Subject: Re: SDL-News: Modeling message loss in SDL
From: Rick Reed TSE (rickreed#tseng.co.uk)
Date: Wed May 28 2003 - 21:48:32 GMT
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-----From Rick Reed TSE <rickreed#tseng.co.uk> to sdlnews -----
Cris Fuhrman at christopher.fuhrman#etsmtl.ca wrote on 28/05/2003 21:35:
> In SDL, is there a way to specify the behavior of a process that simply
> repeats a signal from one route to another? That is, whatever is received on
> route A is repeated on route B, without having to go into the details of the
> signal name and its possible parameters? I have seen the use of a CHOICE
> proposed in Laurent Doldi¹s book SDL Illustrated as a way to have an
> abstraction of a set of signals. This approach is surely better than treating
> each message, but it is still not completely generic and requires defining
> types for each message. One cannot easily reuse the ³network medium² block for
> other applications without redefining the CHOICE and the types.
There is no shorthand way of doing what you want.
You can use an INPUT * to consume any signal (not explicitly mentioned -
which is all signals if none are explicitly mentioned), but
a) in this case the parameters are lost;
b) there is no mechanism for finding out what signal was received.
So the only possibility in SDL at present is to explicitly input each signal
by identity and storing the parameters (if any) in variables followed by an
output using the same identity obtaining the parameter values from
The need to determine the identity of the signal type of the last signal
received and to be able to output this signal without modification has been
recognised for some time and proposals have been discussed by the language
expert group, but so far no feature has been agreed.
So I think the best that can be done with SDL is as suggested by Laurent in
SDL Illustrated - collapse all the signals into one signal type with a
CHOICE data type to distinguish the different cases. The disadvantages of
this is that it is then not possible to save specific messages (except
internally in a process), and determining the actual message received from
the network medium will be a decision rather than an input.
The ideal for a layered protocol would be at the upper layer to treat the
signals as distinct and at the lower layer to treat than all as one signal
type. Unfortunately SDL does not support this (at present).
-- Rick Reed - rickreed#tseng.co.uk Tel:+44 15394 88462 Mob.:+44 7970 50 96 50
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