1epoll_ctl(2)                  System Calls Manual                 epoll_ctl(2)
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NAME

6       epoll_ctl - control interface for an epoll file descriptor
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LIBRARY

9       Standard C library (libc, -lc)
10

SYNOPSIS

12       #include <sys/epoll.h>
13
14       int epoll_ctl(int epfd, int op, int fd,
15                     struct epoll_event *_Nullable event);
16

DESCRIPTION

18       This  system  call is used to add, modify, or remove entries in the in‐
19       terest list of the epoll(7) instance referred to by the file descriptor
20       epfd.   It  requests  that the operation op be performed for the target
21       file descriptor, fd.
22
23       Valid values for the op argument are:
24
25       EPOLL_CTL_ADD
26              Add an entry to the interest list of the epoll file  descriptor,
27              epfd.   The  entry includes the file descriptor, fd, a reference
28              to the corresponding open file  description  (see  epoll(7)  and
29              open(2)), and the settings specified in event.
30
31       EPOLL_CTL_MOD
32              Change  the  settings associated with fd in the interest list to
33              the new settings specified in event.
34
35       EPOLL_CTL_DEL
36              Remove (deregister) the target file descriptor fd from  the  in‐
37              terest list.  The event argument is ignored and can be NULL (but
38              see BUGS below).
39
40       The event argument describes the object linked to the  file  descriptor
41       fd.  The struct epoll_event is described in epoll_event(3type).
42
43       The  data  member  of the epoll_event structure specifies data that the
44       kernel should save and then return (via epoll_wait(2)) when  this  file
45       descriptor becomes ready.
46
47       The  events  member of the epoll_event structure is a bit mask composed
48       by ORing together zero or more event types, returned by  epoll_wait(2),
49       and  input flags, which affect its behaviour, but aren't returned.  The
50       available event types are:
51
52       EPOLLIN
53              The associated file is available for read(2) operations.
54
55       EPOLLOUT
56              The associated file is available for write(2) operations.
57
58       EPOLLRDHUP (since Linux 2.6.17)
59              Stream socket peer closed connection, or shut down writing  half
60              of connection.  (This flag is especially useful for writing sim‐
61              ple code to detect peer shutdown when using edge-triggered moni‐
62              toring.)
63
64       EPOLLPRI
65              There  is  an exceptional condition on the file descriptor.  See
66              the discussion of POLLPRI in poll(2).
67
68       EPOLLERR
69              Error condition happened  on  the  associated  file  descriptor.
70              This event is also reported for the write end of a pipe when the
71              read end has been closed.
72
73              epoll_wait(2) will always report for this event; it is not  nec‐
74              essary to set it in events when calling epoll_ctl().
75
76       EPOLLHUP
77              Hang up happened on the associated file descriptor.
78
79              epoll_wait(2)  will always wait for this event; it is not neces‐
80              sary to set it in events when calling epoll_ctl().
81
82              Note that when reading from a channel such as a pipe or a stream
83              socket, this event merely indicates that the peer closed its end
84              of the channel.  Subsequent reads from the channel will return 0
85              (end of file) only after all outstanding data in the channel has
86              been consumed.
87
88       And the available input flags are:
89
90       EPOLLET
91              Requests edge-triggered notification for the associated file de‐
92              scriptor.   The  default  behavior for epoll is level-triggered.
93              See epoll(7) for more detailed information about  edge-triggered
94              and level-triggered notification.
95
96       EPOLLONESHOT (since Linux 2.6.2)
97              Requests  one-shot notification for the associated file descrip‐
98              tor.  This means that after an event notified for the  file  de‐
99              scriptor  by  epoll_wait(2),  the file descriptor is disabled in
100              the interest list and no other events will be  reported  by  the
101              epoll   interface.    The   user   must  call  epoll_ctl()  with
102              EPOLL_CTL_MOD to rearm the file  descriptor  with  a  new  event
103              mask.
104
105       EPOLLWAKEUP (since Linux 3.5)
106              If  EPOLLONESHOT  and  EPOLLET are clear and the process has the
107              CAP_BLOCK_SUSPEND capability, ensure that the  system  does  not
108              enter  "suspend"  or  "hibernate" while this event is pending or
109              being processed.  The event is considered as  being  "processed"
110              from the time when it is returned by a call to epoll_wait(2) un‐
111              til the next call to epoll_wait(2) on the same epoll(7) file de‐
112              scriptor,  the  closure  of that file descriptor, the removal of
113              the event file descriptor with EPOLL_CTL_DEL, or the clearing of
114              EPOLLWAKEUP  for  the  event file descriptor with EPOLL_CTL_MOD.
115              See also BUGS.
116
117       EPOLLEXCLUSIVE (since Linux 4.5)
118              Sets an exclusive wakeup mode for the epoll file descriptor that
119              is  being  attached  to  the target file descriptor, fd.  When a
120              wakeup event occurs and multiple epoll file descriptors are  at‐
121              tached to the same target file using EPOLLEXCLUSIVE, one or more
122              of the  epoll  file  descriptors  will  receive  an  event  with
123              epoll_wait(2).   The  default in this scenario (when EPOLLEXCLU‐
124              SIVE is not set) is for all epoll file descriptors to receive an
125              event.   EPOLLEXCLUSIVE  is  thus useful for avoiding thundering
126              herd problems in certain scenarios.
127
128              If the same file descriptor is in multiple epoll instances, some
129              with  the  EPOLLEXCLUSIVE  flag, and others without, then events
130              will be provided to all epoll instances  that  did  not  specify
131              EPOLLEXCLUSIVE, and at least one of the epoll instances that did
132              specify EPOLLEXCLUSIVE.
133
134              The following  values  may  be  specified  in  conjunction  with
135              EPOLLEXCLUSIVE:  EPOLLIN,  EPOLLOUT,  EPOLLWAKEUP,  and EPOLLET.
136              EPOLLHUP and EPOLLERR can also be specified, but this is not re‐
137              quired:  as  usual, these events are always reported if they oc‐
138              cur, regardless of whether they are specified  in  events.   At‐
139              tempts to specify other values in events yield the error EINVAL.
140
141              EPOLLEXCLUSIVE  may  be used only in an EPOLL_CTL_ADD operation;
142              attempts to employ it with EPOLL_CTL_MOD  yield  an  error.   If
143              EPOLLEXCLUSIVE has been set using epoll_ctl(), then a subsequent
144              EPOLL_CTL_MOD on the same epfd, fd pair yields an error.  A call
145              to epoll_ctl() that specifies EPOLLEXCLUSIVE in events and spec‐
146              ifies the target file descriptor fd as an  epoll  instance  will
147              likewise fail.  The error in all of these cases is EINVAL.
148

