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EPOLL(7)		   Linux Programmer's Manual		      EPOLL(7)

NAME
       epoll - I/O event notification facility

SYNOPSIS
       #include	<sys/epoll.h>

DESCRIPTION
       The  epoll  API performs	a similar task to poll(2): monitoring multiple
       file descriptors	to see if I/O is possible on any of them.   The	 epoll
       API can be used either as an edge-triggered or a	level-triggered	inter-
       face and	scales well to large numbers of	watched	file descriptors.  The
       following  system  calls	are provided to	create and manage an epoll in-
       stance:

       *  epoll_create(2) creates an epoll instance and	 returns  a  file  de-
	  scriptor  referring  to  that	instance.  (The	more recent epoll_cre-
	  ate1(2) extends the functionality of epoll_create(2).)

       *  Interest in particular  file	descriptors  is	 then  registered  via
	  epoll_ctl(2).	  The  set of file descriptors currently registered on
	  an epoll instance is sometimes called	an epoll set.

       *  epoll_wait(2)	waits for I/O events, blocking the calling  thread  if
	  no events are	currently available.

   Level-triggered and edge-triggered
       The  epoll event	distribution interface is able to behave both as edge-
       triggered (ET) and as level-triggered (LT).  The	difference between the
       two mechanisms can be described as follows.  Suppose that this scenario
       happens:

       1. The file descriptor that represents the read side of a pipe (rfd) is
	  registered on	the epoll instance.

       2. A pipe writer	writes 2 kB of data on the write side of the pipe.

       3. A call to epoll_wait(2) is done that will return rfd as a ready file
	  descriptor.

       4. The pipe reader reads	1 kB of	data from rfd.

       5. A call to epoll_wait(2) is done.

       If the rfd file descriptor has been added to the	epoll interface	 using
       the  EPOLLET  (edge-triggered)  flag, the call to epoll_wait(2) done in
       step 5 will probably hang despite the available data still  present  in
       the  file  input	buffer;	meanwhile the remote peer might	be expecting a
       response	based on the data it already sent.  The	 reason	 for  this  is
       that edge-triggered mode	delivers events	only when changes occur	on the
       monitored file descriptor.  So, in step 5 the caller might end up wait-
       ing  for	some data that is already present inside the input buffer.  In
       the above example, an event on rfd will be  generated  because  of  the
       write  done in 2	and the	event is consumed in 3.	 Since the read	opera-
       tion done in 4 does not consume the whole  buffer  data,	 the  call  to
       epoll_wait(2) done in step 5 might block	indefinitely.

       An  application	that  employs  the EPOLLET flag	should use nonblocking
       file descriptors	to avoid having	a blocking read	or write starve	a task
       that  is	 handling multiple file	descriptors.  The suggested way	to use
       epoll as	an edge-triggered (EPOLLET) interface is as follows:

	      i	  with nonblocking file	descriptors; and

	      ii  by waiting for an event only after read(2) or	 write(2)  re-
		  turn EAGAIN.

       By  contrast,  when  used  as a level-triggered interface (the default,
       when EPOLLET is not specified), epoll is	simply a faster	 poll(2),  and
       can be used wherever the	latter is used since it	shares the same	seman-
       tics.

       Since even with edge-triggered epoll, multiple events can be  generated
       upon  receipt  of multiple chunks of data, the caller has the option to
       specify the EPOLLONESHOT	flag, to tell epoll to disable the  associated
       file descriptor after the receipt of an event with epoll_wait(2).  When
       the EPOLLONESHOT	flag is	specified, it is the  caller's	responsibility
       to rearm	the file descriptor using epoll_ctl(2) with EPOLL_CTL_MOD.

   Interaction with autosleep
       If  the	system	is  in	autosleep mode via /sys/power/autosleep	and an
       event happens which wakes the device from sleep,	the device driver will
       only keep the device awake until	that event is queued.  To keep the de-
       vice awake until	the event has been processed, it is necessary  to  use
       the epoll(7) EPOLLWAKEUP	flag.

       When  the  EPOLLWAKEUP  flag  is	 set  in the events field for a	struct
       epoll_event, the	system will be kept awake from the moment the event is
       queued,	through	 the  epoll_wait(2) call which returns the event until
       the subsequent epoll_wait(2) call.  If the event	should keep the	system
       awake  beyond  that time, then a	separate wake_lock should be taken be-
       fore the	second epoll_wait(2) call.

   /proc interfaces
       The following interfaces	can be used to limit the amount	of kernel mem-
       ory consumed by epoll:

       /proc/sys/fs/epoll/max_user_watches (since Linux	2.6.28)
	      This  specifies  a limit on the total number of file descriptors
	      that a user can register across all epoll	instances on the  sys-
	      tem.   The  limit	is per real user ID.  Each registered file de-
	      scriptor costs roughly 90	bytes on a 32-bit kernel, and  roughly
	      160  bytes on a 64-bit kernel.  Currently, the default value for
	      max_user_watches is 1/25 (4%) of the available low  memory,  di-
	      vided by the registration	cost in	bytes.

