Re: TCP/IP Sockets with GNAT.Sockets

[Adrien Plisson <aplisson-news@stochastique.net>]

fabio de francesco wrote:
> Hello,

hello !

> Today is the first time I try to use sockets in Ada programs, even if I
> have some practical knowledge of programming with TCP/IP sockets in C.
> Because I didn't find any better tutorial on working with GNAT.Sockets
> I am relying on comments in file g-socket.ads from GCC/GNAT sources
> where there is not a complete and detailed coverage of the subject.
> 
> (1) Do you know any better tutorial?

personnally, no. the GNAT reference manual says:
"The chapter here simply gives a brief summary of the facilities 
available. The full documentation is found in the spec file for the 
package. The full sources of these library packages, including both 
spec and body, are provided with all GNAT releases."

so, you can rely on the comments in g-socket.ads (which contains some 
full examples). also, the behavior is identical to other sockets 
implementation (C, Python, or others) so don't hesitate to read 
manuals for other languages.

the only big difference is in Streams, which is Ada specific...

[...]
> I hadn't too many problems with exchanging messages and everything
> works. The problem is that I was only able to use Streams for
> communicating between them and I can't understand at all how to use
> Receive_Socket() and Send_Socket() because they use
> "Stream_Element_Array" and "Stream_Element _Offset" in place of
> Strings. I know that by using Streams they can't communicate with C
> programs and I've read that Streams are everything but efficient.

streams are not specially inefficient (to the contrary, my dear). the 
problem is that many people do not clearly understand them !

so let's explain Ada streams:

streams are defined as a tagged record in Ada.Streams, which is to be 
derived in order to implement stream specific behavior: how to send or 
receive data from stream. Streams define 2 procedures for reading and 
writing data onto them. on this side of streams, data are represented 
as Stream_Element grouped in Stream_Element_Array. a single 
Stream_Element may represent a byte you send over the network for 
example.

at this end of streams, there is no data type: what is important is 
the type of streams you manipulate.

on the other end, you are concerned with data types you read from or 
write to stream, but not with the type of streams. on this end, every 
streams are equal, may it be file, socket, or anything else. this is 
achieved through stream oriented attributes: 'Read, 'Write, 'Input and 
'Output. those attributes allow you to read or write a specific data 
type from/to a stream. for example: Integer'Input( S ) will read an 
integer from the stream S (whatever stream type S is). the behavior of 
those attribute may be overriden through an attribute definition clause.

at this end of streams, there is no stream type: what is important is 
the type of data you manipulate.

in fact, stream attributes are kind of double-dispatching 
operations... at the light of this explaination, you should re-read 
ARM95 13.13, and may better understand them.

so, now you understand this, you may ask yourself if you really need 
to use Receive_Socket() or Send_Socket(): they are intended to 
programmers writing their own stream attributes... (note that this 
could be useful to communicate with a C program through socket streams).

i hope this explaination clarified things a bit and was useful to you 
(or others)... language lawyers will surely push it a little bit further.

-- 
rien