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Linux 2.6 - man page for epoll (linux section 7)

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-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

       *  epoll_create(2)  creates  an	epoll instance and returns a file descriptor referring to
	  that	instance.   (The  more	recent	epoll_create1(2)  extends  the	functionality  of

       *  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 cur-
	  rently 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 waiting 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 operation done in  4  does
       not  consume  the  whole buffer data, the call to epoll_wait(2) done in step 5 might block

       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 semantics.

       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 memory consumed by

       /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 reg-
	      ister  across  all  epoll  instances on the system.  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 registra-
	      tion 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
       listen(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

	   #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

       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  use-
	   ful	technique  for filtering 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 programming 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

       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 descrip-
	   tor 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	automati-

       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 descrip-
	   tor if other file descriptors referring to the same underlying file description remain

       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-oriented

   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

       The  solution  is  to  maintain	a ready list and mark the file descriptor as ready in its
       associated data structure, thereby allowing the application to remember which  files  need
       to  be  processed  but  still round robin amongst all the ready files.  This also supports
       ignoring subsequent 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 previ-
       ous 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  associ-
       ated  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 previously removed and there will be no confusion.

       The  epoll  API was introduced in Linux kernel 2.5.44.  Support was added to glibc in ver-
       sion 2.3.2.

       The epoll API is Linux-specific.  Some other systems provide similar mechanisms, for exam-
       ple, 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 3.55 of the Linux man-pages project.  A description of the
       project,    and	  information	 about	  reporting    bugs,	can    be    found     at

Linux					    2012-04-17					 EPOLL(7)

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