I try to send a few small (8 byte) messages through inproc pair socket. However after a few tries zmq_send() blocks. When I stop debugger I see following stack trace:

libc.so.6!__GI___poll(struct pollfd * fds, nfds_t nfds, int timeout) 
libzmq.so.5!poll(int __timeout, nfds_t __nfds, pollfd * __fds) 
libzmq.so.5!zmq::signaler_t::wait(zmq::signaler_t * const this, int timeout_) 
libzmq.so.5!zmq::mailbox_t::recv(zmq::mailbox_t * const this, zmq::command_t * cmd_, int timeout_) 
libzmq.so.5!zmq::socket_base_t::process_commands(zmq::socket_base_t * const this, int timeout_, bool throttle_) 
libzmq.so.5!zmq::socket_base_t::send(zmq::socket_base_t * const this, zmq::msg_t * msg_, int flags_) 
libzmq.so.5!s_sendmsg(int flags_, zmq_msg_t * msg_, zmq::socket_base_t * s_) 
libzmq.so.5!zmq_send(void * s_, const void * buf_, size_t len_, int flags_)

Why does it happen? What commands does ZMQ try to to process? Why does it call recv()? Is it because of high water mark? I suppose it's something different because I send small amount of data and water mark shouldn't be reached yet. And if water mark is the only explanation then how can I measure it?

CodePudding user response：

Generally, this will happen when the socket reaches some sort of limit on outstanding sent data; it will then block waiting for an acknowledgement message allowing it to continue.

The precise behavior depends on the zmq_socket type as well as any other configuration you have done on it (setting the high-water mark, for example).

CodePudding user response：

It turned out it was bug in logic of my code. After sending some messages, next were sent by different thread using different socket. That socket was not connected so zmq_send() was waiting for connection to be established. This is why its pipe was NULL. Thanks everybody for help.