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 zeromq-dev@lists.zeromq.org http://lists.zeromq.org/mailman/listinfo/zeromq-dev