Re: Finding the end of a stream from a socket

[Andrew Shvets <andrew.shvets@gmail.com>]

On Saturday, October 21, 2017 at 4:44:42 AM UTC-4, Dmitry A. Kazakov wrote:
> On 2017-10-21 04:05, Andrew Shvets wrote:
> 
> > I've been recently trying to make a small TCP server (a toy.) I have
> > a channel that reads Characters and was trying to find the end of
> > the stream. Reading the documentation, I came across the following
> > in  g-socket.ads:
> > 
> > type Stream_Access is access all Ada.Streams.Root_Stream_Type'Class;
> > --  Same interface as Ada.Streams.Stream_IO
> > 
> > I was trying to have a way to find out when the last Character was 
> > read from the stream. I could catch the Ada.IO_Exceptions.End_Error 
> > exception, but I'm wondering if there is a better way.
> 
> End_Error is the best possible way, when applied. GNAT socket streams do 
> not raise End_Error, AFAIK.
> 
> > while not Ada.Streams.Stream_IO.End_Of_File(Channel) loop
> >    Character'Read(Channel, Received_Char);
> >    Ada.Text_IO.Put(Received_Char);
> > end loop;
> 
> This is a bad idea even when if it can work. In the case of sockets 
> there is no file end. When the connection is closed by the peer the 
> stream will stop both returning data and blocking. I suppose that will 
> cause Constraint_Error in Character'Read.
> 
> In practice there is a higher level protocol on top of the socket 
> stream, so that it is always known how many octets to read next.
> 
> Regarding your case, try this:
> 
>     Buffer : Stream_Element_Array (1..Buffer_Size);
>     Last   : Stream_Element_Offset;
> begin
>     loop
>        Receive_Socket (Socket, Buffer, Last);
>        exit when Last < Buffer'First; -- Connection is closed by the peer
>        for Octet in Buffer'First..Last loop -- Dump octets as-is
>           Put (Character'Val (Buffer (Octet)));
>        end loop;
>     end loop;
> 
> -- 
> Regards,
> Dmitry A. Kazakov
> http://www.dmitry-kazakov.de

Thanks for your reply.  I did what you recommended.


  loop
    begin
      GNAT.Sockets.Receive_Socket(Receiver, Buffer, Last);

      -- the connection was reset by peer.
      exit when Last < Buffer'First;

      for Octet in Buffer'First .. Last loop
        Ada.Text_IO.Put(Character'Val(Buffer(Octet)));
      end loop;
    exception
      when Ada.IO_Exceptions.End_Error =>
        Ada.Text_IO.Put_Line(Ada.Text_IO.Standard_Error, " ERROR: Issue encountered while receiving data from user.");
    end;
  end loop;


I get the following error when I make the Receive_Socket procedure call:

raised GNAT.SOCKETS.SOCKET_ERROR : [107] Transport endpoint is not connected