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From: john@nospam.demon.co.uk (John McCabe)
Newsgroups: comp.lang.ada
Subject: Re: Sockets - newbie at a brick wall
Date: Fri, 13 Dec 2002 11:25:59 GMT
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On Fri, 13 Dec 2002 01:23:06 +0900, Toshitaka Kumano
<kumano@cl.cilas.net> wrote:

>Sorry for somewhat off topic from GNAT.Sockets,
>
>John McCabe wrote:
>> >1) Your use of Socket_Datagram with a Connect() call. Perhaps you
>> >should try Socket_Stream sockets.
>> 
>> Datagrams are 'connectionless' sockets so you wouldn't use the
>> Connect() call at all.
>
>This statement seems restrictve.
>
>You can do "connect()" for Datagram socket and get some merits(*).

Now that I've had a look at Unix Network Programming by W Richard
Stevens, I see you are entirely correct. There's an interesting table
in there on page 225 of the Second Edition. It says:

It says that if you use sendto() that specifies a destination on a
connected datagram socket, the error EISCONN (i.e. socket is already
connected) will be raised. This is exactly what was happening with
Ben's original example.

Looking at the source of g-socket.adb, you can see that the write
operation for a datagram socket contains the following...

===========
procedure Write
     (Stream : in out Datagram_Socket_Stream_Type;
      Item   : Ada.Streams.Stream_Element_Array)
is
   :
   :
begin
   loop
      Send_Socket
        (Stream.Socket,
         Item (First .. Max),
         Index,
         Stream.To);
   :
   :
   blah, blah.

Here it is using the Send_Socket operation that requires a destination
parameter (in this case Stream.To). That version of Send_Socket()
calls C_Sendto, which is a pragma import of the C library's sendto()
function.

So the behaviour you got (Ben) is what is programmed into
GNAT.Sockets. I wouldn't necessarily call this a bug, but it is
certainly an oversight for GNAT.Sockets not to adjust what call is
made on a datagram socket depending on whether it is connected or not.
There are various ways of fixing this, but it all depends on how you
determine whether a socket is connected or not. In the simplest
scenario you would change part of Send_Socket from:

Res := C_Sendto
  (C.int (Socket),
   Item (Item'First)'Address,
   Item'Length, 0,
   Sin'Unchecked_Access,
   Len);

to:

if Is_Connected (Socket) then
  (C.int (Socket),
   Item (Item'First)'Address,
   Item'Length, 0,
   Null,
   0);
else
  (C.int (Socket),
   Item (Item'First)'Address,
   Item'Length, 0,
   Sin'Unchecked_Access,
   Len);
end if;

If you're a supported user, I would certainly send in a bug report.

Hope this helps.


Best Regards
John McCabe

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