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

       epoll - I/O event notification facility

       #include <sys/epoll.h>

       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

       *  epoll_create(2)  creates  an	epoll  instance	 and  returns  a  file
	  descriptor  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

       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

       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)
		  return 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-

       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
       keep  the  device  awake	 only until that event is queued.  To keep the
       device awake until the event has been processed, it is necessary to use
       the epoll_ctl(2) 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
       before 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
	      descriptor 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, divided 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
       requires	 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;

	   /* Code to set up listening socket, 'listen_sock',
	      (socket(), bind(), listen()) omitted */

	   epollfd = epoll_create1(0);
	   if (epollfd == -1) {

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

	   for (;;) {
	       nfds = epoll_wait(epollfd, events, MAX_EVENTS, -1);
	       if (nfds == -1) {

	       for (n = 0; n < nfds; ++n) {
		   if (events[n].data.fd == listen_sock) {
		       conn_sock = accept(listen_sock,
					  (struct sockaddr *) &addr, &addrlen);
		       if (conn_sock == -1) {
		       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");
		   } else {

       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
	   duplicate  (dup(2),	dup2(2),  fcntl(2) F_DUPFD) file descriptor to
	   the same epoll instance.  This can be a useful technique  for  fil-
	   tering  events,  if	the  duplicate file descriptors are registered
	   with different 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

       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
	   file	  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 description is created.  An open file description contin-
	   ues 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 file 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	descriptors  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

       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

       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.

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

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

       The set of file descriptors that is being monitored via an  epoll  file
       descriptor can be viewed via the entry for the epoll file descriptor in
       the process's /proc/[pid]/fdinfo directory.  See	 proc(5)  for  further

       epoll_create(2),	   epoll_create1(2),	epoll_ctl(2),	epoll_wait(2),
       poll(2), select(2)

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       description  of	the project, information about reporting bugs, and the
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Linux				  2016-10-08			      EPOLL(7)