1EPOLL(7)                   Linux Programmer's Manual                  EPOLL(7)


6       epoll - I/O event notification facility


9       #include <sys/epoll.h>


12       The  epoll  API performs a similar task to poll(2): monitoring multiple
13       file descriptors to see if I/O is possible on any of them.   The  epoll
14       API can be used either as an edge-triggered or a level-triggered inter‐
15       face and scales well to large numbers of watched file descriptors.
17       The central concept of the epoll API is the epoll instance, an  in-ker‐
18       nel data structure which, from a user-space perspective, can be consid‐
19       ered as a container for two lists:
21       • The interest list (sometimes also called the epoll set): the  set  of
22         file descriptors that the process has registered an interest in moni‐
23         toring.
25       • The ready list: the set of file descriptors that are "ready" for I/O.
26         The  ready  list  is a subset of (or, more precisely, a set of refer‐
27         ences to) the file descriptors in the interest list.  The ready  list
28         is dynamically populated by the kernel as a result of I/O activity on
29         those file descriptors.
31       The following system calls are provided to create and manage  an  epoll
32       instance:
34epoll_create(2)  creates  a new epoll instance and returns a file de‐
35         scriptor referring to that instance.   (The  more  recent  epoll_cre‐
36         ate1(2) extends the functionality of epoll_create(2).)
38       • Interest  in  particular  file  descriptors  is  then  registered via
39         epoll_ctl(2), which adds items to the interest list of the epoll  in‐
40         stance.
42epoll_wait(2) waits for I/O events, blocking the calling thread if no
43         events are currently available.  (This system call can be thought  of
44         as fetching items from the ready list of the epoll instance.)
46   Level-triggered and edge-triggered
47       The  epoll event distribution interface is able to behave both as edge-
48       triggered (ET) and as level-triggered (LT).  The difference between the
49       two mechanisms can be described as follows.  Suppose that this scenario
50       happens:
52       1. The file descriptor that represents the read side of a pipe (rfd) is
53          registered on the epoll instance.
55       2. A pipe writer writes 2 kB of data on the write side of the pipe.
57       3. A call to epoll_wait(2) is done that will return rfd as a ready file
58          descriptor.
60       4. The pipe reader reads 1 kB of data from rfd.
62       5. A call to epoll_wait(2) is done.
64       If the rfd file descriptor has been added to the epoll interface  using
65       the  EPOLLET  (edge-triggered)  flag, the call to epoll_wait(2) done in
66       step 5 will probably hang despite the available data still  present  in
67       the  file  input buffer; meanwhile the remote peer might be expecting a
68       response based on the data it already sent.  The  reason  for  this  is
69       that edge-triggered mode delivers events only when changes occur on the
70       monitored file descriptor.  So, in step 5 the caller might end up wait‐
71       ing  for some data that is already present inside the input buffer.  In
72       the above example, an event on rfd will be  generated  because  of  the
73       write  done in 2 and the event is consumed in 3.  Since the read opera‐
74       tion done in 4 does not consume the whole  buffer  data,  the  call  to
75       epoll_wait(2) done in step 5 might block indefinitely.
77       An  application  that  employs  the EPOLLET flag should use nonblocking
78       file descriptors to avoid having a blocking read or write starve a task
79       that  is  handling multiple file descriptors.  The suggested way to use
80       epoll as an edge-triggered (EPOLLET) interface is as follows:
82       a) with nonblocking file descriptors; and
84       b) by waiting for an event only after read(2) or  write(2)  return  EA‐
85          GAIN.
87       By  contrast,  when  used  as a level-triggered interface (the default,
88       when EPOLLET is not specified), epoll is simply a faster  poll(2),  and
89       can be used wherever the latter is used since it shares the same seman‐
90       tics.
92       Since even with edge-triggered epoll, multiple events can be  generated
93       upon  receipt  of multiple chunks of data, the caller has the option to
94       specify the EPOLLONESHOT flag, to tell epoll to disable the  associated
95       file descriptor after the receipt of an event with epoll_wait(2).  When
