[Haskell-cafe] Behavior of -threaded in GHC 7.4.1?

Michael Craig mkscrg at gmail.com
Tue Feb 14 21:54:12 CET 2012

Hi all,

I'm debugging an issue with multithreading and FFI calls in 7.4.1. The code
in question is the zeromq3-haskell library, which provides an FFI binding
to ZeroMQ.

A little background: ZeroMQ gives the programmer "contexts" and "sockets".
Contexts are thread-safe and generally used one-per-process, but sockets
and not thread-safe so they're generally used one-per-thread.

Have a look at the code in this gist: https://gist.github.com/1829190 It's
just a server that prints requests and replies with "Hello", and a client
that sends the requests ["1", "2", ..., "10"] and prints the responses.

Despite the fact that ZMQ sockets are not thread-safe, it was possible to
compile and run this code with -threaded under 7.0.4. Using MVars or some
other locking mechanism, it was even possible to use the same socket from
multiple parallel threads. (As long as the socket was not actually used in
parallel by multiple threads, all was well.) In 7.4.1, the code will only
run if compiled without -threaded. With -threaded, it croaks with an
"operation on non-socket" error, which generally refers to a socket that's
either been closed or

I've dug through the code in zeromq3-haskell and haven't found anything
suspicious looking. Before I go digging into libzmq itself, can somebody
assure me that nothing has changed in 7.4.1 with regard to -threaded or the
FFI that might cause this breakage?

Mike Craig
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