On Thu, 2017-08-31 at 16:16 +0200, Stephan Opfer wrote: > Hi all, > > I am trying to use zmq_msg_init_data in order to have the zero-copy > advantage. > > Now it is required, that I either pass a function, that can > deallocate the data after zmq sent it, or I pass NULL in order to > keep the ownership. > > In the latter case: How do I know that zmq has done his work and I > can deallocate the data?
Locally, the only sure way is when the socket has been closed and the context has been terminated. But that of course is not ideal in a long- lived application. If your protocol implements ACKS (request-reply pattern for example), then it's implicit that when receiving a response the original request was received. > Another idea was to pass a lambda function (with a capture) to zmq, > in order to make zmq send a signal by calling the lambda function. > The problem is, that it is not allowed to pass lambda functions with > captures as function pointers and without captures, I have no clue > how zmq could pass the signal. > > Maybe somebody of you now already have an idea? > > The general idea is: > > My robotic application keeps some sensor values for a while and sends > some of the values to other robots via zmq. Therefore I don't want > the data to be copied and I don't want zmq to deallocate it. I want > my own application to deallocate it, if it is not needed anymore and > zmq did send everything. > > Greetings, > > Stephan A solution could be to make a shallow-copy of the message via zmq_msg_copy. This will not copy the data buffer, only the message metadata and references, which are a few bytes at most. The data buffer ownership is refcounted. Then when your application is done with the data, you can call zmq_msg_close on the _copy_, which will decrease the refcount. So the last one to be finished with the data, whether it's your application or the library, will deallocate it. -- Kind regards, Luca Boccassi
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