[zeromq-dev] Event callback questions

Lourens Naudé lourens at methodmissing.com
Sun Aug 5 14:37:38 CEST 2012

Hi Bjorn,

Great feedback - digesting and will look into it.

- Lourens

On Sun, Aug 5, 2012 at 12:48 PM, Bjorn Reese <breese at mail1.stofanet.dk>wrote:

> The documentation on zmq_ctx_set_monitor() states that one should "be
> careful with what you're doing in the callback function as excess time
> spent in the handle will block the socket's application thread". This
> does not appear to be fully correct.
> My investigation shows that the callback function can either be called
> by the application thread, e.g. for ZMQ_EVENT_LISTENING, or from a
> context thread, e.g. for ZMQ_EVENT_CONNECTED.
> It may be tempting for users to call zmq_connect() from the callback
> when we receive ZMQ_EVENT_DISCONNECTED, but this may deadlock as the
> event is called from the context thread. It might therefore be a
> good idea to insert a caveat in the documentation that the callback
> function should not call zmq functions.
> Alternative, we could construct a different callback mechanism that
> has a clear separation between application and library threads. Instead
> of a callback, we could create a monitor socket for the context, that
> is, an inproc socket to deliver the events.
> I know that this may sound like overengineering, but in my experience
> errors often occur when we use callbacks in a multithreaded program.
> Furthermore, the ZMQ_EVENT_* macros are given values that are a
> multiple of 2. Does this mean that the "event" parameter of the
> callback may contain two or more events in the same call? If not,
> why aren't the ZMQ_EVENT_* values simply consecutive numbers?
> Oh, and while I am at it, why is the "event" parameter an "int"
> rather than an "unsigned int"?
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