I am trying to understand the threading model of ZeroMQ.
According to their white paper http://zeromq.org/whitepapers:architecture#toc3, each I/O thread that the context is created with maps directly to an OS thread. What I don't understand is why two background threads are spawned by this minimal program:
#include <zmq.hpp>int main() { zmq::context_t context{1}; zmq::socket_t socket(context, ZMQ_SUB); socket.setsockopt(ZMQ_SUBSCRIBE, "", 0); socket.connect("tcp://localhost:5555");}As evidenced by the following gdb output, the line that creates the socket spawns two background threads.
Breakpoint 3, 0x00007f3cbebfbc80 in clone () from /lib64/libc.so.6(gdb) bt#0 0x00007f3cbebfbc80 in clone () from /lib64/libc.so.6#1 0x00007f3cbdff7f7a in do_clone.constprop () from /lib64/libpthread.so.0#2 0x00007f3cbdff9469 in pthread_create@@GLIBC_2.2.5 () from /lib64/libpthread.so.0#3 0x00007f3cbf74547d in zmq::thread_t::start(void (*)(void*), void*) () from /lib64/libzmq.so.5#4 0x00007f3cbf70044a in zmq::ctx_t::start_thread(zmq::thread_t&, void (*)(void*), void*) const () from /lib64/libzmq.so.5#5 0x00007f3cbf7012c1 in zmq::ctx_t::create_socket(int) () from /lib64/libzmq.so.5#6 0x0000000000401251 in zmq::socket_t::init (this=0x7ffd5d0323f0, context_=..., type_=2) at /usr/include/zmq.hpp:649#7 0x00000000004010a1 in zmq::socket_t::socket_t (this=0x7ffd5d0323f0, context_=..., type_=2) at /usr/include/zmq.hpp:463#8 0x0000000000400e2f in main () at sub.cpp:5(gdb) cContinuing.[New Thread 0x7f3cbc7e6700 (LWP 15499)]Breakpoint 3, 0x00007f3cbebfbc80 in clone () from /lib64/libc.so.6(gdb) bt#0 0x00007f3cbebfbc80 in clone () from /lib64/libc.so.6#1 0x00007f3cbdff7f7a in do_clone.constprop () from /lib64/libpthread.so.0#2 0x00007f3cbdff9469 in pthread_create@@GLIBC_2.2.5 () from /lib64/libpthread.so.0#3 0x00007f3cbf74547d in zmq::thread_t::start(void (*)(void*), void*) () from /lib64/libzmq.so.5#4 0x00007f3cbf70044a in zmq::ctx_t::start_thread(zmq::thread_t&, void (*)(void*), void*) const () from /lib64/libzmq.so.5#5 0x00007f3cbf70135f in zmq::ctx_t::create_socket(int) () from /lib64/libzmq.so.5#6 0x0000000000401251 in zmq::socket_t::init (this=0x7ffd5d0323f0, context_=..., type_=2) at /usr/include/zmq.hpp:649#7 0x00000000004010a1 in zmq::socket_t::socket_t (this=0x7ffd5d0323f0, context_=..., type_=2) at /usr/include/zmq.hpp:463#8 0x0000000000400e2f in main () at sub.cpp:5(gdb) cContinuing.[New Thread 0x7f3cbbfe5700 (LWP 15500)]Breakpoint 4, main () at sub.cpp:66 socket.setsockopt(ZMQ_SUBSCRIBE, "", 0);(gdb) info thread Id Target Id Frame 3 Thread 0x7f3cbbfe5700 (LWP 15500) "a" 0x00007f3cbebfc2c3 in epoll_wait () from /lib64/libc.so.6 2 Thread 0x7f3cbc7e6700 (LWP 15499) "a" 0x00007f3cbebfc2c3 in epoll_wait () from /lib64/libc.so.6* 1 Thread 0x7f3cbfb4e840 (LWP 15498) "a" main () at sub.cpp:6(gdb)Using ZeroMQ and cppzmq-devel 4.1.4
Make with: g++ -std=c++11 -g sub.cpp -lzmq
