win32 socket, "Operation not supported"

[David Konerding]

Hello,

I have written an app which opens a TCP connection to a server and uses a protocol to communicate with it.
Specifically, I've written a python IMD client for the molecular dynamics application 'NAMD' (do searches for IMD and NAMD if you want to learn more).

The protocol is very simple: both ends of the TCP connection can send messages to the other side at any time; when data is
available on a read socket, a full packet (with known size) is read in a blocking fashion from the socket.

This is used, on the client side, to make requests of the molecular dynamics engine (such as "speed up", or "slow down", or "quit").
The server side periodically sends updates on the state of the molecular dynamics engine (current step, atomic coords, etc).

This all works fine on linux: I can connect, and send and receive messages as necessary.  

However, on Windows, I can connect and complete the initial handshake, and receive periodic updates, but when I try to send a message
to the server, socket.send gets an "Operation not supported".

>From my reading of the win32 docs, the socket send call returns this in a couple of situations:
when a two-way socket had one half shut down and the send would have used the shut down direction, or
when an out of band message was sent on a socket type which does not support it (such as UDP).

Neither of these situations are true.  When I re-code the app in C, the problem doesn't occur.  Anybody got some suggestions?
(PS: the socket I/O is set to blocking, and the Nagle algorithm is disabled).

Dave