   Example for suggested usage
       While  the  usage of epoll when employed	as a level-triggered interface
       does have the same semantics as poll(2),	the edge-triggered  usage  re-
       quires  more  clarification  to	avoid  stalls in the application event
       loop.  In this example, listener	is a nonblocking socket	on which  lis-
       ten(2)  has  been  called.  The function	do_use_fd() uses the new ready
       file descriptor until EAGAIN is returned	by either read(2) or write(2).
       An event-driven state machine application should, after having received
       EAGAIN,	record	its  current  state  so	 that  at  the	next  call  to
       do_use_fd()  it	will  continue	to  read(2)  or	write(2) from where it
       stopped before.

	   #define MAX_EVENTS 10
	   struct epoll_event ev, events[MAX_EVENTS];
	   int listen_sock, conn_sock, nfds, epollfd;

	   /* Set up listening socket, 'listen_sock' (socket(),
	      bind(), listen())	*/

	   epollfd = epoll_create(10);
	   if (epollfd == -1) {
	       perror("epoll_create");
	       exit(EXIT_FAILURE);
	   }

	   ev.events = EPOLLIN;
	   ev.data.fd =	listen_sock;
	   if (epoll_ctl(epollfd, EPOLL_CTL_ADD, listen_sock, &ev) == -1) {
	       perror("epoll_ctl: listen_sock");
	       exit(EXIT_FAILURE);
	   }

	   for (;;) {
	       nfds = epoll_wait(epollfd, events, MAX_EVENTS, -1);
	       if (nfds	== -1) {
		   perror("epoll_pwait");
		   exit(EXIT_FAILURE);
	       }

	       for (n =	0; n < nfds; ++n) {
		   if (events[n].data.fd == listen_sock) {
		       conn_sock = accept(listen_sock,
				       (struct sockaddr	*) &local, &addrlen);
		       if (conn_sock ==	-1) {
			   perror("accept");
			   exit(EXIT_FAILURE);
		       }
		       setnonblocking(conn_sock);
		       ev.events = EPOLLIN | EPOLLET;
		       ev.data.fd = conn_sock;
		       if (epoll_ctl(epollfd, EPOLL_CTL_ADD, conn_sock,
				   &ev)	== -1) {
			   perror("epoll_ctl: conn_sock");
			   exit(EXIT_FAILURE);
		       }
		   } else {
		       do_use_fd(events[n].data.fd);
		   }
	       }
	   }

       When used as an edge-triggered interface, for performance  reasons,  it
       is  possible  to	 add  the  file	 descriptor inside the epoll interface
       (EPOLL_CTL_ADD) once by specifying (EPOLLIN|EPOLLOUT).  This allows you
       to  avoid  continuously	switching between EPOLLIN and EPOLLOUT calling
       epoll_ctl(2) with EPOLL_CTL_MOD.

   Questions and answers
       Q0  What	is the key used	to distinguish the file	descriptors registered
	   in an epoll set?

       A0  The	key  is	 the combination of the	file descriptor	number and the
	   open	file description (also known as	an  "open  file	 handle",  the
	   kernel's internal representation of an open file).

       Q1  What	 happens  if you register the same file	descriptor on an epoll
	   instance twice?

       A1  You will probably get EEXIST.  However, it is possible to add a du-
	   plicate  (dup(2), dup2(2), fcntl(2) F_DUPFD)	descriptor to the same
	   epoll instance.  This can  be  a  useful  technique	for  filtering
	   events,  if the duplicate file descriptors are registered with dif-
	   ferent events masks.

       Q2  Can two epoll instances wait	for the	same file descriptor?  If  so,
	   are events reported to both epoll file descriptors?

       A2  Yes,	 and  events would be reported to both.	 However, careful pro-
	   gramming may	be needed to do	this correctly.

       Q3  Is the epoll	file descriptor	itself poll/epoll/selectable?

       A3  Yes.	 If an epoll file descriptor has events	waiting, then it  will
	   indicate as being readable.

       Q4  What	 happens  if one attempts to put an epoll file descriptor into
	   its own file	descriptor set?

       A4  The epoll_ctl(2) call will fail (EINVAL).  However, you can add  an
	   epoll file descriptor inside	another	epoll file descriptor set.

       Q5  Can	I  send	 an epoll file descriptor over a UNIX domain socket to
	   another process?

       A5  Yes,	but it does not	make sense to do  this,	 since	the  receiving
	   process  would not have copies of the file descriptors in the epoll
	   set.

       Q6  Will	closing	a file descriptor cause	it  to	be  removed  from  all
	   epoll sets automatically?

