1EPOLL_CTL(2) Linux Programmer's Manual EPOLL_CTL(2)
2
6 epoll_ctl - control interface for an epoll file descriptor
7
9 #include <sys/epoll.h>
10 int epoll_ctl(int epfd, int op, int fd, struct epoll_event *event);
12
14 This system call is used to add, modify, or remove entries in the
15 interest list of the epoll(7) instance referred to by the file descrip‐
16 tor epfd. It requests that the operation op be performed for the tar‐
17 get file descriptor, fd.
18 Valid values for the op argument are:
20 EPOLL_CTL_ADD
22 Add fd to the interest list and associate the settings specified
23 in event with the internal file linked to fd.
24 EPOLL_CTL_MOD
26 Change the settings associated with fd in the interest list to
27 the new settings specified in event.
28 EPOLL_CTL_DEL
30 Remove (deregister) the target file descriptor fd from the
31 interest list. The event argument is ignored and can be NULL
32 (but see BUGS below).
33 The event argument describes the object linked to the file descriptor
35 fd. The struct epoll_event is defined as:
36 typedef union epoll_data {
38 void *ptr;
39 int fd;
40 uint32_t u32;
41 uint64_t u64;
42 } epoll_data_t;
43 struct epoll_event {
45 uint32_t events; /* Epoll events */
46 epoll_data_t data; /* User data variable */
47 };
48 The events member is a bit mask composed by ORing together zero or more
50 of the following available event types:
51 EPOLLIN
53 The associated file is available for read(2) operations.
54 EPOLLOUT
56 The associated file is available for write(2) operations.
57 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 EPOLLPRI
65 There is an exceptional condition on the file descriptor. See
66 the discussion of POLLPRI in poll(2).
67 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. epoll_wait(2) will always report for
72 this event; it is not necessary to set it in events.
73 EPOLLHUP
75 Hang up happened on the associated file descriptor.
76 epoll_wait(2) will always wait for this event; it is not neces‐
77 sary to set it in events.
78 Note that when reading from a channel such as a pipe or a stream
80 socket, this event merely indicates that the peer closed its end
81 of the channel. Subsequent reads from the channel will return 0
82 (end of file) only after all outstanding data in the channel has
83 been consumed.
84 EPOLLET
86 Sets the Edge Triggered behavior for the associated file
87 descriptor. The default behavior for epoll is Level Triggered.
88 See epoll(7) for more detailed information about Edge and Level
89 Triggered event distribution architectures.
90 EPOLLONESHOT (since Linux 2.6.2)
92 Sets the one-shot behavior for the associated file descriptor.
93 This means that after an event is pulled out with epoll_wait(2)
94 the associated file descriptor is internally disabled and no
95 other events will be reported by the epoll interface. The user
96 must call epoll_ctl() with EPOLL_CTL_MOD to rearm the file
97 descriptor with a new event mask.
98 EPOLLWAKEUP (since Linux 3.5)
100 If EPOLLONESHOT and EPOLLET are clear and the process has the
101 CAP_BLOCK_SUSPEND capability, ensure that the system does not
102 enter "suspend" or "hibernate" while this event is pending or
103 being processed. The event is considered as being "processed"
104 from the time when it is returned by a call to epoll_wait(2)
105 until the next call to epoll_wait(2) on the same epoll(7) file
106 descriptor, the closure of that file descriptor, the removal of
107 the event file descriptor with EPOLL_CTL_DEL, or the clearing of
108 EPOLLWAKEUP for the event file descriptor with EPOLL_CTL_MOD.
109 See also BUGS.
110 EPOLLEXCLUSIVE (since Linux 4.5)
112 Sets an exclusive wakeup mode for the epoll file descriptor that
113 is being attached to the target file descriptor, fd. When a
114 wakeup event occurs and multiple epoll file descriptors are
115 attached to the same target file using EPOLLEXCLUSIVE, one or
116 more of the epoll file descriptors will receive an event with
117 epoll_wait(2). The default in this scenario (when EPOLLEXCLU‐
118 SIVE is not set) is for all epoll file descriptors to receive an
119 event. EPOLLEXCLUSIVE is thus useful for avoiding thundering
120 herd problems in certain scenarios.
121 If the same file descriptor is in multiple epoll instances, some
123 with the EPOLLEXCLUSIVE flag, and others without, then events
124 will be provided to all epoll instances that did not specify
125 EPOLLEXCLUSIVE, and at least one of the epoll instances that did
126 specify EPOLLEXCLUSIVE.
