ZeroMQ handling interrupt in multithreaded application - c++

Question

Graceful exit in ZeroMQ in multithreaded environment
Specs : ubuntu 16.04 with c++11,libzmq : 4.2.3
Sample code
static int s_interrupted = 0;
static void s_signal_handler (int signal_value)
{
s_interrupted = 1;
//some code which will tell main thread to exit
}
static void s_catch_signals (void)
{
struct sigaction action;
action.sa_handler = s_signal_handler;
action.sa_flags = 0;
sigemptyset (&action.sa_mask);
sigaction (SIGINT, &action, NULL);
sigaction (SIGTERM, &action, NULL);
}
static void Thread((zsock_t *pipe, void *)
{
zmq::context_t context(1);
zmq::socket_t requester1(context,ZMQ_DEALER);
zmq::socket_t requester2(context,ZMQ_DEALER);
requester1.connect(address1);
requester2.connect(address2);
zmq_pollitem_t items []=
{{requester1,0,ZMQ_POLLIN,0},
{requester2,0,ZMQ_POLLIN,0}};
while(true)
{
zmq::message_t message;
zmq::poll (items, 2, -1);
if (items [0].revents & ZMQ_POLLIN)
{
requester1.recv(&message);
}
if (items [1].revents & ZMQ_POLLIN)
{
requester2.recv(&message);
}
}
}
int main()
{
.
//some code
.
zactor_t *actor = zactor_new (Threaded, nullptr);
s_catch_signals();
.
//continue
.
//wait till thread finishes to exit
return 0;
}
Now when the interrupt occurs it will call the signal handler from the main thread. I somehow need to tell the thread (poller) to exit from the signal handler. Any ideas how to achieve this?

Answer 1

From ZMQ documentation you have 2 "idiomatic" way of dealing with this :
Polling on a pipe, and writing on the pipe in the signal handler.
Catching exception thrown in recv when a signal is sent.
After testing it, seems that zmq::poll does not throw an exception on SIGINT.
Therefore the solution seem to be to use a socket dedicated to closing.
The solution looks like this :
#include <iostream>
#include <thread>
#include <signal.h>
#include <zmq.hpp>
zmq::context_t* ctx;
static void s_signal_handler (int signal_value)
{
std::cout << "Signal received" << std::endl;
zmq::socket_t stop_socket(*ctx, ZMQ_PAIR);
stop_socket.connect("inproc://stop_address");
zmq::message_t msg("0", 1);
stop_socket.send(msg);
std::cout << "end sighandler" << std::endl;
}
static void s_catch_signals (void)
{
struct sigaction action;
action.sa_handler = s_signal_handler;
action.sa_flags = 0;
sigemptyset (&action.sa_mask);
sigaction (SIGINT, &action, NULL);
sigaction (SIGTERM, &action, NULL);
}
void thread(void)
{
std::cout << "Thread Begin" << std::endl;
zmq::context_t context (1);
ctx = &context;
zmq::socket_t requester1(context,ZMQ_DEALER);
zmq::socket_t requester2(context,ZMQ_DEALER);
zmq::socket_t stop_socket(context, ZMQ_PAIR);
requester1.connect("tcp://127.0.0.1:36483");
requester2.connect("tcp://127.0.0.1:36483");
stop_socket.bind("inproc://stop_address");
zmq_pollitem_t items []=
{
{requester1,0,ZMQ_POLLIN,0},
{requester2,0,ZMQ_POLLIN,0},
{stop_socket,0,ZMQ_POLLIN,0}
};
while ( true )
{
// Blocking read will throw on a signal
int rc = 0;
std::cout << "Polling" << std::endl;
rc = zmq::poll (items, 3, -1);
zmq::message_t message;
if(rc > 0)
{
if (items [0].revents & ZMQ_POLLIN)
{
requester1.recv(&message);
}
if (items [1].revents & ZMQ_POLLIN)
{
requester2.recv(&message);
}
if(items [2].revents & ZMQ_POLLIN)
{
std::cout << "message stop received " << std::endl;
break;
}
}
}
requester1.setsockopt(ZMQ_LINGER, 0);
requester2.setsockopt(ZMQ_LINGER, 0);
stop_socket.setsockopt(ZMQ_LINGER, 0);
requester1.close();
requester2.close();
stop_socket.close();
std::cout << "Thread end" << std::endl;
}
int main(void)
{
std::cout << "Begin" << std::endl;
s_catch_signals ();
zmq::context_t context (1);
zmq::socket_t router(context,ZMQ_ROUTER);
router.bind("tcp://127.0.0.1:36483");
std::thread t(&thread);
t.join();
std::cout << "end join" << std::endl;
}
Note that if you do not want to share the context to the signal handler you could use "ipc://..." .

Answer 2

If you wish to preserve the feel of ZMQ's Actor model in handling signals, you could use the signalfd interface on Linux: signalfd manpage. That way you could use zmq poll to wait for the signal to be delivered, instead of having a signal handler.
It has the added advantage that when handling a signal delivered through a file descriptor, you can call any function you like, because you're handling it synchronously, not asynchronously.