1、ev_loop是libev用来描述事件循环的结构体。在libev中的定义比较绕，这里把它摘抄出来，做下注释，方便学习。libev的定义如下
struct ev_loop
{
ev_tstamp ev_rt_now;
#define ev_rt_now ((loop)->ev_rt_now)
#define VAR(name,decl) decl;
#include "ev_vars.h"
#undef VAR
};
#include "ev_wrap.h"
static struct ev_loop default_loop_struct;
EV_API_DECL struct ev_loop *ev_default_loop_ptr = 0;
ev_tstamp 是double类型，是一个时间事件的计数类型，这里使用define来定义ev_rt_now为((loop)->ev_rt_now)，程序里可以直接使用ev_rt_now，后边结构体的定义都是这样，这可能是作者的一些编程技巧。
#define VAR(name,decl) decl; #define VARx(type,name) VAR(name, type name)
这两行表示了一个意思，用type来定义name，就是等于 type name；如 int backend；相当于在结构体中定义了一个变量。具体变量在ev_vars.h头文件中定义。
ev_warp.h文件的作用和#define ev_rt_now ((loop)->ev_rt_now) 相同。具体两个文件代码如下：
#define VARx(type,name) VAR(name, type name)
VARx(ev_tstamp, now_floor) /* last time we refreshed rt_time */
VARx(ev_tstamp, mn_now) /* monotonic clock "now" */
VARx(ev_tstamp, rtmn_diff) /* difference realtime - monotonic time */
/* for reverse feeding of events */
VARx(W *, rfeeds)
VARx(int, rfeedmax)
VARx(int, rfeedcnt)
VAR (pendings, ANPENDING *pendings [NUMPRI])
VAR (pendingmax, int pendingmax [NUMPRI])
VAR (pendingcnt, int pendingcnt [NUMPRI])
VARx(int, pendingpri) /* highest priority currently pending */
VARx(ev_prepare, pending_w) /* dummy pending watcher */
VARx(ev_tstamp, io_blocktime)
VARx(ev_tstamp, timeout_blocktime)
VARx(int, backend)
VARx(int, activecnt) /* total number of active events ("refcount") */
VARx(EV_ATOMIC_T, loop_done) /* signal by ev_break */
VARx(int, backend_fd)
VARx(ev_tstamp, backend_mintime) /* assumed typical timer resolution */
VAR (backend_modify, void (*backend_modify)(EV_P_ int fd, int oev, int nev))
VAR (backend_poll , void (*backend_poll)(EV_P_ ev_tstamp timeout))
VARx(ANFD *, anfds)
VARx(int, anfdmax)
VAR (evpipe, int evpipe [2])
VARx(ev_io, pipe_w)
VARx(EV_ATOMIC_T, pipe_write_wanted)
VARx(EV_ATOMIC_T, pipe_write_skipped)
#if !defined(_WIN32) || EV_GENWRAP
VARx(pid_t, curpid)
#endif
VARx(char, postfork) /* true if we need to recreate kernel state after fork */
#if EV_USE_SELECT || EV_GENWRAP
VARx(void *, vec_ri)
VARx(void *, vec_ro)
VARx(void *, vec_wi)
VARx(void *, vec_wo)
#if defined(_WIN32) || EV_GENWRAP
VARx(void *, vec_eo)
#endif
VARx(int, vec_max)
#endif
#if EV_USE_POLL || EV_GENWRAP
VARx(struct pollfd *, polls)
VARx(int, pollmax)
VARx(int, pollcnt)
VARx(int *, pollidxs) /* maps fds into structure indices */
VARx(int, pollidxmax)
#endif
#if EV_USE_EPOLL || EV_GENWRAP
VARx(struct epoll_event *, epoll_events)
VARx(int, epoll_eventmax)
VARx(int *, epoll_eperms)
VARx(int, epoll_epermcnt)
VARx(int, epoll_epermmax)
#endif
#if EV_USE_KQUEUE || EV_GENWRAP
VARx(pid_t, kqueue_fd_pid)
VARx(struct kevent *, kqueue_changes)
VARx(int, kqueue_changemax)
VARx(int, kqueue_changecnt)
VARx(struct kevent *, kqueue_events)
VARx(int, kqueue_eventmax)
#endif
#if EV_USE_PORT || EV_GENWRAP
