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From: "Mattias Lindblad" <ml_news@matli.net>
Newsgroups: comp.lang.ada
Subject: POSIX_Generic_Shared_Memory
Date: 20 Sep 2004 10:23:10 -0700
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Date: 2004-09-20T10:23:10-07:00
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Hi,

I have found some problems when experimenting with the
POSIX_Generic_Shared_Memory package. Let's start with a code example to
illustrate my point:

---[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 is 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(Descriptor);
Data_Access.I := 1;
loop
Ada.Text_IO.Put_Line(Integer'Image(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;
package 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_Access.I + 1;
delay(1.0);
end loop;
end Shmcli;
---[end shmcli.adb]---

The purpose of this example code is to let one process change data in a
shared memory area, while the other process monitors the shared data.
It accomplishes this by using the convenient
Open_(Or_Create_)And_Map_Shared_Memory functions.

However, the client raises a Storage_Error when trying to access the
shared data. I have found that this is because the Protection_Options
are passed straight through to Map_Memory, and in my case this leads to
a memory region that is write-only.

The obvious solution would of course be to set Protection_Options to
(Allow_Write + Allow_Read). Unfortunately, by doing this the
Open_Shared_Memory will be called with a mode of Read_Only. This feels
a bit weird, but follows the POSIX 1003.5b standard, 12.5.1.2(1): "If
the value of Protection is set to Allow_Write, Mode is Read_Write;
otherwise Mode is Read_Only." (From the 1996 version, I'm afraid I
haven't access to an updated version).

Is this really how it is supposed to work? In my opinion the standard
text should read "If the value of Protection *includes* Allow_Write
[...]".

I have only tried this with GNAT 3.15p and Florist. So I don't know if
this might be specific to this system or if it is more general.

Any ideas about this? Is it a bug in GNAT, the standard, both of them
or none of them (i.e. I'm not doing things the way I'm supposed to)?
Thanks,
Mattias