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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: Ada x <whatever> Datagram Sockets
Date: Thu, 07 Feb 2019 10:08:48 +0000
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Xref: reader01.eternal-september.org comp.lang.ada:55456
Date: 2019-02-07T10:08:48+00:00
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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:

> Well, using streams directly with UDP is calling for trouble (and
> highly non-portable too). You should probably never use streams with
> TCP either. Certainly not if socket options like NO_DELAY are
> planned.

The reason for this is to be found in ARM 13.13.2(9)[1] for 'Write:

   For composite types, the Write or Read attribute for each component
   is called in canonical order

which will result in multiple calls to the underlying OS services; bad
for TCP_NODELAY, very very bad for UDP.

See GCC PR9535[2]. Note, the change applied really doesn't address the
problem at all, in spite of being essentially my patch! I lost interest,
having already decided to do as Dmitry says ...

> Anyway, the way to do it is to have an outgoing packet buffer of
> Stream_Element_Array. You put data there and then send all buffer to
> the socket (write to the file etc), in one piece.
>
> If you want to use streams. Put a memory-mapped stream on top of the
> Stream_Element_Array. Write packet whole. Flush the buffer contents to
> the socket/file.

... see, in particular, comments 2, 6 in [2].

[1] http://www.ada-auth.org/standards/rm12_w_tc1/html/RM-13-13-2.html#p9
[2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=9535