VARx(struct port_event *, port_events)
VARx(int, port_eventmax)
#endif
#if EV_USE_IOCP || EV_GENWRAP
VARx(HANDLE, iocp)
#endif
VARx(int *, fdchanges)
VARx(int, fdchangemax)
VARx(int, fdchangecnt)
VARx(ANHE *, timers)
VARx(int, timermax)
VARx(int, timercnt)
#if EV_PERIODIC_ENABLE || EV_GENWRAP
VARx(ANHE *, periodics)
VARx(int, periodicmax)
VARx(int, periodiccnt)
#endif
#if EV_IDLE_ENABLE || EV_GENWRAP
VAR (idles, ev_idle **idles [NUMPRI])
VAR (idlemax, int idlemax [NUMPRI])
VAR (idlecnt, int idlecnt [NUMPRI])
#endif
VARx(int, idleall) /* total number */
VARx(struct ev_prepare **, prepares)
VARx(int, preparemax)
VARx(int, preparecnt)
VARx(struct ev_check **, checks)
VARx(int, checkmax)
VARx(int, checkcnt)
#if EV_FORK_ENABLE || EV_GENWRAP
VARx(struct ev_fork **, forks)
VARx(int, forkmax)
VARx(int, forkcnt)
#endif
#if EV_CLEANUP_ENABLE || EV_GENWRAP
VARx(struct ev_cleanup **, cleanups)
VARx(int, cleanupmax)
VARx(int, cleanupcnt)
#endif
#if EV_ASYNC_ENABLE || EV_GENWRAP
VARx(EV_ATOMIC_T, async_pending)
VARx(struct ev_async **, asyncs)
VARx(int, asyncmax)
VARx(int, asynccnt)
#endif
#if EV_USE_INOTIFY || EV_GENWRAP
VARx(int, fs_fd)
VARx(ev_io, fs_w)
VARx(char, fs_2625) /* whether we are running in linux 2.6.25 or newer */
VAR (fs_hash, ANFS fs_hash [EV_INOTIFY_HASHSIZE])
#endif
VARx(EV_ATOMIC_T, sig_pending)
#if EV_USE_SIGNALFD || EV_GENWRAP
VARx(int, sigfd)
VARx(ev_io, sigfd_w)
VARx(sigset_t, sigfd_set)
#endif
VARx(unsigned int, origflags) /* original loop flags */
#if EV_FEATURE_API || EV_GENWRAP
VARx(unsigned int, loop_count) /* total number of loop iterations/blocks */
VARx(unsigned int, loop_depth) /* #ev_run enters - #ev_run leaves */
VARx(void *, userdata)
/* C++ doesn't support the ev_loop_callback typedef here. stinks. */
VAR (release_cb, void (*release_cb)(EV_P) EV_THROW)
VAR (acquire_cb, void (*acquire_cb)(EV_P) EV_THROW)
VAR (invoke_cb , ev_loop_callback invoke_cb)
#endif
#undef VARx
#define acquire_cb ((loop)->acquire_cb) #define activecnt ((loop)->activecnt) #define anfdmax ((loop)->anfdmax) #define anfds ((loop)->anfds) #define async_pending ((loop)->async_pending) #define asynccnt ((loop)->asynccnt) #define asyncmax ((loop)->asyncmax) #define asyncs ((loop)->asyncs) #define backend ((loop)->backend) #define backend_fd ((loop)->backend_fd) #define backend_mintime ((loop)->backend_mintime) #define backend_modify ((loop)->backend_modify) #define backend_poll ((loop)->backend_poll) #define checkcnt ((loop)->checkcnt) #define checkmax ((loop)->checkmax) #define checks ((loop)->checks) #define cleanupcnt ((loop)->cleanupcnt) #define cleanupmax ((loop)->cleanupmax) #define cleanups ((loop)->cleanups) #define curpid ((loop)->curpid) #define epoll_epermcnt ((loop)->epoll_epermcnt) #define epoll_epermmax ((loop)->epoll_epermmax) #define epoll_eperms ((loop)->epoll_eperms) #define epoll_eventmax ((loop)->epoll_eventmax) #define epoll_events ((loop)->epoll_events) #define evpipe ((loop)->evpipe) #define fdchangecnt ((loop)->fdchangecnt) #define fdchangemax ((loop)->fdchangemax) #define fdchanges ((loop)->fdchanges) #define forkcnt ((loop)->forkcnt) #define forkmax ((loop)->forkmax) #define forks ((loop)->forks) #define fs_2625 ((loop)->fs_2625) #define fs_fd ((loop)->fs_fd) #define fs_hash ((loop)->fs_hash) #define fs_w ((loop)->fs_w) #define idleall ((loop)->idleall) #define idlecnt ((loop)->idlecnt) #define idlemax ((loop)->idlemax) #define idles ((loop)->idles) #define invoke_cb ((loop)->invoke_cb) #define io_blocktime ((loop)->io_blocktime) #define iocp ((loop)->iocp) #define kqueue_changecnt ((loop)->kqueue_changecnt) #define kqueue_changemax ((loop)->kqueue_changemax) #define kqueue_changes ((loop)->kqueue_changes) #define kqueue_eventmax ((loop)->kqueue_eventmax) #define kqueue_events ((loop)->kqueue_events) #define kqueue_fd_pid ((loop)->kqueue_fd_pid) #define loop_count ((loop)->loop_count) #define loop_depth ((loop)->loop_depth) #define loop_done ((loop)->loop_done) #define mn_now ((loop)->mn_now) #define now_floor ((loop)->now_floor) #define origflags ((loop)->origflags) #define pending_w ((loop)->pending_w) #define pendingcnt ((loop)->pendingcnt) #define pendingmax ((loop)->pendingmax) #define pendingpri ((loop)->pendingpri) #define pendings ((loop)->pendings) #define periodiccnt ((loop)->periodiccnt) #define periodicmax ((loop)->periodicmax) #define periodics ((loop)->periodics) #define pipe_w ((loop)->pipe_w) #define pipe_write_skipped ((loop)->pipe_write_skipped) #define pipe_write_wanted ((loop)->pipe_write_wanted) #define pollcnt ((loop)->pollcnt) #define pollidxmax ((loop)->pollidxmax) #define pollidxs ((loop)->pollidxs) #define pollmax ((loop)->pollmax) #define polls ((loop)->polls) #define port_eventmax ((loop)->port_eventmax) #define port_events ((loop)->port_events) #define postfork ((loop)->postfork) #define preparecnt ((loop)->preparecnt) #define preparemax ((loop)->preparemax) #define prepares ((loop)->prepares) #define release_cb ((loop)->release_cb) #define rfeedcnt ((loop)->rfeedcnt) #define rfeedmax ((loop)->rfeedmax) #define rfeeds ((loop)->rfeeds) #define rtmn_diff ((loop)->rtmn_diff) #define sig_pending ((loop)->sig_pending) #define sigfd ((loop)->sigfd) #define sigfd_set ((loop)->sigfd_set) #define sigfd_w ((loop)->sigfd_w) #define timeout_blocktime ((loop)->timeout_blocktime) #define timercnt ((loop)->timercnt) #define timermax ((loop)->timermax) #define timers ((loop)->timers) #define userdata ((loop)->userdata) #define vec_eo ((loop)->vec_eo) #define vec_max ((loop)->vec_max) #define vec_ri ((loop)->vec_ri) #define vec_ro ((loop)->vec_ro) #define vec_wi ((loop)->vec_wi) #define vec_wo ((loop)->vec_wo)