127 The following values may be specified in conjunction with
129 EPOLLEXCLUSIVE: EPOLLIN, EPOLLOUT, EPOLLWAKEUP, and EPOLLET.
130 EPOLLHUP and EPOLLERR can also be specified, but this is not
131 required: as usual, these events are always reported if they
132 occur, regardless of whether they are specified in events.
133 Attempts to specify other values in events yield the error EIN‐
134 VAL.
135 EPOLLEXCLUSIVE may be used only in an EPOLL_CTL_ADD operation;
137 attempts to employ it with EPOLL_CTL_MOD yield an error. If
138 EPOLLEXCLUSIVE has been set using epoll_ctl(), then a subsequent
139 EPOLL_CTL_MOD on the same epfd, fd pair yields an error. A call
140 to epoll_ctl() that specifies EPOLLEXCLUSIVE in events and spec‐
141 ifies the target file descriptor fd as an epoll instance will
142 likewise fail. The error in all of these cases is EINVAL.
143
145 When successful, epoll_ctl() returns zero. When an error occurs,
146 epoll_ctl() returns -1 and errno is set appropriately.
147
149 EBADF epfd or fd is not a valid file descriptor.
150 EEXIST op was EPOLL_CTL_ADD, and the supplied file descriptor fd is
152 already registered with this epoll instance.
153 EINVAL epfd is not an epoll file descriptor, or fd is the same as epfd,
155 or the requested operation op is not supported by this inter‐
156 face.
157 EINVAL An invalid event type was specified along with EPOLLEXCLUSIVE in
159 events.
160 EINVAL op was EPOLL_CTL_MOD and events included EPOLLEXCLUSIVE.
162 EINVAL op was EPOLL_CTL_MOD and the EPOLLEXCLUSIVE flag has previously
164 been applied to this epfd, fd pair.
165 EINVAL EPOLLEXCLUSIVE was specified in event and fd refers to an epoll
167 instance.
168 ELOOP fd refers to an epoll instance and this EPOLL_CTL_ADD operation
170 would result in a circular loop of epoll instances monitoring
171 one another.
172 ENOENT op was EPOLL_CTL_MOD or EPOLL_CTL_DEL, and fd is not registered
174 with this epoll instance.
175 ENOMEM There was insufficient memory to handle the requested op control
177 operation.
178 ENOSPC The limit imposed by /proc/sys/fs/epoll/max_user_watches was
180 encountered while trying to register (EPOLL_CTL_ADD) a new file
181 descriptor on an epoll instance. See epoll(7) for further
182 details.
183 EPERM The target file fd does not support epoll. This error can occur
185 if fd refers to, for example, a regular file or a directory.
186
188 epoll_ctl() was added to the kernel in version 2.6.
189
191 epoll_ctl() is Linux-specific. Library support is provided in glibc
192 starting with version 2.3.2.
193
195 The epoll interface supports all file descriptors that support poll(2).
196
198 In kernel versions before 2.6.9, the EPOLL_CTL_DEL operation required a
199 non-null pointer in event, even though this argument is ignored. Since
200 Linux 2.6.9, event can be specified as NULL when using EPOLL_CTL_DEL.
201 Applications that need to be portable to kernels before 2.6.9 should
202 specify a non-null pointer in event.
203 If EPOLLWAKEUP is specified in flags, but the caller does not have the
205 CAP_BLOCK_SUSPEND capability, then the EPOLLWAKEUP flag is silently
206 ignored. This unfortunate behavior is necessary because no validity
207 checks were performed on the flags argument in the original implementa‐
208 tion, and the addition of the EPOLLWAKEUP with a check that caused the
209 call to fail if the caller did not have the CAP_BLOCK_SUSPEND capabil‐
210 ity caused a breakage in at least one existing user-space application
211 that happened to randomly (and uselessly) specify this bit. A robust
212 application should therefore double check that it has the
213 CAP_BLOCK_SUSPEND capability if attempting to use the EPOLLWAKEUP flag.
214
216 epoll_create(2), epoll_wait(2), poll(2), epoll(7)
217
219 This page is part of release 5.04 of the Linux man-pages project. A
220 description of the project, information about reporting bugs, and the
221 latest version of this page, can be found at
222 https://www.kernel.org/doc/man-pages/.
223Linux 2019-03-06 EPOLL_CTL(2)