RETURN VALUE

150       When  successful,  epoll_ctl()  returns  zero.   When  an error occurs,
151       epoll_ctl() returns -1 and errno is set to indicate the error.
152

ERRORS

154       EBADF  epfd or fd is not a valid file descriptor.
155
156       EEXIST op was EPOLL_CTL_ADD, and the supplied file descriptor fd is al‐
157              ready registered with this epoll instance.
158
159       EINVAL epfd is not an epoll file descriptor, or fd is the same as epfd,
160              or the requested operation op is not supported  by  this  inter‐
161              face.
162
163       EINVAL An invalid event type was specified along with EPOLLEXCLUSIVE in
164              events.
165
166       EINVAL op was EPOLL_CTL_MOD and events included EPOLLEXCLUSIVE.
167
168       EINVAL op was EPOLL_CTL_MOD and the EPOLLEXCLUSIVE flag has  previously
169              been applied to this epfd, fd pair.
170
171       EINVAL EPOLLEXCLUSIVE  was specified in event and fd refers to an epoll
172              instance.
173
174       ELOOP  fd refers to an epoll instance and this EPOLL_CTL_ADD  operation
175              would  result  in  a circular loop of epoll instances monitoring
176              one another or a nesting depth of epoll instances  greater  than
177              5.
178
179       ENOENT op  was EPOLL_CTL_MOD or EPOLL_CTL_DEL, and fd is not registered
180              with this epoll instance.
181
182       ENOMEM There was insufficient memory to handle the requested op control
183              operation.
184
185       ENOSPC The limit imposed by /proc/sys/fs/epoll/max_user_watches was en‐
186              countered while trying to register (EPOLL_CTL_ADD)  a  new  file
187              descriptor  on  an epoll instance.  See epoll(7) for further de‐
188              tails.
189
190       EPERM  The target file fd does not support epoll.  This error can occur
191              if fd refers to, for example, a regular file or a directory.
192

STANDARDS

194       Linux.
195

HISTORY

197       Linux 2.6, glibc 2.3.2.
198

NOTES

200       The epoll interface supports all file descriptors that support poll(2).
201

BUGS

203       Before  Linux  2.6.9,  the  EPOLL_CTL_DEL operation required a non-null
204       pointer in event, even though this argument is  ignored.   Since  Linux
205       2.6.9, event can be specified as NULL when using EPOLL_CTL_DEL.  Appli‐
206       cations that need to be portable to kernels before Linux  2.6.9  should
207       specify a non-null pointer in event.
208
209       If  EPOLLWAKEUP is specified in flags, but the caller does not have the
210       CAP_BLOCK_SUSPEND capability, then the EPOLLWAKEUP flag is silently ig‐
211       nored.   This  unfortunate  behavior  is  necessary because no validity
212       checks were performed on the flags argument in the original implementa‐
213       tion,  and the addition of the EPOLLWAKEUP with a check that caused the
214       call to fail if the caller did not have the CAP_BLOCK_SUSPEND  capabil‐
215       ity  caused  a breakage in at least one existing user-space application
216       that happened to randomly (and uselessly) specify this bit.   A  robust
217       application   should   therefore   double   check   that   it  has  the
218       CAP_BLOCK_SUSPEND capability if attempting to use the EPOLLWAKEUP flag.
219

SEE ALSO

221       epoll_create(2), epoll_wait(2), poll(2), epoll(7)
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225Linux man-pages 6.04              2023-04-03                      epoll_ctl(2)
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