.\"  Copyright (C) 2003  Davide Libenzi

.\"  Davide Libenzi <davidel@xmailserver.org>

.\"

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.\"

.TH EPOLL_CTL 2 2017-09-15 "Linux" "Linux Programmer's Manual"

.SH NAME

epoll_ctl \- control interface for an epoll file descriptor

.SH SYNOPSIS

.B #include <sys/epoll.h>

.PP

.BI "int epoll_ctl(int " epfd ", int " op ", int " fd \

", struct epoll_event *" event );

.SH DESCRIPTION

This system call performs control operations on the

.BR epoll (7)

instance

referred to by the file descriptor

.IR epfd .

It requests that the operation

.I op

be performed for the target file descriptor,

.IR fd .

.PP

Valid values for the

.I op

argument are:

.TP

.B EPOLL_CTL_ADD

Register the target file descriptor

.I fd

on the

.B epoll

instance referred to by the file descriptor

.I epfd

and associate the event

.I event

with the internal file linked to

.IR fd .

.TP

.B EPOLL_CTL_MOD

Change the event

.I event

associated with the target file descriptor

.IR fd .

.TP

.B EPOLL_CTL_DEL

Remove (deregister) the target file descriptor

.I fd

from the

.B epoll

instance referred to by

.IR epfd .

The

.I event

is ignored and can be NULL (but see BUGS below).

.PP

The

.I event

argument describes the object linked to the file descriptor

.IR fd .

The

.I struct epoll_event

is defined as:

.PP

.in +4n

.EX

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 */

};

.EE

.in

.PP

The

.I events

member is a bit mask composed by ORing together zero or more of

the following available event types:

.TP

.B EPOLLIN

The associated file is available for

.BR read (2)

operations.

.TP

.B EPOLLOUT

The associated file is available for

.BR write (2)

operations.

.TP

.BR 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.)

.TP

.B EPOLLPRI

There is an exceptional condition on the file descriptor.

See the discussion of

.B POLLPRI

in

.BR poll (2).

.TP

.B 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.

.BR epoll_wait (2)

will always report for this event; it is not necessary to set it in

.IR events .

.TP

.B EPOLLHUP

Hang up happened on the associated file descriptor.

.BR epoll_wait (2)

will always wait for this event; it is not necessary to set it in

.IR events .

.IP

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.

.TP

.B EPOLLET

Sets the Edge Triggered behavior for the associated file descriptor.

The default behavior for

.B epoll

is Level Triggered.

See

.BR epoll (7)

for more detailed information about Edge and Level Triggered event

distribution architectures.

.TP

.BR 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

.BR epoll_wait (2)

the associated file descriptor is internally disabled and no other events

will be reported by the

.B epoll

interface.

The user must call

.BR epoll_ctl ()

with

.B EPOLL_CTL_MOD

to rearm the file descriptor with a new event mask.

.TP

.BR EPOLLWAKEUP " (since Linux 3.5)"

.\" commit 4d7e30d98939a0340022ccd49325a3d70f7e0238

If

.B EPOLLONESHOT

and

.B EPOLLET

are clear and the process has the

.B 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

.BR epoll_wait (2)

until the next call to

.BR epoll_wait (2)

on the same

.BR epoll (7)

file descriptor,

the closure of that file descriptor,

the removal of the event file descriptor with

.BR EPOLL_CTL_DEL ,

or the clearing of

.B EPOLLWAKEUP

for the event file descriptor with

.BR EPOLL_CTL_MOD .

See also BUGS.

.TP

.BR EPOLLEXCLUSIVE " (since Linux 4.5)"

Sets an exclusive wakeup mode for the epoll file descriptor that is being

attached to the target file descriptor,

.IR fd .

When a wakeup event occurs and multiple epoll file descriptors

are attached to the same target file using

.BR EPOLLEXCLUSIVE ,

one or more of the epoll file descriptors will receive an event with

.BR epoll_wait (2).

The default in this scenario (when

.BR EPOLLEXCLUSIVE

is not set) is for all epoll file descriptors to receive an event.

