[zeromq-dev] Extending zmq_msg_t API

[Goswin von Brederlow]

On Tue, Jul 08, 2014 at 10:42:41AM -0500, Thomas Rodgers wrote:
> tl;dr; Is there any objection to adding some sort of accessor to the API to
> determine if a given zmq_msg_t is_shared()?
> 
> Background/Rationale:
> 
> Something I encountered while writing a "high level" C++ wrapper for
> zmq_msg_t and it's API is the following set of behaviors -
> 
> zmq_msg_init(&msg_vsm, 20);
> 
> Results in a type_vsm message, the body of which is held entirely within
> the space allocated to zmq_msg_t
> 
> zmq_msg_init(&msg_lmsg, 1024);
> 
> Results in a type_lmsg message, the body is held as a reference to a block
> of size bytes.
> 
> memcpy(zmq_msg_data(&msg_vsm), "VSM", 3);
> memcpy(zmq_msg_data(&msg_lmsg), "LMSG", 4);
> 
> So far so good.  Now copy -
> 
> zmq_msg_copy(&msg_vsm2, &msg_vsm);
> zmq_msg_copy(&msg_lmsg2, &msg_lmsg);
> 
> Now change contents -
> 
> memcpy(zmq_msg_data(&msg_vsm2), "vsm", 3);
> memcpy(zmq_msg_data(&msg_lmsg2), "lmsg", 4);
> 
> assert(memcmp(&msg_vsm, &msg_vsm2, 3) != 0); // ok
> assert(memcmp(&msg_lmsg, &msg_lmsg2, 4) != 0); // fail
> 
> This happens by design (lmsg's are refcounted on copy, not deep copied).
> But it results in a situation where a zmq_msg_t is sometimes a Value and
> sometimes a Reference.  This could lead to astonishment for the unwary.
> 
> >From the perspective of a wrapper (particularly one that takes a strong
> stand on value semantics and local reasoning), this behavior is ungood.  So
> my options are deep copy always or implement copy-on-write.
> 
> For efficiency I prefer the latter approach in the case of type_lmsg
> messages.  I have implemented the copy-on-write logic through a horrible
> brittle hack that examines the last byte of zmq_msg_t.  I would prefer a
> less brittle solution.

"lmsg's are refcounted on copy" Can't you access the refcount?
Or is that the API call you want to add?

Maybe instead of is_shared() an unshare() call would be more usefull,
which would copy the message payload if it is shared. Or both?

MfG
	Goswin