------------------------------------------------------------------------------ -- -- -- GNAT COMPILER COMPONENTS -- -- -- -- S Y S T E M . S H A R E D _ S T O R A G E -- -- -- -- S p e c -- -- -- -- Copyright (C) 1998-2008, Free Software Foundation, Inc. -- -- -- -- GNAT is free software; you can redistribute it and/or modify it under -- -- terms of the GNU General Public License as published by the Free Soft- -- -- ware Foundation; either version 2, or (at your option) any later ver- -- -- sion. GNAT is distributed in the hope that it will be useful, but WITH- -- -- OUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY -- -- or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License -- -- for more details. You should have received a copy of the GNU General -- -- Public License distributed with GNAT; see file COPYING. If not, write -- -- to the Free Software Foundation, 51 Franklin Street, Fifth Floor, -- -- Boston, MA 02110-1301, USA. -- -- -- -- As a special exception, if other files instantiate generics from this -- -- unit, or you link this unit with other files to produce an executable, -- -- this unit does not by itself cause the resulting executable to be -- -- covered by the GNU General Public License. This exception does not -- -- however invalidate any other reasons why the executable file might be -- -- covered by the GNU Public License. -- -- -- -- GNAT was originally developed by the GNAT team at New York University. -- -- Extensive contributions were provided by Ada Core Technologies Inc. -- -- -- ------------------------------------------------------------------------------ -- This package manages the shared/persistent storage required for -- full implementation of variables in Shared_Passive packages, more -- precisely variables whose enclosing dynamic scope is a shared -- passive package. This implementation is specific to GNAT and GLADE -- provides a more general implementation not dedicated to file -- storage. -- This unit (and shared passive partitions) are supported on all -- GNAT implementations except on OpenVMS (where problems arise from -- trying to share files, and with version numbers of files) -- -------------------------- -- -- Shared Storage Model -- -- -------------------------- -- The basic model used is that each partition that references the -- Shared_Passive package has a local copy of the package data that -- is initialized in accordance with the declarations of the package -- in the normal manner. The routines in System.Shared_Storage are -- then used to ensure that the values in these separate copies are -- properly synchronized with the state of the overall system. -- In the GNAT implementation, this synchronization is ensured by -- maintaining a set of files, in a designated directory. The -- directory is designated by setting the environment variable -- SHARED_MEMORY_DIRECTORY. This variable must be set for all -- partitions. If the environment variable is not defined, then the -- current directory is used. -- There is one storage for each variable. The name is the fully -- qualified name of the variable with all letters forced to lower -- case. For example, the variable Var in the shared passive package -- Pkg results in the storage name pkg.var. -- If the storage does not exist, it indicates that no partition has -- assigned a new value, so that the initial value is the correct -- one. This is the critical component of the model. It means that -- there is no system-wide synchronization required for initializing -- the package, since the shared storages need not (and do not) -- reflect the initial state. There is therefore no issue of -- synchronizing initialization and read/write access. -- ----------------------- -- -- Read/Write Access -- -- ----------------------- -- The approach is as follows: -- For each shared variable, var, an instantiation of the below generic -- package is created which provides Read and Write supporting procedures. -- The routine Read in package System.Shared_Storage.Shared_Var_Procs -- ensures to assign variable V to the last written value among processes -- referencing it. A call to this procedure is generated by the expander -- before each read access to the shared variable. -- The routine Write in package System.Shared_Storage.Shared_Var_Proc -- set a new value to the shared variable and, according to the used -- implementation, propagate this value among processes referencing it. -- A call to this procedure is generated by the expander after each -- assignment of the shared variable. -- Note: a special circuit allows the use of stream attributes Read and -- Write for limited types (using the corresponding attribute for the -- full type), but there are limitations on the data that can be placed -- in shared passive partitions. See sem_smem.ads/adb for details. -- ---------------------------------------------------------------- -- -- Handling of Protected Objects in Shared Passive Partitions -- -- ---------------------------------------------------------------- -- In the context of GNAT, during the execution of a protected -- subprogram call, access is locked out using a locking mechanism -- per protected object, as provided by the GNAT.Lock_Files -- capability in the specific case of GNAT. This package contains the -- lock and unlock calls, and the expander generates a call to the -- lock routine before the protected call and a call to the unlock -- routine after the protected call. -- Within the code of the protected subprogram, the access to the -- protected object itself uses the local copy, without any special -- synchronization. Since global access is locked out, no other task -- or partition can attempt to read or write this data as long as the -- lock is held. -- The data in the local copy does however need synchronizing with -- the global values in the shared storage. This is achieved as -- follows: -- The protected object generates a read and assignment routine as -- described for other shared passive variables. The code for the -- 'Read and 'Write attributes (not normally allowed, but allowed -- in this special case) simply reads or writes the values of the -- components in the protected record. -- The lock call is followed by a call to the shared read routine to -- synchronize the local copy to contain the proper global value. -- The unlock call in the procedure case only is preceded by a call -- to the shared assign routine to synchronize the global shared -- storages with the (possibly modified) local copy. -- These calls to the read and assign routines, as well as the lock -- and unlock routines, are inserted by the expander (see exp_smem.adb). package System.Shared_Storage is procedure Shared_Var_Lock (Var : String); -- This procedure claims the shared storage lock. It is used for -- protected types in shared passive packages. A call to this -- locking routine is generated as the first operation in the code -- for the body of a protected subprogram, and it busy waits if -- the lock is busy. procedure Shared_Var_Unlock (Var : String); -- This procedure releases the shared storage lock obtained by a -- prior call to the Shared_Var_Lock procedure, and is to be -- generated as the last operation in the body of a protected -- subprogram. -- This generic package is instantiated for each shared passive -- variable. It provides supporting procedures called upon each -- read or write access by the expanded code. generic type Typ is limited private; -- Shared passive variable type V : in out Typ; -- Shared passive variable Full_Name : String; -- Shared passive variable storage name package Shared_Var_Procs is procedure Read; -- Shared passive variable access routine. Each reference to the -- shared variable, V, is preceded by a call to the corresponding -- Read procedure, which either leaves the initial value unchanged -- if the storage does not exist, or reads the current value from -- the shared storage. procedure Write; -- Shared passive variable assignment routine. Each assignment to -- the shared variable, V, is followed by a call to the corresponding -- Write procedure, which writes the new value to the shared storage. end Shared_Var_Procs; end System.Shared_Storage;