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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON |
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EPOLL_CTL(2) Linux Programmer's Manual EPOLL_CTL(2)
epoll_ctl - control interface for an epoll file descriptor
#include <sys/epoll.h>
int epoll_ctl(int epfd, int op, int fd, struct epoll_event *event);
This system call performs control operations on 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
Register the target file descriptor fd on the epoll instance
referred to by the file descriptor epfd and associate the
event event with the internal file linked to fd.
EPOLL_CTL_MOD
Change the event event associated with the target file
descriptor fd.
EPOLL_CTL_DEL
Remove (deregister) the target file descriptor fd from the
epoll instance referred to by epfd. The event 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 writ‐
ing simple code to detect peer shutdown when using Edge Trig‐
gered 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 nec‐
essary 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 Trig‐
gered. 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 call epoll_ctl() with EPOLL_CTL_MOD
to rearm the file descriptor with a new event mask.
EPOLLWAKEUP (since Linux 3.5)
If EPOLLONESHOT and EPOLLET are clear and the process has the
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 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 with EPOLL_CTL_DEL, or the clear‐
ing of EPOLLWAKEUP for the event file descriptor with
EPOLL_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 using EPOLLEXCLUSIVE, one
or more of the epoll file descriptors will receive an event
with epoll_wait(2). The default in this scenario (when
EPOLLEXCLUSIVE is not set) is for all epoll file descriptors
to receive an event. EPOLLEXCLUSIVE is thus useful for avoid‐
ing 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
specify EPOLLEXCLUSIVE, and at least one of the epoll
instances that did specify EPOLLEXCLUSIVE.
The following values may be specified in conjunction with
EPOLLEXCLUSIVE: EPOLLIN, EPOLLOUT, EPOLLWAKEUP, and EPOLLET.
EPOLLHUP and 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 events.
Attempts to specify other values in events yield an error.
EPOLLEXCLUSIVE may be used only in an EPOLL_CTL_ADD operation;
attempts to employ it with EPOLL_CTL_MOD yield an error. If
EPOLLEXCLUSIVE has been set using epoll_ctl(), then a subse‐
quent EPOLL_CTL_MOD on the same epfd, fd pair yields an error.
A call to epoll_ctl() that specifies EPOLLEXCLUSIVE in events
and specifies the target file descriptor fd as an epoll
instance will likewise fail. The error in all of these cases
is EINVAL.
When successful, epoll_ctl() returns zero. When an error occurs,
epoll_ctl() returns -1 and errno is set appropriately.
EBADF epfd or fd is not a valid file descriptor.
EEXIST op was EPOLL_CTL_ADD, and the supplied file descriptor fd is
already registered with this epoll instance.
EINVAL epfd is not an epoll file descriptor, or fd is the same as
epfd, or the requested operation op is not supported by this
interface.
EINVAL An invalid event type was specified along with EPOLLEXCLUSIVE
in events.
EINVAL op was EPOLL_CTL_MOD and events included EPOLLEXCLUSIVE.
EINVAL op was EPOLL_CTL_MOD and the EPOLLEXCLUSIVE flag has
previously been applied to this epfd, fd pair.
EINVAL EPOLLEXCLUSIVE was specified in event and fd refers to an
epoll instance.
ELOOP fd refers to an epoll instance and this EPOLL_CTL_ADD
operation would result in a circular loop of epoll instances
monitoring one another.
ENOENT op was EPOLL_CTL_MOD or EPOLL_CTL_DEL, and fd 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 support epoll. This error can
occur if fd refers to, for example, a regular file or a
directory.
epoll_ctl() was added to the kernel in version 2.6.
epoll_ctl() is Linux-specific. Library support is provided in glibc
starting with version 2.3.2.
The epoll interface supports all file descriptors that support
poll(2).
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.
epoll_create(2), epoll_wait(2), poll(2), epoll(7)
This page is part of release 4.15 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/.
Linux 2017-09-15 EPOLL_CTL(2)
Pages that refer to this page: epoll_create(2), epoll_wait(2), syscalls(2), sd_event_add_io(3), sd_event_get_fd(3), sd_notify(3), proc(5), epoll(7)
Copyright and license for this manual page
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