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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Problems with Ada.Streams.Read (blocking)
Newsgroups: comp.lang.ada
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Date: Fri, 8 Aug 2008 15:56:27 +0200
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Date: 2008-08-08T15:56:27+02:00
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On Fri, 08 Aug 2008 15:24:43 +0200, Dennis Hoppe wrote:

> 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?

You have to implement the application layer protocol above the stream.
Stream is a flow of stream elements, nothing more. If you have a
record-oriented layer above it, that segments the stream into records, then
you define a procedure to receive one record according to the definition
of. For instance, a sequence of stream elements until CR/LF:

procedure Read_Record
   (  Channel : in out Socket_Type;
       Buffer : in out Stream_Element_Array;
      Last : out Stream_Element_Offset
   )  is
   -- Reads the stream until CR/LF filling Buffer. Last is the index of
   -- of the last element of the packet. It is Buffer'First - 1 when
   -- the record body is empty.
   Index : Stream_Element_Offset;
   Has_CR : Boolean := False;
begin
   while Index in Buffer'Range loop
      Read (Channel, Buffer (Index));
      if Has_CR and then
         Storage_Element'Pos (Buffer (Index) = Character'Pos (LF)
      then
         Last := Index - 2;
         return;
      else
         Has_CR :=
            Storage_Element'Pos (Buffer (Index) = Character'Pos (CR);
      end if;
   end loop;
   raise Data_Overrun_Error; -- Too large packet
end Read_Record;

> How can I query the stream, if new elements are ready to read?

You need not, because record-oriented protocols let you know either the
record size in advance or else provide a way to determine the record end.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de