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From: Dennis Hoppe <dennis.hoppe@hoppinet.de>
Newsgroups: comp.lang.ada
Subject: Problems with Ada.Streams.Read (blocking)
Date: Fri, 08 Aug 2008 15:24:43 +0200
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Date: 2008-08-08T15:24:43+02:00
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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?

Many thanks in advance,
   Dennis