|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|
|
NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | BUGS | SEE ALSO | COLOPHON |
|
|
|
epoll_ctl(2) System Calls Manual epoll_ctl(2)
epoll_ctl - control interface for an epoll file descriptor
Standard C library (libc, -lc)
#include <sys/epoll.h>
int epoll_ctl(int epfd, int op, int fd,
struct epoll_event *_Nullable event);
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 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
interest 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 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 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
necessary 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 descriptor. 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
descriptor. This means that after an event notified for
the file descriptor 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) 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 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 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 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 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 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 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 to indicate the error.
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 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 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.
Linux.
Linux 2.6, glibc 2.3.2.
The epoll interface supports all file descriptors that support
poll(2).
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. Applications 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 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), ioctl_eventpoll(2), poll(2),
epoll(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.15 2025-05-17 epoll_ctl(2)
Pages that refer to this page: epoll_create(2), epoll_wait(2), io_uring_enter2(2), io_uring_enter(2), signalfd(2), syscalls(2), epoll_event(3type), sd_event_add_io(3), sd_event_get_fd(3), sd_notify(3), proc_pid_fdinfo(5), epoll(7)
|
HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|