[erlang-questions] Is ei_xreceive_msg() thread safe?

[Serge Aleynikov]

e
i_send/receive family of functions are not thread safe, and receive
functions do handle heartbeats internally.  The way the
ei_{connect,send,receive} functions are written it's not possible to do any
non-blocking I/O with them or use them in multi-threaded code in the manner
you described without modifying the functions.

You can take a look at the alternative C++ library that doesn't have such
limitations:
https://github.com/saleyn/eixx
, and offers almost all functionality that ei has.

Regards,

Serge


On Tue, Aug 27, 2013 at 3:30 AM, Peter Membrey <peter@REDACTED> wrote:

> Hi guys,
>
> I've got a fairly basic C Node set up where I have the main thread running
> in a loop with ei_xreceive_msg() and a number of "callback" threads that
> execute functions and write data using ei_send() to the shared socket
> (connecting to the Erlang node).
>
> Originally I had a lot of data corruption (the Erlang node crashing due to
> corrupt data) because of incorrect locking on socket writes. I added
> mutexes to the ei_send() calls and this problem seemed to go away.
>
> However I've had a couple of occasions where the system has been quiet and
> then suddenly become busy where corrupt data has still been sent to the
> Erlang node. I'm positive all the places where I do ei_send() are
> protected, but that got me wondering about ei_xreceive_msg().
>
> From what I can find, ei_xreceive_msg() automatically handles heartbeats
> for you and I guess that means it will send some sort of reply on that
> socket. If the heart beat is being sent at the same time as some other
> process tries to write to the socket, is it possible that the two could get
> interleaved or something? I would honestly have thought it unlikely but I'm
> running out of ideas.
>
> Assuming it's possible, how could I add a mutex in this case? The call
> itself blocks, so I can't wrap the whole call in the mutex else nothing
> else will be able to send data, and there's no way to pass a mutex into the
> call itself. So as far as I can tell, there's no way to protect these
> writes and prevent them from getting mixed up with other writes on that
> socket.
>
> Does anyone have any ideas? I'm quite willing to accept I could be doing
> something pretty stupid, but I'm really out of ideas as to what that might
> be...
>
> Thanks in advance!
>
> Kind Regards,
>
> Peter Membrey
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