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

       epoll - I/O event notification facility

       #include <sys/epoll.h>

       epoll  is a variant of poll(2) that can be used either as an edge-trig‐
       gered or a level-triggered interface and scales well to	large  numbers
       of  watched  file descriptors.  The following system calls are provided
       to create and manage an epoll instance:

       *  An epoll instance created by epoll_create(2), which returns  a  file
	  descriptor  referring	 to  the  epoll	 instance.   (The  more recent
	  epoll_create1(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.

       *  Finally, the actual wait is started by epoll_wait(2).

   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 only delivers events 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  non-blocking
       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 non-blocking 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.

   /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 non-blocking 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) {

	   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 *) &local, &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) 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

       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
	   description	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() 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 re-read 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 (non-blocking) 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.

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

       This page is part of release 3.22 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux				  2009-02-01			      EPOLL(7)

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