.\" Copyright (C) 2003 Davide Libenzi .\" Davide Libenzi .\" .\" %%%LICENSE_START(GPLv2+_SW_3_PARA) .\" This program is free software; you can redistribute it and/or modify .\" it under the terms of the GNU General Public License as published by .\" the Free Software Foundation; either version 2 of the License, or .\" (at your option) any later version. .\" .\" This program is distributed in the hope that it will be useful, .\" but WITHOUT ANY WARRANTY; without even the implied warranty of .\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the .\" GNU General Public License for more details. .\" .\" You should have received a copy of the GNU General Public .\" License along with this manual; if not, see .\" . .\" %%%LICENSE_END .\" .TH EPOLL_CTL 2 2016-10-08 "Linux" "Linux Programmer's Manual" .SH NAME epoll_ctl \- control interface for an epoll file descriptor .SH SYNOPSIS .B #include .sp .BI "int epoll_ctl(int " epfd ", int " op ", int " fd \ ", struct epoll_event *" event ); .SH DESCRIPTION This system call performs control operations on 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 . Valid values for the .I op argument are: .TP .B EPOLL_CTL_ADD Register the target file descriptor .I fd on the .B epoll instance referred to by the file descriptor .I epfd and associate the event .I event with the internal file linked to .IR fd . .TP .B EPOLL_CTL_MOD Change the event .I event associated with the target file descriptor .IR fd . .TP .B EPOLL_CTL_DEL Remove (deregister) the target file descriptor .I fd from the .B epoll instance referred to by .IR epfd . The .I event 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 defined as: .sp .in +4n .nf typedef union epoll_data { void *ptr; int fd; uint32_t u32; uint64_t u64; } epoll_data_t; struct epoll_event { uint32_t events; /* Epoll events */ epoll_data_t data; /* User data variable */ }; .fi .in The .I events member is a bit mask composed using the following available event types: .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 urgent data available for .BR read (2) operations. .TP .B EPOLLERR Error condition happened on the associated file descriptor. .BR epoll_wait (2) will always wait for this event; it is not necessary to set it in .IR events . .TP .B EPOLLHUP Hang up happened on the associated file descriptor. .BR epoll_wait (2) will always wait for this event; it is not necessary to set it in .IR events . 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. .TP .B EPOLLET Sets the Edge Triggered behavior for the associated file descriptor. The default behavior for .B epoll is Level Triggered. See .BR epoll (7) for more detailed information about Edge and Level Triggered event distribution architectures. .TP .BR EPOLLONESHOT " (since Linux 2.6.2)" Sets the one-shot behavior for the associated file descriptor. This means that after an event is pulled out with .BR epoll_wait (2) the associated file descriptor is internally disabled 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 .BR EPOLLEXCLUSIVE is not set) is for all epoll file descriptors to receive an event. .BR EPOLLEXCLUSIVE is thus useful for avoiding thundering herd problems in certain scenarios. If the same file descriptor is in multiple epoll instances, some with the .BR 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 . The following values may be specified in conjunction with .BR EPOLLEXCLUSIVE : .BR EPOLLIN , .BR EPOLLOUT , .BR EPOLLWAKEUP, and .BR EPOLLET . .BR EPOLLHUP and .BR 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 an error. .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 appropriately. .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 .IR events included .BR EPOLLEXCLUSIVE . .TP .B EINVAL .I op was .B EPOLL_CTL_MOD and the .BR EPOLLEXCLUSIVE flag has previously been applied to this .IR epfd ",\ " fd pair. .TP .B EINVAL .BR EPOLLEXCLUSIVE was specified in .IR 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. .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 VERSIONS .BR epoll_ctl () was added to the kernel in version 2.6. .\" To be precise: kernel 2.5.44. .\" The interface should be finalized by Linux kernel 2.5.66. .SH CONFORMING TO .BR epoll_ctl () is Linux-specific. Library support is provided in glibc starting with version 2.3.2. .SH NOTES The .B epoll interface supports all file descriptors that support .BR poll (2). .SH BUGS In kernel versions before 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 2.6.9 should specify a non-null pointer in .IR event . If .B EPOLLWAKEUP is specified in .IR flags , but the caller does not have the .BR 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 .IR 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) .SH COLOPHON This page is part of release 4.10 of the Linux .I man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at \%https://www.kernel.org/doc/man\-pages/.