is send() from the multipart_t intentionally broken? about cppzmq HOT 1 OPEN

Hi there - I ran across this comment just looking for some good examples of using zmq_addon.h for multipart messaging. I am not a 0MQ expert, but I've done plenty of sockets programming and have been steeping in the 0MQ guide.

From what I understand, I believe this is not actually an issue... because the underlying calls to socket.send() with the ZMQ_SENDMORE flag indicated do not actually send any data yet. Only at the final call, in which the "more" flag is deasserted, is the entire multipart message atomically sent to the underlying socket.

A cool side-effect of this, which I'm fairly certain is correct (I've inferred this myself, but I've built network interface and DMA logic for FPGAs, and had to write Linux drivers for them) is this: because of the way this works, when used in conjunction with zero-copy tactics, messages are sent down to the OS's network stack in such a way that, if the actual network adapter hardware has SGDMA at its disposal, it can implement true zero-copy. That is, all the various multiple parts of the message are gathered from their disparate memory locations by DMA at the point in which the packet is actually being enqueued for presentation to the MAC layer... super efficient.

Feel free to poke holes, anyone. Just my two cents.

• Eldridge

from cppzmq.