epoll_ctl(2) System Calls Manual epoll_ctl(2)
NAME
epoll_ctl - control interface for an epoll file descriptor
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <sys/epoll.h>
int epoll_ctl(int epfd, int op, int fd, struct epoll_event *_Nullable event);
DESCRIPTION
This system call is used to add, modify, or remove entries in the in-
terest 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 an entry to the interest list of the epoll file descriptor,
epfd. The entry includes the file descriptor, fd, a reference
to the corresponding open file description (see epoll(7) and
open(2)), and the settings specified in event.
EPOLL_CTL_MOD
Change the settings associated with fd in the interest list to
the new settings specified in event.
EPOLL_CTL_DEL
Remove (deregister) the target file descriptor fd from the in-
terest list. The event 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 described in epoll_event(3type).
The data member of the epoll_event structure specifies data that the
kernel should save and then return (via epoll_wait(2)) when this file
descriptor becomes ready.
The events member of the epoll_event structure is a bit mask composed
by ORing together zero or more event types, returned by epoll_wait(2),
and input flags, which affect its behaviour, but aren't returned. The
available event types are:
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 sim-
ple code to detect peer shutdown when using edge-triggered moni-
toring.)
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 nec-
essary to set it in events when calling epoll_ctl().
EPOLLHUP
Hang up happened on the associated file descriptor.
epoll_wait(2) will always wait for this event; it is not neces-
sary to set it in events when calling epoll_ctl().
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.
And the available input flags are:
EPOLLET
Requests edge-triggered notification for the associated file de-
scriptor. The default behavior for epoll is level-triggered.
See epoll(7) for more detailed information about edge-triggered
and level-triggered notification.
EPOLLONESHOT (since Linux 2.6.2)
Requests one-shot notification for the associated file descrip-
tor. This means that after an event notified for the file de-
scriptor by epoll_wait(2), the file descriptor is disabled in
the interest list 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) un-
til the next call to epoll_wait(2) on the same epoll(7) file de-
scriptor, the closure of that file descriptor, the removal of
the event file descriptor with EPOLL_CTL_DEL, or the clearing 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 at-
tached 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 EPOLLEXCLU-
SIVE 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 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 re-
quired: as usual, these events are always reported if they oc-
cur, regardless of whether they are specified in events. At-
tempts to specify other values in events yield the error EINVAL.
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 subsequent
EPOLL_CTL_MOD on the same epfd, fd pair yields an error. A call
to epoll_ctl() that specifies EPOLLEXCLUSIVE in events and spec-
ifies the target file descriptor fd 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 to indicate the error.
ERRORS
EBADF epfd or fd is not a valid file descriptor.
EEXIST op was EPOLL_CTL_ADD, and the supplied file descriptor fd is al-
ready 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 inter-
face.
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 or a nesting depth of epoll instances greater than
5.
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 en-
countered while trying to register (EPOLL_CTL_ADD) a new file
descriptor on an epoll instance. See epoll(7) for further de-
tails.
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.
VERSIONS
epoll_ctl() was added to in Linux 2.6. Library support is provided in
glibc 2.3.2.
STANDARDS
epoll_ctl() is Linux-specific.
NOTES
The epoll interface supports all file descriptors that support poll(2).
BUGS
Before Linux 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. Appli-
cations that need to be portable to kernels before Linux 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 ig-
nored. This unfortunate behavior is necessary because no validity
checks were performed on the flags argument in the original implementa-
tion, 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 capabil-
ity 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.
SEE ALSO
epoll_create(2), epoll_wait(2), poll(2), epoll(7)
Linux man-pages 6.03 2022-12-04 epoll_ctl(2)
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