Re: Ada x <whatever> Datagram Sockets

[Egil H H <ehh.public@gmail.com>]

On Thursday, February 7, 2019 at 7:41:28 AM UTC+1, Rego, P. wrote:
> On Thursday, February 7, 2019 at 4:00:11 AM UTC-2, Egil H H wrote:
> > > > On Wednesday, February 6, 2019 at 6:10:37 PM UTC-5, Rego, P. wrote:
> > > 
> > > Just the string "test". I also confirmed that the MQL container (uchar data[]) has really the size 5.
> > > 
> > 
> > Right, Ada strings are not zero-terminated, and the size of the string needs 
> > to come from somewhere, which is why String'Input will interpret the first x bits (probably 32, but may depend on your platform) as the length. So in this
> > case, "test" becomes a big number and 'Input will most likely try to allocate
> > just under 2GB  on your stack (and probably at least twice, in order to copy
> > the result into your string).
> A C++ uchar in Windows machine is usually 1 byte, so assuming that both use the same signature, both messages (from C++ and from Ada) should be of same size, right? 
> 

The *message* may be of the same size, but that doesn't matter, as long as
it's interpreted differently in C++ vs Ada.
C++ apparently puts an extra character in your message (you said "test"
was length 5...)
Ada interprets your string (presumably the first 4 bytes, which is usually
the size of Positive/Integer) as a very big number.

> > 
> > Don't use Ada stream attributes to communicate with other languages.
> > 
> > And, as some will tell you, don't use them at all, especially for arrays, as
> > each array element may end up being transferred as a single data packet, destroying performance.
> 
> This is quite controversial, in this case. I'd usually agree with you, however the use of Ada stream is not by choice. The whole GNAT Ada package uses Ada streams, and actually the above code was entirely extracted from the comments from g-sockets.ads with minimum modification. I don't think it would be wise to re-implement Ada Sockets pkg to be more efficient.


How is it controversial? The example works fine when the server and client
are both written in Ada, as they would then agree on the protocol.
But when dealing with cross-language communication, you're better off using
Send_Socket/Receive_Socket directly. No need to re-implement

-- 
~egilhh