       A6  Yes,	 but  be aware of the following	point.	A file descriptor is a
	   reference to	an open	file description (see  open(2)).   Whenever  a
	   descriptor  is duplicated via dup(2), dup2(2), fcntl(2) F_DUPFD, or
	   fork(2), a new file descriptor referring to the same	open file  de-
	   scription  is created.  An open file	description continues to exist
	   until all file descriptors referring	to it  have  been  closed.   A
	   file	 descriptor  is	 removed  from an epoll	set only after all the
	   file	descriptors referring to the underlying	open file  description
	   have	been closed (or	before if the descriptor is explicitly removed
	   using epoll_ctl(2) EPOLL_CTL_DEL).  This means that	even  after  a
	   file	 descriptor  that  is  part  of	 an epoll set has been closed,
	   events may be reported for that file	descriptor if other  file  de-
	   scriptors  referring	to the same underlying file description	remain
	   open.

       Q7  If more than	one event occurs between epoll_wait(2) calls, are they
	   combined or reported	separately?

       A7  They	will be	combined.

       Q8  Does	an operation on	a file descriptor affect the already collected
	   but not yet reported	events?

       A8  You can do two operations on	an existing file  descriptor.	Remove
	   would  be  meaningless for this case.  Modify will reread available
	   I/O.

       Q9  Do I	need to	continuously read/write	a file descriptor until	EAGAIN
	   when	using the EPOLLET flag (edge-triggered behavior) ?

       A9  Receiving  an  event	 from epoll_wait(2) should suggest to you that
	   such	file descriptor	is ready for the requested I/O operation.  You
	   must	 consider  it  ready  until  the next (nonblocking) read/write
	   yields EAGAIN.  When	and how	you will use the  file	descriptor  is
	   entirely up to you.

	   For packet/token-oriented files (e.g., datagram socket, terminal in
	   canonical mode), the	only way to detect the end of  the  read/write
	   I/O space is	to continue to read/write until	EAGAIN.

	   For	stream-oriented	 files	(e.g., pipe, FIFO, stream socket), the
	   condition that the read/write I/O space is exhausted	 can  also  be
	   detected  by	checking the amount of data read from /	written	to the
	   target file descriptor.  For	example, if you	call read(2) by	asking
	   to read a certain amount of data and	read(2)	returns	a lower	number
	   of bytes, you can be	sure of	having exhausted the  read  I/O	 space
	   for	the  file  descriptor.	 The  same  is true when writing using
	   write(2).  (Avoid this latter technique  if	you  cannot  guarantee
	   that	 the  monitored	file descriptor	always refers to a stream-ori-
	   ented file.)

   Possible pitfalls and ways to avoid them
       o Starvation (edge-triggered)

       If there	is a large amount of I/O space,	it is possible that by	trying
       to  drain it the	other files will not get processed causing starvation.
       (This problem is	not specific to	epoll.)

       The solution is to maintain a ready list	and mark the  file  descriptor
       as  ready in its	associated data	structure, thereby allowing the	appli-
       cation to remember which	files need to be  processed  but  still	 round
       robin  amongst all the ready files.  This also supports ignoring	subse-
       quent events you	receive	for file descriptors that are already ready.

       o If using an event cache...

       If you use an event cache or store all the  file	 descriptors  returned
       from epoll_wait(2), then	make sure to provide a way to mark its closure
       dynamically (i.e., caused by a previous event's	processing).   Suppose
       you receive 100 events from epoll_wait(2), and in event #47 a condition
       causes event #13	to  be	closed.	  If  you  remove  the	structure  and
       close(2)	the file descriptor for	event #13, then	your event cache might
       still say there are events waiting for  that  file  descriptor  causing
       confusion.

       One  solution  for  this	is to call, during the processing of event 47,
       epoll_ctl(EPOLL_CTL_DEL)	to delete file	descriptor  13	and  close(2),
       then  mark  its	associated  data structure as removed and link it to a
       cleanup list.  If you find another event	for file descriptor 13 in your
       batch processing, you will discover the file descriptor had been	previ-
       ously removed and there will be no confusion.

VERSIONS
       The epoll API was introduced in Linux kernel 2.5.44.  Support was added
       to glibc	in version 2.3.2.

CONFORMING TO
       The  epoll  API	is Linux-specific.  Some other systems provide similar
       mechanisms, for example,	FreeBSD	has kqueue, and	Solaris	has /dev/poll.

SEE ALSO
       epoll_create(2),	epoll_create1(2), epoll_ctl(2),	epoll_wait(2)

COLOPHON
       This page is part of release 3.74 of the	Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at
       http://www.kernel.org/doc/man-pages/.

Linux				  2014-07-08			      EPOLL(7)

NAME | SYNOPSIS | DESCRIPTION | VERSIONS | CONFORMING TO | SEE ALSO | COLOPHON

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