96       the EPOLLONESHOT flag is specified, it is the  caller's  responsibility
97       to rearm the file descriptor using epoll_ctl(2) with EPOLL_CTL_MOD.
99       If  multiple  threads  (or processes, if child processes have inherited
100       the epoll file descriptor across fork(2)) are blocked in  epoll_wait(2)
101       waiting  on the same epoll file descriptor and a file descriptor in the
102       interest list that is marked for edge-triggered (EPOLLET)  notification
103       becomes  ready,  just  one of the threads (or processes) is awoken from
104       epoll_wait(2).  This provides a useful optimization for avoiding "thun‐
105       dering herd" wake-ups in some scenarios.
107   Interaction with autosleep
108       If  the  system  is  in  autosleep mode via /sys/power/autosleep and an
109       event happens which wakes the device from sleep, the device driver will
110       keep the device awake only until that event is queued.  To keep the de‐
111       vice awake until the event has been processed, it is necessary  to  use
112       the epoll_ctl(2) EPOLLWAKEUP flag.
114       When  the  EPOLLWAKEUP  flag  is  set  in the events field for a struct
115       epoll_event, the system will be kept awake from the moment the event is
116       queued,  through  the  epoll_wait(2) call which returns the event until
117       the subsequent epoll_wait(2) call.  If the event should keep the system
118       awake  beyond  that time, then a separate wake_lock should be taken be‐
119       fore the second epoll_wait(2) call.
121   /proc interfaces
122       The following interfaces can be used to limit the amount of kernel mem‐
123       ory consumed by epoll:
125       /proc/sys/fs/epoll/max_user_watches (since Linux 2.6.28)
126              This  specifies  a limit on the total number of file descriptors
127              that a user can register across all epoll instances on the  sys‐
128              tem.   The  limit is per real user ID.  Each registered file de‐
129              scriptor costs roughly 90 bytes on a 32-bit kernel, and  roughly
130              160  bytes on a 64-bit kernel.  Currently, the default value for
131              max_user_watches is 1/25 (4%) of the available low  memory,  di‐
132              vided by the registration cost in bytes.
134   Example for suggested usage
135       While  the  usage of epoll when employed as a level-triggered interface
136       does have the same semantics as poll(2), the edge-triggered  usage  re‐
137       quires  more  clarification  to  avoid  stalls in the application event
138       loop.  In this example, listener is a nonblocking socket on which  lis‐
139       ten(2)  has  been  called.  The function do_use_fd() uses the new ready
140       file descriptor until EAGAIN is returned by either read(2) or write(2).
141       An event-driven state machine application should, after having received
142       EAGAIN,  record  its  current  state  so  that  at  the  next  call  to
143       do_use_fd()  it  will  continue  to  read(2)  or write(2) from where it
144       stopped before.
146           #define MAX_EVENTS 10
147           struct epoll_event ev, events[MAX_EVENTS];
148           int listen_sock, conn_sock, nfds, epollfd;
150           /* Code to set up listening socket, 'listen_sock',
151              (socket(), bind(), listen()) omitted. */
153           epollfd = epoll_create1(0);
154           if (epollfd == -1) {
155               perror("epoll_create1");
156               exit(EXIT_FAILURE);
157           }
159           ev.events = EPOLLIN;
160           ev.data.fd = listen_sock;
161           if (epoll_ctl(epollfd, EPOLL_CTL_ADD, listen_sock, &ev) == -1) {
162               perror("epoll_ctl: listen_sock");
163               exit(EXIT_FAILURE);
164           }
166           for (;;) {
167               nfds = epoll_wait(epollfd, events, MAX_EVENTS, -1);
168               if (nfds == -1) {
169                   perror("epoll_wait");
170                   exit(EXIT_FAILURE);
171               }
173               for (n = 0; n < nfds; ++n) {
174                   if (events[n].data.fd == listen_sock) {
175                       conn_sock = accept(listen_sock,
176                                          (struct sockaddr *) &addr, &addrlen);
177                       if (conn_sock == -1) {
178                           perror("accept");
179                           exit(EXIT_FAILURE);
180                       }
181                       setnonblocking(conn_sock);
182                       ev.events = EPOLLIN | EPOLLET;
