.\" Copyright (C) 2003 Davide Libenzi .\" Davide Libenzi .\" and Copyright 2009, 2014, 2016, 2018, 2019 Michael Kerrisk .\" .\" SPDX-License-Identifier: GPL-2.0-or-later .\" .TH epoll_ctl 2 2023-04-03 "Linux man-pages 6.04" .SH NAME epoll_ctl \- control interface for an epoll file descriptor .SH LIBRARY Standard C library .RI ( libc ", " \-lc ) .SH SYNOPSIS .nf .B #include .PP .BI "int epoll_ctl(int " epfd ", int " op ", int " fd , .BI " struct epoll_event *_Nullable " event ); .fi .SH DESCRIPTION This system call is used to add, modify, or remove entries in the interest list of the .BR epoll (7) instance referred to by the file descriptor .IR epfd . It requests that the operation .I op be performed for the target file descriptor, .IR fd . .PP Valid values for the .I op argument are: .TP .B EPOLL_CTL_ADD Add an entry to the interest list of the epoll file descriptor, .IR epfd . The entry includes the file descriptor, .IR fd , a reference to the corresponding open file description (see .BR epoll (7) and .BR open (2)), and the settings specified in .IR event . .TP .B EPOLL_CTL_MOD Change the settings associated with .I fd in the interest list to the new settings specified in .IR event . .TP .B EPOLL_CTL_DEL Remove (deregister) the target file descriptor .I fd from the interest list. The .I event argument is ignored and can be NULL (but see BUGS below). .PP The .I event argument describes the object linked to the file descriptor .IR fd . The .I struct epoll_event is described in .BR epoll_event (3type). .PP The .I data member of the .I epoll_event structure specifies data that the kernel should save and then return (via .BR epoll_wait (2)) when this file descriptor becomes ready. .PP The .I events member of the .I epoll_event structure is a bit mask composed by ORing together zero or more event types, returned by .BR epoll_wait (2), and input flags, which affect its behaviour, but aren't returned. The available event types are: .TP .B EPOLLIN The associated file is available for .BR read (2) operations. .TP .B EPOLLOUT The associated file is available for .BR write (2) operations. .TP .BR EPOLLRDHUP " (since Linux 2.6.17)" Stream socket peer closed connection, or shut down writing half of connection. (This flag is especially useful for writing simple code to detect peer shutdown when using edge-triggered monitoring.) .TP .B EPOLLPRI There is an exceptional condition on the file descriptor. See the discussion of .B POLLPRI in .BR poll (2). .TP .B EPOLLERR Error condition happened on the associated file descriptor. This event is also reported for the write end of a pipe when the read end has been closed. .IP .BR epoll_wait (2) will always report for this event; it is not necessary to set it in .I events when calling .BR epoll_ctl (). .TP .B EPOLLHUP Hang up happened on the associated file descriptor. .IP .BR epoll_wait (2) will always wait for this event; it is not necessary to set it in .I events when calling .BR epoll_ctl (). .IP Note that when reading from a channel such as a pipe or a stream socket, this event merely indicates that the peer closed its end of the channel. Subsequent reads from the channel will return 0 (end of file) only after all outstanding data in the channel has been consumed. .PP And the available input flags are: .TP .B EPOLLET Requests edge-triggered notification for the associated file descriptor. The default behavior for .B epoll is level-triggered. See .BR epoll (7) for more detailed information about edge-triggered and level-triggered notification. .TP .BR EPOLLONESHOT " (since Linux 2.6.2)" Requests one-shot notification for the associated file descriptor. This means that after an event notified for the file descriptor by .BR epoll_wait (2), the file descriptor is disabled in the interest list and no other events will be reported by the .B epoll interface. The user must call .BR epoll_ctl () with .B EPOLL_CTL_MOD to rearm the file descriptor with a new event mask. .TP .BR EPOLLWAKEUP " (since Linux 3.5)" .\" commit 4d7e30d98939a0340022ccd49325a3d70f7e0238 If .B EPOLLONESHOT and .B EPOLLET are clear and the process has the .B CAP_BLOCK_SUSPEND capability, ensure that the system does not enter "suspend" or "hibernate" while this event is pending or being processed. The event is considered as being "processed" from the time when it is returned by a call to .BR epoll_wait (2) until the next call to .BR epoll_wait (2) on the same .BR epoll (7) file descriptor, the closure of that file descriptor, the removal of the event file descriptor with .BR EPOLL_CTL_DEL , or the clearing of .B EPOLLWAKEUP for the event file descriptor with .BR EPOLL_CTL_MOD . See also BUGS. .TP .BR EPOLLEXCLUSIVE " (since Linux 4.5)" Sets an exclusive wakeup mode for the epoll file descriptor that is being attached to the target file descriptor, .IR fd . When a wakeup event occurs and multiple epoll file descriptors are attached to the same target file using .BR EPOLLEXCLUSIVE , one or more of the epoll file descriptors will receive an event with .BR epoll_wait (2). The default in this scenario (when .B EPOLLEXCLUSIVE is not set) is for all epoll file descriptors to receive an event. .B EPOLLEXCLUSIVE is thus useful for avoiding thundering herd problems in certain scenarios. .IP If the same file descriptor is in multiple epoll instances, some with the .B EPOLLEXCLUSIVE flag, and others without, then events will be provided to all epoll instances that did not specify .BR EPOLLEXCLUSIVE , and at least one of the epoll instances that did specify .BR EPOLLEXCLUSIVE . .IP The following values may be specified in conjunction with .BR EPOLLEXCLUSIVE : .BR EPOLLIN , .BR EPOLLOUT , .BR EPOLLWAKEUP , and .BR EPOLLET . .B EPOLLHUP and .B EPOLLERR can also be specified, but this is not required: as usual, these events are always reported if they occur, regardless of whether they are specified in .IR events . Attempts to specify other values in .I events yield the error .BR EINVAL . .IP .B EPOLLEXCLUSIVE may be used only in an .B EPOLL_CTL_ADD operation; attempts to employ it with .B EPOLL_CTL_MOD yield an error. If .B EPOLLEXCLUSIVE has been set using .BR epoll_ctl (), then a subsequent .B EPOLL_CTL_MOD on the same .IR epfd ,\~ fd pair yields an error. A call to .BR epoll_ctl () that specifies .B EPOLLEXCLUSIVE in .I events and specifies the target file descriptor .I fd as an epoll instance will likewise fail. The error in all of these cases is .BR EINVAL . .SH RETURN VALUE When successful, .BR epoll_ctl () returns zero. When an error occurs, .BR epoll_ctl () returns \-1 and .I errno is set to indicate the error. .SH ERRORS .TP .B EBADF .I epfd or .I fd is not a valid file descriptor. .TP .B EEXIST .I op was .BR EPOLL_CTL_ADD , and the supplied file descriptor .I fd is already registered with this epoll instance. .TP .B EINVAL .I epfd is not an .B epoll file descriptor, or .I fd is the same as .IR epfd , or the requested operation .I op is not supported by this interface. .TP .B EINVAL An invalid event type was specified along with .B EPOLLEXCLUSIVE in .IR events . .TP .B EINVAL .I op was .B EPOLL_CTL_MOD and .I events included .BR EPOLLEXCLUSIVE . .TP .B EINVAL .I op was .B EPOLL_CTL_MOD and the .B EPOLLEXCLUSIVE flag has previously been applied to this .IR epfd ,\~ fd pair. .TP .B EINVAL .B EPOLLEXCLUSIVE was specified in .I event and .I fd refers to an epoll instance. .TP .B ELOOP .I fd refers to an epoll instance and this .B EPOLL_CTL_ADD operation would result in a circular loop of epoll instances monitoring one another or a nesting depth of epoll instances greater than 5. .TP .B ENOENT .I op was .B EPOLL_CTL_MOD or .BR EPOLL_CTL_DEL , and .I fd is not registered with this epoll instance. .TP .B ENOMEM There was insufficient memory to handle the requested .I op control operation. .TP .B ENOSPC The limit imposed by .I /proc/sys/fs/epoll/max_user_watches was encountered while trying to register .RB ( EPOLL_CTL_ADD ) a new file descriptor on an epoll instance. See .BR epoll (7) for further details. .TP .B EPERM The target file .I fd does not support .BR epoll . This error can occur if .I fd refers to, for example, a regular file or a directory. .SH STANDARDS Linux. .SH HISTORY Linux 2.6, .\" To be precise: Linux 2.5.44. .\" The interface should be finalized by Linux 2.5.66. glibc 2.3.2. .SH NOTES The .B epoll interface supports all file descriptors that support .BR poll (2). .SH BUGS Before Linux 2.6.9, the .B EPOLL_CTL_DEL operation required a non-null pointer in .IR event , even though this argument is ignored. Since Linux 2.6.9, .I event can be specified as NULL when using .BR EPOLL_CTL_DEL . Applications that need to be portable to kernels before Linux 2.6.9 should specify a non-null pointer in .IR event . .PP If .B EPOLLWAKEUP is specified in .IR flags , but the caller does not have the .B CAP_BLOCK_SUSPEND capability, then the .B EPOLLWAKEUP flag is .IR "silently ignored" . This unfortunate behavior is necessary because no validity checks were performed on the .I flags argument in the original implementation, and the addition of the .B EPOLLWAKEUP with a check that caused the call to fail if the caller did not have the .B CAP_BLOCK_SUSPEND capability caused a breakage in at least one existing user-space application that happened to randomly (and uselessly) specify this bit. .\" commit a8159414d7e3af7233e7a5a82d1c5d85379bd75c (behavior change) .\" https://lwn.net/Articles/520198/ A robust application should therefore double check that it has the .B CAP_BLOCK_SUSPEND capability if attempting to use the .B EPOLLWAKEUP flag. .SH SEE ALSO .BR epoll_create (2), .BR epoll_wait (2), .BR poll (2), .BR epoll (7)