探讨epoll_wait

NAME        epoll_wait, epoll_pwait - wait for an I/O event on an epoll file descriptor SYNOPSIS        #include        int epoll_wait(int epfd, struct epoll_event *events,                       int maxevents, int timeout);        int epoll_pwait(int epfd, struct epoll_event *events,                       int maxevents, int timeout,                       const sigset_t *sigmask); DESCRIPTION        The  epoll_wait()  system  call  waits for events on the epoll file descriptor epfd for a maximum time of timeout milliseconds.  The memory area pointed to by events        will contain the events that will be available for the caller.  Up to maxevents are returned by epoll_wait(2).  The maxevents parameter must be  greater  than  zero.        Specifying  a  timeout  of  -1  makes  epoll_wait(2) wait indefinitely, while specifying a timeout equal to zero makes epoll_wait(2) to return immediately even if no        events are available (return code equal to zero).  The struct epoll_event is defined as :             typedef union epoll_data {                 void *ptr;                 int fd;                 __uint32_t u32;                 __uint64_t u64;             } epoll_data_t;             struct epoll_event {                 __uint32_t events;      /* Epoll events */                 epoll_data_t data;      /* User data variable */             };        The data of each returned structure will contain the same data the user set with a epoll_ctl(2) (EPOLL_CTL_ADD,EPOLL_CTL_MOD) while the events  member  will  contain        the returned event bit field.    epoll_pwait()        The  relationship  between epoll_wait() and epoll_pwait() is analogous to the relationship between select(2) and pselect(2): like pselect(2), epoll_pwait() allows an        application to safely wait until either a file descriptor becomes ready or until a signal is caught.   可是，epoll_wait的maxevents该设置成多少呢？是不是如果有超过maxevents的事件就绪，就抛弃了？先看epoll_wait系统实现的调用栈。 epoll_wait     ep_poll         ep_events_transfer             ep_collect_ready_items             ep_send_events 简单分析： (1) ep_collect_ready_items 可以看出，事件之前是挂在一个列表ep->rdllist上的，这个函数就是把指定数量（该数量就是epoll_wait的参数maxevents）的事件挂到另一个列表txlist，也就是没有取出来的 事件仍然挂在ep->rdllist上，不会丢失，下个epoll_wait再通知用户 (2) ep_send_events 执行的动作就是把列表txlist的内容放到缓冲区，然后复制到用户缓冲区   maxevents 是epoll_wait可以处理的连接事件的最大限度值，这个值一般要小于或等于epoll_create的那个size，当然如果设置成比size还大的话也无所谓，size是epoll整体可以监听的最大fd数量。maxevents的意义是防止epoll的API在填写你传进去的指针events的时候，超过指针指向的内存的大小从而导致内存溢出。