183                       ev.data.fd = conn_sock;
184                       if (epoll_ctl(epollfd, EPOLL_CTL_ADD, conn_sock,
185                                   &ev) == -1) {
186                           perror("epoll_ctl: conn_sock");
187                           exit(EXIT_FAILURE);
188                       }
189                   } else {
190                       do_use_fd(events[n].data.fd);
191                   }
192               }
193           }
195       When used as an edge-triggered interface, for performance  reasons,  it
196       is  possible  to  add  the  file  descriptor inside the epoll interface
197       (EPOLL_CTL_ADD) once by specifying (EPOLLIN|EPOLLOUT).  This allows you
198       to  avoid  continuously  switching between EPOLLIN and EPOLLOUT calling
199       epoll_ctl(2) with EPOLL_CTL_MOD.
201   Questions and answers
202       0.  What is the key used to distinguish the file descriptors registered
203           in an interest list?
205           The  key  is  the combination of the file descriptor number and the
206           open file description (also known as an  "open  file  handle",  the
207           kernel's internal representation of an open file).
209       1.  What  happens  if you register the same file descriptor on an epoll
210           instance twice?
212           You will probably get EEXIST.  However, it is possible to add a du‐
213           plicate  (dup(2), dup2(2), fcntl(2) F_DUPFD) file descriptor to the
214           same epoll instance.  This can be a useful technique for  filtering
215           events,  if the duplicate file descriptors are registered with dif‐
216           ferent events masks.
218       2.  Can two epoll instances wait for the same file descriptor?  If  so,
219           are events reported to both epoll file descriptors?
221           Yes,  and  events would be reported to both.  However, careful pro‐
222           gramming may be needed to do this correctly.
224       3.  Is the epoll file descriptor itself poll/epoll/selectable?
226           Yes.  If an epoll file descriptor has events waiting, then it  will
227           indicate as being readable.
229       4.  What  happens  if one attempts to put an epoll file descriptor into
230           its own file descriptor set?
232           The epoll_ctl(2) call fails (EINVAL).   However,  you  can  add  an
233           epoll file descriptor inside another epoll file descriptor set.
235       5.  Can  I  send  an epoll file descriptor over a UNIX domain socket to
236           another process?
238           Yes, but it does not make sense to do  this,  since  the  receiving
239           process would not have copies of the file descriptors in the inter‐
240           est list.
242       6.  Will closing a file descriptor cause it  to  be  removed  from  all
243           epoll interest lists?
245           Yes,  but  be aware of the following point.  A file descriptor is a
246           reference to an open file description (see  open(2)).   Whenever  a
247           file   descriptor  is  duplicated  via  dup(2),  dup2(2),  fcntl(2)
248           F_DUPFD, or fork(2), a new file descriptor referring  to  the  same
249           open file description is created.  An open file description contin‐
250           ues to exist until all file descriptors referring to it  have  been
251           closed.
253           A  file  descriptor is removed from an interest list only after all
254           the file descriptors referring to the underlying open file descrip‐
255           tion  have been closed.  This means that even after a file descrip‐
256           tor that is part of an interest list has been closed, events may be
257           reported  for that file descriptor if other file descriptors refer‐
258           ring to the same underlying file description remain open.  To  pre‐
259           vent this happening, the file descriptor must be explicitly removed
260           from the interest list (using epoll_ctl(2) EPOLL_CTL_DEL) before it
261           is duplicated.  Alternatively, the application must ensure that all
262           file descriptors are closed (which may be  difficult  if  file  de‐
263           scriptors  were  duplicated  behind the scenes by library functions
264           that used dup(2) or fork(2)).
266       7.  If more than one event occurs between epoll_wait(2) calls, are they
267           combined or reported separately?
269           They will be combined.
271       8.  Does an operation on a file descriptor affect the already collected
272           but not yet reported events?
