if the
 187    --  corresponding shared storage does not exist, and otherwise, if
 188    --  the storage does exist, a Stream_Access value that references
 189    --  the shared storage, ready to read the current value.
 190
 191    function Shared_Var_WOpen (Var : String) return SIO.Stream_Access;
 192    --  As described above, this routine returns a Stream_Access value
 193    --  that references the shared storage, ready to write the new
 194    --  value. The storage is created by this call if it does not
 195    --  already exist.
 196
 197    procedure Shared_Var_Close (Var : in SIO.Stream_Access);
 198    --  This routine signals the end of a read/assign operation. It can
 199    --  be useful to embrace a read/write operation between a call to
 200    --  open and a call to close which protect the whole operation.
 201    --  Otherwise, two simultaneous operations can result in the
 202    --  raising of exception Data_Error by setting the access mode of
 203    --  the variable in an incorrect mode.
 204
 205    procedure Shared_Var_Lock (Var : String);
 206    --  This procedure claims the shared storage lock. It is used for
 207    --  protected types in shared passive packages. A call to this
 208    --  locking routine is generated as the first operation in the code
 209    --  for the body of a protected subprogram, and it busy waits if
 210    --  the lock is busy.
 211
 212    procedure Shared_Var_Unlock (Var : String);
 213    --  This procedure releases the shared storage lock obtaind by a
 214    --  prior call to the Shared_Mem_Lock procedure, and is to be
 215    --  generated as the last operation in the body of a protected
 216    --  subprogram.
 217
 218 end System.Shared_Storage;