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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
       instance:

       *  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
       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)
		  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‐
       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.

   /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;

	   /* 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
	   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
	   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
	   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(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
	   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 3.54 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				  2012-04-17			      EPOLL(7)
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