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EPOLL(7)		   Linux Programmers Manual		     EPOLL(7)

       epoll - I/O event notification facility


       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.  Three system calls are provided to set up
       and control an epoll set: epoll_create(2), epoll_ctl(2), epoll_wait(2).

       An  epoll  set  is connected to a file descriptor created by epoll_cre
       ate(2).	Interest for certain file descriptors is then  registered  via
       epoll_ctl(2).  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 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
	  added inside the epoll device.

       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 the edge-triggered epoll multiple events can be	gener
       ated upon receipt of multiple chunks of data, the caller has the option
       to specify the EPOLLONESHOT flag, to tell epoll to disable the  associ
       ated  file descriptor after the receipt of an event with epoll_wait(2).
       When the EPOLLONESHOT flag is specified, it is the  callers  responsi
       bility	to   rearm   the   file  descriptor  using  epoll_ctl(2)  with

   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.

	   struct epoll_event ev, *events;

	   for (;;) {
	       nfds = epoll_wait(kdpfd, events, maxevents, -1);

	       for (n = 0; n < nfds; ++n) {
		   if (events[n].data.fd == listener) {
		       client = accept(listener, (struct sockaddr *) &local,
		       if (client < 0){
		       ev.events = EPOLLIN | EPOLLET;
		       ev.data.fd = client;
		       if (epoll_ctl(kdpfd, EPOLL_CTL_ADD, client, &ev)
			       == -1) {
				   "epoll set insertion error: fd=%d\n",
			   return -1;
		   } 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 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
	   kernels internal representation of an open file).

       Q1  What happens if you add the same file descriptor to	an  epoll  set

       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	set.   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 sets 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 the epoll file descriptor is put 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 events 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.  Its interface
       should be finalized in Linux kernel 2.5.66.

       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_ctl(2), epoll_wait(2)

       This page is part of release 3.05 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				  2008-02-28			      EPOLL(7)

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