Section (2) epoll_ctl
Name
epoll_ctl — control interface for an epoll file descriptor
Synopsis
#include <sys/epoll.h>
int
epoll_ctl( |
int epfd, |
int op, | |
int fd, | |
struct epoll_event *event) ; |
DESCRIPTION
This system call is used to add, modify, or remove entries
in the interest list of the epoll(7) instance referred
to by the file descriptor epfd
. It requests that the
operation op
be
performed for the target file descriptor, fd
.
Valid values for the op
argument are:
EPOLL_CTL_ADD
-
Add
fd
to the interest list and associate the settings specified inevent
with the internal file linked tofd
. EPOLL_CTL_MOD
-
Change the settings associated with
fd
in the interest list to the new settings specified inevent
. EPOLL_CTL_DEL
-
Remove (deregister) the target file descriptor
fd
from the interest list. Theevent
argument is ignored and can be NULL (but see BUGS below).
The event
argument
describes the object linked to the file descriptor fd
. The struct epoll_event is defined
as:
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 */};
The events
member
is a bit mask composed by ORing together zero or more of the
following available event types:
EPOLLIN
-
The associated file is available for read(2) operations.
EPOLLOUT
-
The associated file is available for write(2) operations.
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.)
EPOLLPRI
-
There is an exceptional condition on the file descriptor. See the discussion of
POLLPRI
in poll(2). 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. epoll_wait(2) will always report for this event; it is not necessary to set it in
events
. EPOLLHUP
-
Hang up happened on the associated file descriptor. epoll_wait(2) will always wait for this event; it is not necessary to set it in
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.
EPOLLET
-
Sets the Edge Triggered behavior for the associated file descriptor. The default behavior for
epoll
is Level Triggered. See epoll(7) for more detailed information about Edge and Level Triggered event distribution architectures. 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 epoll_wait(2) the associated file descriptor is internally disabled and no other events will be reported by the
epoll
interface. The user must callepoll_ctl
() withEPOLL_CTL_MOD
to rearm the file descriptor with a new event mask. EPOLLWAKEUP
(since Linux 3.5)-
If
EPOLLONESHOT
andEPOLLET
are clear and the process has theCAP_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 epoll_wait(2) until the next call to epoll_wait(2) on the same epoll(7) file descriptor, the closure of that file descriptor, the removal of the event file descriptor withEPOLL_CTL_DEL
, or the clearing ofEPOLLWAKEUP
for the event file descriptor withEPOLL_CTL_MOD
. See also BUGS. EPOLLEXCLUSIVE
(since Linux 4.5)-
Sets an exclusive wakeup mode for the epoll file descriptor that is being attached to the target file descriptor,
fd
. When a wakeup event occurs and multiple epoll file descriptors are attached to the same target file usingEPOLLEXCLUSIVE
, one or more of the epoll file descriptors will receive an event with epoll_wait(2). The default in this scenario (whenEPOLLEXCLUSIVE
is not set) is for all epoll file descriptors to receive an event.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
EPOLLEXCLUSIVE
flag, and others without, then events will be provided to all epoll instances that did not specifyEPOLLEXCLUSIVE
, and at least one of the epoll instances that did specifyEPOLLEXCLUSIVE
.The following values may be specified in conjunction with
EPOLLEXCLUSIVE
:EPOLLIN
,EPOLLOUT
,EPOLLWAKEUP
, andEPOLLET
.EPOLLHUP
andEPOLLERR
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 inevents
. Attempts to specify other values inevents
yield the error EINVAL.EPOLLEXCLUSIVE
may be used only in anEPOLL_CTL_ADD
operation; attempts to employ it withEPOLL_CTL_MOD
yield an error. IfEPOLLEXCLUSIVE
has been set usingepoll_ctl
(), then a subsequentEPOLL_CTL_MOD
on the sameepfd
,fd
pair yields an error. A call toepoll_ctl
() that specifiesEPOLLEXCLUSIVE
inevents
and specifies the target file descriptorfd
as an epoll instance will likewise fail. The error in all of these cases is EINVAL.
RETURN VALUE
When successful, epoll_ctl
()
returns zero. When an error occurs, epoll_ctl
() returns −1 and
errno
is set appropriately.
ERRORS
- EBADF
-
epfd
orfd
is not a valid file descriptor. - EEXIST
-
op
wasEPOLL_CTL_ADD
, and the supplied file descriptorfd
is already registered with this epoll instance. - EINVAL
-
epfd
is not anepoll
file descriptor, orfd
is the same asepfd
, or the requested operationop
is not supported by this interface. - EINVAL
-
An invalid event type was specified along with
EPOLLEXCLUSIVE
inevents
. - EINVAL
-
op
wasEPOLL_CTL_MOD
andevents
includedEPOLLEXCLUSIVE
. - EINVAL
-
op
wasEPOLL_CTL_MOD
and theEPOLLEXCLUSIVE
flag has previously been applied to thisepfd
,fd
pair. - EINVAL
-
EPOLLEXCLUSIVE
was specified inevent
andfd
refers to an epoll instance. - ELOOP
-
fd
refers to an epoll instance and thisEPOLL_CTL_ADD
operation would result in a circular loop of epoll instances monitoring one another. - ENOENT
-
op
wasEPOLL_CTL_MOD
orEPOLL_CTL_DEL
, andfd
is not registered with this epoll instance. - ENOMEM
-
There was insufficient memory to handle the requested
op
control operation. - ENOSPC
-
The limit imposed by
/proc/sys/fs/epoll/max_user_watches
was encountered while trying to register (EPOLL_CTL_ADD
) a new file descriptor on an epoll instance. See epoll(7) for further details. - EPERM
-
The target file
fd
does not supportepoll
. This error can occur iffd
refers to, for example, a regular file or a directory.
CONFORMING TO
epoll_ctl
() is
Linux-specific. Library support is provided in glibc starting
with version 2.3.2.
BUGS
In kernel versions before 2.6.9, the EPOLL_CTL_DEL
operation required a non-null
pointer in event
,
even though this argument is ignored. Since Linux 2.6.9,
event
can be
specified as NULL when using EPOLL_CTL_DEL
. Applications that need to be
portable to kernels before 2.6.9 should specify a non-null
pointer in event
.
If EPOLLWAKEUP
is specified
in flags
, but the
caller does not have the CAP_BLOCK_SUSPEND
capability, then the
EPOLLWAKEUP
flag is
silently ignored. This
unfortunate behavior is necessary because no validity checks
were performed on the flags
argument in the
original implementation, and the addition of the EPOLLWAKEUP
with a check that caused the
call to fail if the caller did not have the CAP_BLOCK_SUSPEND
capability caused a
breakage in at least one existing user-space application that
happened to randomly (and uselessly) specify this bit. A
robust application should therefore double check that it has
the CAP_BLOCK_SUSPEND
capability if attempting to use the EPOLLWAKEUP
flag.
COLOPHON
This page is part of release 5.04 of the Linux 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/.
Copyright (C) 2003 Davide Libenzi Davide Libenzi <davidelxmailserver.org> %%%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 <http://www.gnu.org/licenses/>. %%%LICENSE_END |