epoll_ctl(2) - Linux manual page (original) (raw)
epollctl(2) System Calls Manual epollctl(2)
NAME top
epoll_ctl - control interface for an epoll file descriptorLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#include <sys/epoll.h>**
**int epoll_ctl(int** _epfd_**, int** _op_**, int** _fd_**,**
**struct epoll_event *_Nullable** _event_**);**DESCRIPTION top
This system call is used to add, modify, or remove entries in the
interest list of the [epoll(7)](../man7/epoll.7.html) 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)](../man7/epoll.7.html) and [open(2)](../man2/open.2.html)), 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 epollevent_ is described in
[epoll_event(3type)](../man3/epoll%5Fevent.3type.html).
The _data_ member of the _epollevent_ structure specifies data that
the kernel should save and then return (via [epoll_wait(2)](../man2/epoll%5Fwait.2.html)) when
this file descriptor becomes ready.
The _events_ member of the _epollevent_ structure is a bit mask
composed by ORing together zero or more event types, returned by
[epoll_wait(2)](../man2/epoll%5Fwait.2.html), 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)](../man2/read.2.html) operations.
**EPOLLOUT**
The associated file is available for [write(2)](../man2/write.2.html) 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)](../man2/poll.2.html).
**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)](../man2/epoll%5Fwait.2.html) 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)](../man2/epoll%5Fwait.2.html) 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)](../man7/epoll.7.html) 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)](../man2/epoll%5Fwait.2.html), 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)](../man2/epoll%5Fwait.2.html) until the next call to [epoll_wait(2)](../man2/epoll%5Fwait.2.html)
on the same [epoll(7)](../man7/epoll.7.html) 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)](../man2/epoll%5Fwait.2.html). 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**.RETURN VALUE top
When successful, **epoll_ctl**() returns zero. When an error occurs,
**epoll_ctl**() returns -1 and _[errno](../man3/errno.3.html)_ is set to indicate the error.ERRORS top
**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/maxuserwatches_
was encountered while trying to register (**EPOLL_CTL_ADD**) a
new file descriptor on an epoll instance. See [epoll(7)](../man7/epoll.7.html) 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.STANDARDS top
Linux.HISTORY top
Linux 2.6, glibc 2.3.2.NOTES top
The **epoll** interface supports all file descriptors that support
[poll(2)](../man2/poll.2.html).BUGS top
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.SEE ALSO top
[epoll_create(2)](../man2/epoll%5Fcreate.2.html), [epoll_wait(2)](../man2/epoll%5Fwait.2.html), [ioctl_eventpoll(2)](../man2/ioctl%5Feventpoll.2.html), [poll(2)](../man2/poll.2.html),
[epoll(7)](../man7/epoll.7.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-07-23 epollctl(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)