1 .\" Copyright (C) 2003 Davide Libenzi
2 .\" Davide Libenzi <davidel@xmailserver.org>
4 .\" %%%LICENSE_START(GPLv2+_SW_3_PARA)
5 .\" This program is free software; you can redistribute it and/or modify
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13 .\" GNU General Public License for more details.
15 .\" You should have received a copy of the GNU General Public
16 .\" License along with this manual; if not, see
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20 .TH EPOLL_CTL 2 2015-05-07 "Linux" "Linux Programmer's Manual"
22 epoll_ctl \- control interface for an epoll file descriptor
24 .B #include <sys/epoll.h>
26 .BI "int epoll_ctl(int " epfd ", int " op ", int " fd \
27 ", struct epoll_event *" event );
29 This system call performs control operations on the
32 referred to by the file descriptor
34 It requests that the operation
36 be performed for the target file descriptor,
44 Register the target file descriptor
48 instance referred to by the file descriptor
50 and associate the event
52 with the internal file linked to
58 associated with the target file descriptor
62 Remove (deregister) the target file descriptor
66 instance referred to by
70 is ignored and can be NULL (but see BUGS below).
74 argument describes the object linked to the file descriptor
82 typedef union epoll_data {
90 uint32_t events; /* Epoll events */
91 epoll_data_t data; /* User data variable */
98 member is a bit mask composed using the following available event
102 The associated file is available for
107 The associated file is available for
111 .BR EPOLLRDHUP " (since Linux 2.6.17)"
112 Stream socket peer closed connection,
113 or shut down writing half of connection.
114 (This flag is especially useful for writing simple code to detect
115 peer shutdown when using Edge Triggered monitoring.)
118 There is urgent data available for
123 Error condition happened on the associated file descriptor.
125 will always wait for this event; it is not necessary to set it in
129 Hang up happened on the associated file descriptor.
131 will always wait for this event; it is not necessary to set it in
133 Note that when reading from a channel such as a pipe or a stream socket,
134 this event merely indicates that the peer closed its end of the channel.
135 Subsequent reads from the channel will return 0 (end of file)
136 only after all outstanding data in the channel has been consumed.
139 Sets the Edge Triggered behavior for the associated file descriptor.
140 The default behavior for
145 for more detailed information about Edge and Level Triggered event
146 distribution architectures.
148 .BR EPOLLONESHOT " (since Linux 2.6.2)"
149 Sets the one-shot behavior for the associated file descriptor.
150 This means that after an event is pulled out with
152 the associated file descriptor is internally disabled and no other events
153 will be reported by the
160 to rearm the file descriptor with a new event mask.
162 .BR EPOLLWAKEUP " (since Linux 3.5)"
163 .\" commit 4d7e30d98939a0340022ccd49325a3d70f7e0238
168 are clear and the process has the
171 ensure that the system does not enter "suspend" or
172 "hibernate" while this event is pending or being processed.
173 The event is considered as being "processed" from the time
174 when it is returned by a call to
176 until the next call to
181 the closure of that file descriptor,
182 the removal of the event file descriptor with
186 for the event file descriptor with
190 .BR EPOLLEXCLUSIVE " (since Linux 4.5)"
191 Sets an exclusive wakeup mode for the epoll file descriptor that is being
192 attached to the target file descriptor,
194 When a wakeup event occurs and multiple epoll file descriptors
195 are attached to the same target file using
197 one or more of the epoll file descriptors will receive an event with
199 The default in this scenario (when
201 is not set) is for all epoll file descriptors to receive an event.
203 is thus useful for avoiding thundering herd problems in certain scenarios.
205 If the same file descriptor is in multiple epoll instances,
208 flag, and others without, then events will provided to all epoll
209 instances that did not specify
211 and at least one of the epoll instances that did specify
214 The following values may be specified in conjunction with
224 can also be specified, but this is not required:
225 as usual, these events are always reported if they occur,
226 regardless of whether they are specified in
228 Attempts to specify other values in
232 may be used only in an
234 operation; attempts to employ it with
245 pair yields an error.
252 and specifies the target file descriptor
254 as an epoll instance will likewise fail.
255 The error in all of these cases is
261 When an error occurs,
265 is set appropriately.
272 is not a valid file descriptor.
278 and the supplied file descriptor
280 is already registered with this epoll instance.
291 or the requested operation
293 is not supported by this interface.
297 refers to an epoll instance and this
299 operation would result in a circular loop of epoll instances
300 monitoring one another.
303 An invalid event type was specified along with
323 flag has previously been applied to this
333 refers to an epoll instance.
343 is not registered with this epoll instance.
346 There was insufficient memory to handle the requested
352 .I /proc/sys/fs/epoll/max_user_watches
353 was encountered while trying to register
354 .RB ( EPOLL_CTL_ADD )
355 a new file descriptor on an epoll instance.
365 This error can occur if
367 refers to, for example, a regular file or a directory.
370 was added to the kernel in version 2.6.
371 .\" To be precise: kernel 2.5.44.
372 .\" The interface should be finalized by Linux kernel 2.5.66.
376 Library support is provided in glibc starting with version 2.3.2.
380 interface supports all file descriptors that support
383 In kernel versions before 2.6.9, the
385 operation required a non-null pointer in
387 even though this argument is ignored.
390 can be specified as NULL
393 Applications that need to be portable to kernels before 2.6.9
394 should specify a non-null pointer in
401 but the caller does not have the
402 .BR CAP_BLOCK_SUSPEND
406 .IR "silently ignored" .
407 This unfortunate behavior is necessary because no validity
408 checks were performed on the
410 argument in the original implementation, and the addition of the
412 with a check that caused the call to fail if the caller did not have the
414 capability caused a breakage in at least one existing user-space
415 application that happened to randomly (and uselessly) specify this bit.
416 .\" commit a8159414d7e3af7233e7a5a82d1c5d85379bd75c (behavior change)
417 .\" https://lwn.net/Articles/520198/
418 A robust application should therefore double check that it has the
420 capability if attempting to use the
424 .BR epoll_create (2),