http://man7.org/linux/man-pages/man2/epoll_wait.2.html

epoll_wait, epoll_pwait - wait for an I/O event on an epoll file
descriptor
以上两个系统调用等待epoll fd上的IO 事件

#include <sys/epoll.h>

       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);

The epoll_wait() system call waits for events on the epoll(7)
instance referred to by the file descriptor epfd. 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(). The maxevents argument must be greater than zero.

The timeout argument specifies the number of milliseconds that
epoll_wait() will block. Time is measured against the
CLOCK_MONOTONIC clock. The call will block until either:
timout参数的时间单位是毫秒，epoll_wait 系统调用将被阻塞。时间由CLOCK_MONOTONIC测量，直到下列几种情况该调用将一直被阻塞。

• a file descriptor delivers an event;
• the call is interrupted by a signal handler;
• the timeout expires.

Note
that the timeout interval will be rounded up to the system clock
granularity, and kernel scheduling delays mean that the blocking
interval may overrun by a small amount.

• Specifying a timeout of -1 causes epoll_wait() to block indefinitely,
  timeout=-1 将一直被阻塞，直到有 event 到来
• while specifying a timeout equal to zero cause epoll_wait() to return immediately, even if no events are available.
  timeout=0 则立即返回，即使没有可用的event

RETURN VALUE

When successful, epoll_wait() returns the number of file descriptors
ready for the requested I/O, or zero if no file descriptor became
ready during the requested timeout milliseconds. When an error
occurs, epoll_wait() returns -1 and errno is set appropriately.

ERROR

• EBADF epfd is not a valid file descriptor.
• EFAULT The memory area pointed to by events is not accessible with
  write permissions.
• EINTR The call was interrupted by a signal handler before either (1)
  any of the requested events occurred or (2) the timeout
  expired; see signal(7).
• EINVAL epfd is not an epoll file descriptor, or maxevents is less
  than or equal to zero.