zmq_msg_get() could be extended to give either the refcount or an indicator on whether a message was share; based on other refcounted designs I'm hesitant to promote surfacing the actual count. Similarly, zmq_msg_set() could allow 'unsharing' by adding a ZMQ_SHARED property #define and setting it's value to 0 (no effect on non-shared messages).
So the only API surface area change is an additional message property. This seems the cleanest to me. On Wednesday, July 9, 2014, Goswin von Brederlow <[email protected]> wrote: > 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 > _______________________________________________ > zeromq-dev mailing list > [email protected] <javascript:;> > http://lists.zeromq.org/mailman/listinfo/zeromq-dev >
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