Hints for networking

Hints for networking

[Jano]

Hello everybody.

I'm about to program a networking package for the first time in Ada and 
am somewhat undecided the way to go. I'd like to hear some pointers for 
examples or hints.

The protocol has some fixed-size packets and others of unknown size (but 
with known headers which contains the full size). I'm wondering what 
representation clauses could help me, if any. Also, I've my doubts about 
streams, because in the past I've tried them to write files and in Gnat 
I couldn't manage to output exact sized types except uncheckedconverting 
them into byte arrays. For example an integer subtype with 
representation clauses for 16 bits was always dumped as its base type, 
i.e. 32 bits. Frankly, the RM is too dense for me so some beginner 
documentation would be very useful.

I've read some tutorials and books from adapower lately but can't find 
something more on the subject. Clues welcomed.

TIA,

-- 
-------------------------
Jano
402450[at]cepsz.unizar.es
-------------------------

Re: Hints for networking

[tmoran]

> Also, I've my doubts about streams, because in the past I've tried them
> to write files and in Gnat I couldn't manage to output exact sized types
> except uncheckedconverting
  Streams can be quite convenient, but they are like programming without
rep clauses.  If the representation doesn't matter, fine.  If it does,
you have to write your own 'read/'write routines to and from
Stream_Element_Array.  But you probably want your IO to be in terms of
objects at a higher level than Integer or Boolean, etc, so you would
want to write your own Some_Record 'Write routines anyway.
At the low level, your code will contain things like
  type Some_Record is record
  ...
  subtype External_Form_Some_Record is Ada.Streams.Stream_Element_Array(1 .. 6);
  function External_Some_Record is new Ada.Unchecked_Conversion
    (Source=>Some_Record, Target=>External_Form_Some_Record);
but at higher levels writing a complex object just becomes a series of
calls to 'Write on lower level subrecords, and, if there's nothing fancy
going on, you can let the compiler create the series of subrecord 'Writes.

Re: Hints for networking

[David C. Hoos]

Hi Jano,

I have written many many network protocol representations
using Ada95 streams.  The elegance of Ada Streams, and
the simplicity of their use once you have made the effort to
properly define your types and their stream attributes really
makes the effort worthwhile.  Here are some hints and rules
needed to do this successfully: These rules are based on
the presumption that there is a specific network representation
required by your protocol.

   1,  This rule number 1 is very important.  NEVER base
         any value written to a stream on the standard Ada type
         Integer.  
         You should always define project-specific types that
         do not depend on implementations that are subject to
         variation between compilers or platform types.  For
         example, you can base integer types on those available
         in the package Interfaces -- Integer_16, Unsigned_16, etc.
         That's what the Interface package is for -- i.e. to permit
         writing portable code for interfaces.

    2.  You need to write stream attributes for multi-octet numbers
         that respect the bendiness of your platform.  Even when
         you've been told that "this only has to run on Sparc," sooner
         or later your code will also need to run on Intel, so I've
         always taken the trouble to deal with bendiness.

   3.  All objects written to a stream must have a 'Size that is an
        integer multiple of 8.  So, if you have an octet with two four-bit
        fields, I make them into a record type, and write the stream
        attributes for that record.  I've done things like a record that
        has a 31-bit field, and a 1-bit field.  

Happy streaming!

David Hoos
 
----- Original Message ----- 
From: "Jano" <402450@cepsz.unizar.es>
Newsgroups: comp.lang.ada
To: <comp.lang.ada@ada.eu.org>
Sent: Friday, January 31, 2003 10:37 AM
Subject: Hints for networking


> Hello everybody.
> 
> I'm about to program a networking package for the first time in Ada and 
> am somewhat undecided the way to go. I'd like to hear some pointers for 
> examples or hints.
> 
> The protocol has some fixed-size packets and others of unknown size (but 
> with known headers which contains the full size). I'm wondering what 
> representation clauses could help me, if any. Also, I've my doubts about 
> streams, because in the past I've tried them to write files and in Gnat 
> I couldn't manage to output exact sized types except uncheckedconverting 
> them into byte arrays. For example an integer subtype with 
> representation clauses for 16 bits was always dumped as its base type, 
> i.e. 32 bits. Frankly, the RM is too dense for me so some beginner 
> documentation would be very useful.
> 
> I've read some tutorials and books from adapower lately but can't find 
> something more on the subject. Clues welcomed.
> 
> TIA,
> 
> -- 
> -------------------------
> Jano
> 402450[at]cepsz.unizar.es
> -------------------------
> _______________________________________________
> comp.lang.ada mailing list
> comp.lang.ada@ada.eu.org
> http://ada.eu.org/mailman/listinfo/comp.lang.ada
> 

Re: Hints for networking

[Jano]

En el mensaje <mailman.4.1044048101.3911.comp.lang.ada@ada.eu.org>, 
david.c.hoos.sr@ada95.com dice...

Hello David,
 
>     2.  You need to write stream attributes for multi-octet numbers
>          that respect the bendiness of your platform.  Even when
>          you've been told that "this only has to run on Sparc," sooner
>          or later your code will also need to run on Intel, so I've
>          always taken the trouble to deal with bendiness.

I'm going to work on Intel but the protocol is using big endian fields, 
so I was already concerned about that. The only related thing I've found 
is the 'bit_order and system.default_bit_order, but I suspect that's 
related to storage representation and not to real byte ordering. 

Must I detect machine endingness manually? Are there helping packages 
for networking in the wild?

>    3.  All objects written to a stream must have a 'Size that is an
>         integer multiple of 8.  So, if you have an octet with two four-bit
>         fields, I make them into a record type, and write the stream
>         attributes for that record.  I've done things like a record that
>         has a 31-bit field, and a 1-bit field.  

I think that will not be a problem. I don't foresee streams of bits, all 
I'll need is 8-bit multiple types.

So to conclude: I must define my "atomic" types and implement their own 
stream input/output attributes. Then construct the more complex packets 
using them.

> Happy streaming!

Thanks to all who answered!

-- 
-------------------------
Jano
402450[at]cepsz.unizar.es
-------------------------

Re: Hints for networking

[Simon Wright]

Jano <402450@cepsz.unizar.es> writes:

> Must I detect machine endingness manually? Are there helping
> packages for networking in the wild?

I think that there are theoretical circumstances in which it is not OK
to check whether system.default_bit_order is low_order_first
(little-endian) or high_order_first (big-endian), but that has worked
for me on Sparc, Alpha, PowerPC, Intel (GNAT in all cases).

Re: Hints for networking

[Alfred Hilscher]

Jano schrieb:
> 
> Hello everybody.
> 
> I'm about to program a networking package for the first time in Ada and
> am somewhat undecided the way to go. I'd like to hear some pointers for
> examples or hints.

Have a look at http://alfred.hilscher.bei.t-online.de/sophi.zip