274           You can do two operations on an existing file  descriptor.   Remove
275           would  be  meaningless for this case.  Modify will reread available
276           I/O.
278       9.  Do I need to continuously read/write a file descriptor until EAGAIN
279           when using the EPOLLET flag (edge-triggered behavior)?
281           Receiving  an  event  from epoll_wait(2) should suggest to you that
282           such file descriptor is ready for the requested I/O operation.  You
283           must  consider  it  ready  until  the next (nonblocking) read/write
284           yields EAGAIN.  When and how you will use the  file  descriptor  is
285           entirely up to you.
287           For packet/token-oriented files (e.g., datagram socket, terminal in
288           canonical mode), the only way to detect the end of  the  read/write
289           I/O space is to continue to read/write until EAGAIN.
291           For  stream-oriented  files  (e.g., pipe, FIFO, stream socket), the
292           condition that the read/write I/O space is exhausted  can  also  be
293           detected  by checking the amount of data read from / written to the
294           target file descriptor.  For example, if you call read(2) by asking
295           to read a certain amount of data and read(2) returns a lower number
296           of bytes, you can be sure of having exhausted the  read  I/O  space
297           for  the  file  descriptor.   The  same  is true when writing using
298           write(2).  (Avoid this latter technique  if  you  cannot  guarantee
299           that  the  monitored file descriptor always refers to a stream-ori‐
300           ented file.)
302   Possible pitfalls and ways to avoid them
303       o Starvation (edge-triggered)
305       If there is a large amount of I/O space, it is possible that by  trying
306       to  drain it the other files will not get processed causing starvation.
307       (This problem is not specific to epoll.)
309       The solution is to maintain a ready list and mark the  file  descriptor
310       as  ready in its associated data structure, thereby allowing the appli‐
311       cation to remember which files need to be  processed  but  still  round
312       robin  amongst all the ready files.  This also supports ignoring subse‐
313       quent events you receive for file descriptors that are already ready.
315       o If using an event cache...
317       If you use an event cache or store all the  file  descriptors  returned
318       from epoll_wait(2), then make sure to provide a way to mark its closure
319       dynamically (i.e., caused by a previous event's  processing).   Suppose
320       you receive 100 events from epoll_wait(2), and in event #47 a condition
321       causes event #13 to  be  closed.   If  you  remove  the  structure  and
322       close(2) the file descriptor for event #13, then your event cache might
323       still say there are events waiting for  that  file  descriptor  causing
324       confusion.
326       One  solution  for  this is to call, during the processing of event 47,
327       epoll_ctl(EPOLL_CTL_DEL) to delete file  descriptor  13  and  close(2),
328       then  mark  its  associated  data structure as removed and link it to a
329       cleanup list.  If you find another event for file descriptor 13 in your
330       batch processing, you will discover the file descriptor had been previ‐
331       ously removed and there will be no confusion.


334       The epoll API was introduced in Linux kernel 2.5.44.  Support was added
335       to glibc in version 2.3.2.


338       The  epoll  API  is Linux-specific.  Some other systems provide similar
339       mechanisms, for example, FreeBSD has kqueue, and Solaris has /dev/poll.


342       The set of file descriptors that is being monitored via an  epoll  file
343       descriptor can be viewed via the entry for the epoll file descriptor in
344       the process's /proc/[pid]/fdinfo directory.  See  proc(5)  for  further
345       details.
347       The kcmp(2) KCMP_EPOLL_TFD operation can be used to test whether a file
348       descriptor is present in an epoll instance.


351       epoll_create(2),   epoll_create1(2),    epoll_ctl(2),    epoll_wait(2),
352       poll(2), select(2)


355       This  page  is  part of release 5.13 of the Linux man-pages project.  A
356       description of the project, information about reporting bugs,  and  the
357       latest     version     of     this    page,    can    be    found    at
358       https://www.kernel.org/doc/man-pages/.
362Linux                             2021-03-22                          EPOLL(7)