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EPOLL_WAIT(2)		   Linux Programmer's Manual		 EPOLL_WAIT(2)

       epoll_wait,  epoll_pwait	 -  wait  for  an  I/O	event on an epoll file

       #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:

       *  a file descriptor delivers an event;

       *  the call is interrupted by a signal handler; or

       *  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 inter-
       val may overrun by a small amount.  Specifying a timeout of  -1	causes
       epoll_wait() to block indefinitely, while specifying a timeout equal to
       zero cause epoll_wait() to return immediately, even if  no  events  are

       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	 an  epoll_ctl(2)  (EPOLL_CTL_ADD,  EPOLL_CTL_MOD)  while  the
       events member will contain the returned event bit field.

       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.

       The following epoll_pwait() call:

	   ready = epoll_pwait(epfd, &events, maxevents, timeout, &sigmask);

       is equivalent to atomically executing the following calls:

	   sigset_t origmask;

	   pthread_sigmask(SIG_SETMASK, &sigmask, &origmask);
	   ready = epoll_wait(epfd, &events, maxevents, timeout);
	   pthread_sigmask(SIG_SETMASK, &origmask, NULL);

       The  sigmask  argument  may  be	specified  as  NULL,  in  which	  case
       epoll_pwait() is equivalent to epoll_wait().

       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.

       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.

       epoll_wait() was added to the kernel in version 2.6.   Library  support
       is provided in glibc starting with version 2.3.2.

       epoll_pwait()  was added to Linux in kernel 2.6.19.  Library support is
       provided in glibc starting with version 2.6.

       epoll_wait() is Linux-specific.

       While one thread is blocked in a call to epoll_pwait(), it is  possible
       for  another  thread  to add a file descriptor to the waited-upon epoll
       instance.  If the new file descriptor becomes ready, it will cause  the
       epoll_wait() call to unblock.

       For  a  discussion  of what may happen if a file descriptor in an epoll
       instance being monitored by epoll_wait() is closed in  another  thread,
       see select(2).

       In  kernels  before  2.6.37,  a timeout value larger than approximately
       LONG_MAX / HZ milliseconds is treated as -1  (i.e.,  infinity).	 Thus,
       for  example, on a system where the sizeof(long) is 4 and the kernel HZ
       value is 1000, this means that timeouts greater than 35.79 minutes  are
       treated as infinity.

   C library/kernel differences
       The  raw epoll_pwait() system call has a sixth argument, size_t sigset-
       size, which specifies the size in bytes of the sigmask  argument.   The
       glibc epoll_pwait() wrapper function specifies this argument as a fixed
       value (equal to sizeof(sigset_t)).

       epoll_create(2), epoll_ctl(2), epoll(7)

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Linux				  2016-10-08			 EPOLL_WAIT(2)