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From: Eli Zaretskii <eliz@gnu.org>
Newsgroups: gmane.emacs.devel
Subject: Re: datagram source port?
Date: Wed, 15 Jan 2020 18:27:38 +0200
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To: Robert Pluim <rpluim@gmail.com>
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> From: Robert Pluim <rpluim@gmail.com>
> Cc: mlang@delysid.org,  emacs-devel@gnu.org
> Date: Wed, 15 Jan 2020 10:49:33 +0100
> 
> I took another quick look at this, and noticed that the windows port
> doesn't support datagram sockets at all, although the underlying
> Winsock does. Is there a reason for that?

This comment from nt/inc/ms-w32.h explains that:

  /* And the select implementation does 1-byte read-ahead waiting
     for received packets, so datagrams are broken too.  */
  #define BROKEN_DATAGRAM_SOCKETS 1

(Let me know if you want me to explain what it means by "1-byte
read-ahead".  Or just read the large commentary that starts around
line 870 of w32proc.c.)

I don't know enough about datagram sockets, but I hope you do, in
which case the above comment ought to be the answer.

Thanks.