Re: Problems with Ada.Streams.Read (blocking)

[Adam Beneschan <adam@irvine.com>]

On Aug 8, 6:24 am, Dennis Hoppe <dennis.ho...@hoppinet.de> wrote:
> Hi,
>
> I've written a minimal example to access an ftp server (like FileZilla).
> First, let's have a look at the code snippet:
>
> -- START
> with Ada.Text_IO;
> with Ada.Streams;
> with GNAT.Sockets; use GNAT.Sockets;
> use type Ada.Streams.Stream_Element_Count;
>
> procedure Test is
>    Client: Socket_Type;
>    Address: Sock_Addr_Type;
>    Channel: Stream_Access;
>    Data   : Ada.Streams.Stream_Element_Array (1 .. 1);
>    Offset : Ada.Streams.Stream_Element_Count;
>
> begin
>    Initialize;
>    Create_Socket(Client);
>    Address.Addr := Inet_Addr("127.0.0.1");
>    Address.Port := 21;
>    Connect_Socket (Client,Address);
>    Channel := Stream(Client);
>
>    loop  -- reads in the welcome message
>      Ada.Streams.Read (Channel.all, Data(1..1), Offset);
>      exit when Offset = 0;
>      -- alternative: exit when Offset /= Data'Last
>      for I in 1 .. Offset loop
>         Ada.Text_IO.Put (Character'Val (Data (I)));
>      end loop;
>    end loop;
> end Test;
> -- END
>
> The problem is, that Ada.Streams.Read is blocking, if the end of the
> stream is reached. I found many examples, that outline, that the
> variable Offset will be 0, if no further elements are on the stream.
> But this seems not to be the case, unfortunately.
>
> For a ftp server/client situation, each command is terminate by <CRLF>,
> so I enhanced the exit condition to:
>
> loop
>    Ada.Streams.Read (Channel.all, Data(1..2), Offset);
>    exit when (Character'Val (Data(1)) = ASCII.CR and Character'Val
> (Data(2)) = ASCII.LF);
>    -- code omitted
> end loop;
>
> Of course, the Stream_Element_Array is enhanced to (1..2).
>
> This approach works very well, but some ftp commands send a messages
> over several lines. I do not know in advance, how many lines I should
> read in. Subsequently, Ada.Streams.Read has to be called in a loop,
> which will eventually block, again.
>
> How can I query the stream, if new elements are ready to read?

I'm not familiar with GNAT.Sockets, but looking at the spec it appears
that there are a couple routines that might help: Control_Socket which
lets you specify non-blocking I/O, and Check_Selector which I think
can be used to query whether data is available, if you give it a
Timeout of zero.  Anyway, I haven't tried anything and I have no idea
whether it's appropriate for your problem, but it seems like it might
help.  If not, my apologies.

                                  -- Adam