1、IEEE-SA - IEEE Std 1003.5-1992 interpretationsIEEE Standards Interpretations for IEEE Std 1003.5 (1999 Edition) - IEEE Standard for Information Technology-POSIX Ada Language Interfaces-PART 1: Binding for System Application Program Interface Copyright 2006 by the Institute of Electrical and Electron
2、ics Engineers, Inc. All Rights Reserved.This is an interpretation of IEEE Std 1003.5 (1999 Edition). Interpretations are issued to explain and clarify the intent of a standard and are not intended to constitute an alteration to the original standard or to supply consulting information. Permission is
3、 hereby granted to download and print one copy of this document. Individuals seeking permission to reproduce and/or distribute this document in its entirety or portions of this document must contact the IEEE Standard Department for the appropriate license. Use of the information contained in this do
4、cument is at your own risk.IEEE Standards Department Copyrights and Permissions 445 Hoes Lane, P. O. Box 1331 Piscataway, New Jersey 08855-1331, USADecember 2006 Interpretation Number: #1 Topic: Open_And_Map_Shared_Memory and Open_Or_Create_And_Map_Shared_Memory functions. Relevant Clauses: 12.5.1.2
5、Interpretation request #1 This standard states that these functions are equivalent to a sequence of calls. This includes a call to Open_Shared_Memory, with the Mode parameter set as follows: “If the value of Protection is set to Allow_Write, Mode is Read_Write; otherwise Mode is Read_Only.“However,
6、Protection is a /set/ of options and may be set to “Allow_Write + Allow_Read“. By following the standard literally, this would lead to a Mode of Read_Only, which was clearly not what was intended.This interpretation would make these functions unusable for opening or creating a shared memory mapping
7、with both read and write access, which this user supposes is the most commonly desired mode of operation.However, this user sees two possible interpretations that would make the functions usable:1. The quoted sentence above should be interpreted as “If the value of Protection /includes/ Allow_Write,
8、 Mode is Read_Write; otherwise Mode is Read_Only.“2. As it is not specified exactly how the Protection parameter should be used in the subsequent call to Map_Memory, a possible interpretation would be that one should always add the Allow_Read option in the call to Map_Memory.http:/standards.ieee.org
9、/findstds/interps/1003.5-1999.html (1 of 3)11/4/2011 10:16:17 PMIEEE-SA - IEEE Std 1003.5-1992 interpretationsIn this users opinion, number #2 (above) is counterintuitive, as one would expect the protection parameters passed on to Map_Memory being the same as those initially submitted. And besides t
10、hat, this would decrease the flexibility by not allowing the user to create a write-only mapping.To illustrate the problematics, a simple example will follow. The example consists of two Ada functions which communicate via a shared memory area. With a common implementation of the IEEE 1003.5 (Floris
11、t), the problem arises in the form of a Storage_Error when trying to access the shared area:-shmserv.adb- with POSIX_Generic_Shared_Memory, POSIX_IO, POSIX_Memory_Mapping, POSIX.Permissions; with Ada.Text_IO;procedure Shmserv is type Data is record I : Integer; J : Integer; end record; package GSM i
12、s new Posix_Generic_Shared_Memory(Data); Descriptor : POSIX_IO.File_Descriptor; Data_Access : GSM.Shared_Access; begin Descriptor := GSM.Open_Or_Create_And_Map_Shared_Memory (“/foo“, POSIX_Memory_Mapping.Allow_Write, POSIX.Permissions.Access_Permission_Set); Data_Access := GSM.Access_Shared_Memory(D
13、escriptor); Data_Access.I := 1; loop Ada.Text_IO.Put_Line(IntegerImage(Data_Access.I); delay(0.5); end loop; end Shmserv; -end shmserv.adb-shmcli.adb- with POSIX_Generic_Shared_Memory, POSIX_IO, POSIX_Memory_Mapping; procedure Shmcli is type Data is record I : Integer; J : Integer; end record; packa
14、ge GSM is new Posix_Generic_Shared_Memory(Data); Descriptor : POSIX_IO.File_Descriptor; Data_Access : GSM.Shared_Access; begin Descriptor := GSM.Open_And_Map_Shared_Memory (“/foo“, POSIX_Memory_Mapping.Allow_Write); Data_Access := GSM.Access_Shared_Memory(Descriptor); loop Data_Access.I := Data_Acce
15、ss.I + 1; delay(1.0); end loop; end Shmcli; -end shmcli.adb-The current Florist implementation of Open_And_Map_Shared_Memory is defined as follows:- function Open_And_Map_Shared_Memory (Name : POSIX.POSIX_String; Protection : POSIX.Memory_Mapping.Protection_Options; Masked_Signals : POSIX.Signal_Mas
16、king := POSIX.RTS_Signals) return POSIX.IO.File_Descriptor is FD : POSIX.IO.File_Descriptor; Mode : POSIX.IO.File_Mode; begin if Protection = POSIX.Memory_Mapping.Allow_Write then Mode := POSIX.IO.Read_Write; else Mode := POSIX.IO.Read_Only; end if; Begin_Critical_Section; begin FD := POSIX.Shared_M
17、emory_Objects.Open_Shared_Memory (Name, Mode, POSIX.IO.Empty_Set, Masked_Signals); POSIX.IO.Truncate_File (FD, Length); Insert_Node (FD, POSIX.Memory_Mapping.Map_Memory (System.Storage_Elements.Storage_Offset (Length), Protection, POSIX.Memory_Mapping.Map_Shared, FD, 0); End_Critical_Section; except
18、ion when others = End_Critical_Section; raise; end; return FD; end Open_And_Map_Shared_Memory; -Interpretation Response #1 There is a problem with the wording of 12.5.1.2, and that your suggested intepretation (item 1 above) is correct. That is where the standard says http:/standards.ieee.org/findst
19、ds/interps/1003.5-1999.html (2 of 3)11/4/2011 10:16:17 PMIEEE-SA - IEEE Std 1003.5-1992 interpretations“If the value of Protection is set to Allow_Write, Mode is Read_Write; otherwise Mode is Read_Only to convey the intended meaning it should instead say“If the value of Protection *includes* Allow_W
20、rite, Mode is Read_Write; otherwise Mode is Read_Only“.Accordingly, it seems the “=“ test in the Florist implementation code below should be changed to “=“.if Protection = POSIX.Memory_Mapping.Allow_Write then Mode := POSIX.IO.Read_Write; else Mode := POSIX.IO.Read_Only; end if;http:/standards.ieee.org/findstds/interps/1003.5-1999.html (3 of 3)11/4/2011 10:16:17 PM
