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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-triggered 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  hap-

       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 only delivers events 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 indefinitely.

       An application that employs the EPOLLET flag should use non-blocking file descriptors to avoid having a  block-
       ing  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 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 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 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 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 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 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) {

  = 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) {
              = EPOLLIN | EPOLLET;
              = 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 different events masks.

       Q2  Can  two  epoll instances wait for the same file descriptor?  If so, are events reported to both epoll file

       A2  Yes, and events would be reported to both.  However, careful programming may be  needed  to  do  this  cor-

       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 con-
           tinues 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  descrip-
           tion 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.  Mod-
           ify will re-read available I/O.

       Q9  Do I need to continuously read/write a file descriptor until EAGAIN when using the EPOLLET flag (edge-trig-
           gered 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-oriented 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  struc-
       ture,  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 pro-
       vide 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 descrip-
       tor 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 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 informa-
       tion about reporting bugs, can be found at

Linux                             2009-02-01                          EPOLL(7)