网络&线程模型
zmq封装了select，poll，epoll，queue，kqueue等各个平台上基础的网络模型，但是在windows上没有封装IOCP模型，而是使用select，这对zmq在windows上会造成一些性能影响，毕竟select模型的性能相对较低。虽然这些模型的原理和操作都不相同，但是zmq封装了这些差异，抽象出统一的实现接口，下面我们以linux下的epoll模型为例进行分析，zmq中封装epoll模型的类为epoll_t，他继承自poller_base_t：
class poller_base_t
{
public:
poller_base_t ();
virtual ~poller_base_t ();
// Returns load of the poller. Note that this function can be
// invoked from a different thread!
int get_load ();
// Add a timeout to expire in timeout_ milliseconds. After the
// expiration timer_event on sink_ object will be called with
// argument set to id_.
void add_timer (int timeout_, zmq::i_poll_events *sink_, int id_);
// Cancel the timer created by sink_ object with ID equal to id_.
void cancel_timer (zmq::i_poll_events *sink_, int id_);
protected:
// Called by individual poller implementations to manage the load.
void adjust_load (int amount_);
// Executes any timers that are due. Returns number of milliseconds
// to wait to match the next timer or 0 meaning "no timers".
uint64_t execute_timers ();
private:
// Clock instance private to this I/O thread.
clock_t clock;
// List of active timers.
struct timer_info_t
{
zmq::i_poll_events *sink;
int id;
};
typedef std::multimap <uint64_t, timer_info_t> timers_t;
timers_t timers;
// Load of the poller. Currently the number of file descriptors
// registered.
atomic_counter_t load;
poller_base_t (const poller_base_t&);
const poller_base_t &operator = (const poller_base_t&);
};poller_base主要是封装定时器操作，他用一个multimap来记录所有的timer，在execute_timers从头开始遍历timer看是否有到时间的定时器，由于multimap是有序的，所以每次遍历检查到第一个没有到达的定时器之后就可以停止遍历了。
class epoll_t : public poller_base_t
{
public:
typedef void* handle_t;
epoll_t (const ctx_t &ctx_);
~epoll_t ();
// "poller" concept.
handle_t add_fd (fd_t fd_, zmq::i_poll_events *events_);
void rm_fd (handle_t handle_);
void set_pollin (handle_t handle_);
void reset_pollin (handle_t handle_);
void set_pollout (handle_t handle_);
void reset_pollout (handle_t handle_);
void start ();
void stop ();
static int max_fds ();
private:
// Main worker thread routine.
static void worker_routine (void *arg_);
// Main event loop.
void loop ();
// Reference to ZMQ context.
const ctx_t &ctx;
// Main epoll file descriptor
fd_t epoll_fd;
struct poll_entry_t
{
fd_t fd;
epoll_event ev;
zmq::i_poll_events *events;
};
// List of retired event sources.
typedef std::vector <poll_entry_t*> retired_t;
retired_t retired;
// If true, thread is in the process of shutting down.
bool stopping;
// Handle of the physical thread doing the I/O work.
thread_t worker;
epoll_t (const epoll_t&);
const epoll_t &operator = (const epoll_t&);
};epoll_t的变量中包含一个worker作为工作线程，zmq的线程操作主要封装在thread_t中，在windows上使用原生的线程库，在linux等其他平台上是使用pthread来现实。epoll_t 定义了一个poll_entry_t结构体作为handle_t，当需要使用epoll监听一个描述符时需要首先调用add_fd方法:
zmq::epoll_t::handle_t zmq::epoll_t::add_fd (fd_t fd_, i_poll_events *events_)
{
poll_entry_t *pe = new (std::nothrow) poll_entry_t;
alloc_assert (pe);
// The memset is not actually needed. It's here to prevent debugging
// tools to complain about using uninitialised memory.
memset (pe, 0, sizeof (poll_entry_t));
pe->fd = fd_;
pe->ev.events = 0;
pe->ev.data.ptr = pe;
pe->events = events_;
int rc = epoll_ctl (epoll_fd, EPOLL_CTL_ADD, fd_, &pe->ev);
errno_assert (rc != -1);
// Increase the load metric of the thread.
adjust_load (1);
return pe;
}epoll方法的内部会构造一个poll_entry_t对象，然后将对应的描述符和epoll_event添加进epoll_fd中，之后对该描述符的所有操作都是通过handle_t来进行的：
void set_pollin (handle_t handle_);
void reset_pollin (handle_t handle_);
void set_pollout (handle_t handle_);
void reset_pollout (handle_t handle_);每次在工作线程中调用epoll_wait后都会根据描述符的具体事件调用对用的event事件，event事件是在add_fd方法内注册的，任何想要监听描述符事件的类都需要继承i_poll_events，比如之后要讲的listener（用于监听连接），conneter（检测连接），socket_base（监听mailbox中的命令）,stream_engine（监听网络数据）等。
struct i_poll_events
{
virtual ~i_poll_events () {}
// Called by I/O thread when file descriptor is ready for reading.
virtual void in_event () = 0;
// Called by I/O thread when file descriptor is ready for writing.
virtual void out_event () = 0;
// Called when timer expires.
virtual void timer_event (int id_) = 0;
};在zmq把各种网络模型typedef为poller_t，又把poller_t封装在一个io_thread中，io_thread中除了一个poller外还包含一个mailbox，mailbox的作用将在之后进行分析。下图是zmq线程模型示意图，这里先简单看一下，之后在分析socket，session时会具体分析。