.BR EPOLLEXCLUSIVE

is thus useful for avoiding thundering herd problems in certain scenarios.

.IP

If the same file descriptor is in multiple epoll instances,

some with the

.BR EPOLLEXCLUSIVE

flag, and others without, then events will be provided to all epoll

instances that did not specify

.BR EPOLLEXCLUSIVE ,

and at least one of the epoll instances that did specify

.BR EPOLLEXCLUSIVE .

.IP

The following values may be specified in conjunction with

.BR EPOLLEXCLUSIVE :

.BR EPOLLIN ,

.BR EPOLLOUT ,

.BR EPOLLWAKEUP,

and

.BR EPOLLET .

.BR EPOLLHUP

and

.BR 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

.IR events .

Attempts to specify other values in

.I events

yield an error.

.B EPOLLEXCLUSIVE

may be used only in an

.B EPOLL_CTL_ADD

operation; attempts to employ it with

.B EPOLL_CTL_MOD

yield an error.

If

.B EPOLLEXCLUSIVE

has been set using

.BR epoll_ctl (),

then a subsequent

.B EPOLL_CTL_MOD

on the same

.IR epfd ",\ " fd

pair yields an error.

A call to

.BR epoll_ctl ()

that specifies

.B EPOLLEXCLUSIVE

in

.I events

and specifies the target file descriptor

.I fd

as an epoll instance will likewise fail.

The error in all of these cases is

.BR EINVAL .

.SH RETURN VALUE

When successful,

.BR epoll_ctl ()

returns zero.

When an error occurs,

.BR epoll_ctl ()

returns \-1 and

.I errno

is set appropriately.

.SH ERRORS

.TP

.B EBADF

.I epfd

or

.I fd

is not a valid file descriptor.

.TP

.B EEXIST

.I op

was

.BR EPOLL_CTL_ADD ,

and the supplied file descriptor

.I fd

is already registered with this epoll instance.

.TP

.B EINVAL

.I epfd

is not an

.B epoll

file descriptor,

or

.I fd

is the same as

.IR epfd ,

or the requested operation

.I op

is not supported by this interface.

.TP

.B EINVAL

An invalid event type was specified along with

.B EPOLLEXCLUSIVE

in

.IR events .

.TP

.B EINVAL

.I op

was

.B EPOLL_CTL_MOD

and

.IR events

included

.BR EPOLLEXCLUSIVE .

.TP

.B EINVAL

.I op

was

.B EPOLL_CTL_MOD

and the

.BR EPOLLEXCLUSIVE

flag has previously been applied to this

.IR epfd ",\ " fd

pair.

.TP

.B EINVAL

.BR EPOLLEXCLUSIVE

was specified in

.IR event

and

.I fd

refers to an epoll instance.

.TP

.B ELOOP

.I fd

refers to an epoll instance and this

.B EPOLL_CTL_ADD

operation would result in a circular loop of epoll instances

monitoring one another.

.TP

.B ENOENT

.I op

was

.B EPOLL_CTL_MOD

or

.BR EPOLL_CTL_DEL ,

and

.I fd

is not registered with this epoll instance.

.TP

.B ENOMEM

There was insufficient memory to handle the requested

.I op

control operation.

.TP

.B ENOSPC

The limit imposed by

.I /proc/sys/fs/epoll/max_user_watches

was encountered while trying to register

.RB ( EPOLL_CTL_ADD )

a new file descriptor on an epoll instance.

See

.BR epoll (7)

for further details.

.TP

.B EPERM

The target file

.I fd

does not support

.BR epoll .

This error can occur if

.I fd

refers to, for example, a regular file or a directory.

.SH VERSIONS

.BR epoll_ctl ()

was added to the kernel in version 2.6.

.\" To be precise: kernel 2.5.44.

.\" The interface should be finalized by Linux kernel 2.5.66.

.SH CONFORMING TO

.BR epoll_ctl ()

is Linux-specific.

Library support is provided in glibc starting with version 2.3.2.

.SH NOTES

The

.B epoll

interface supports all file descriptors that support

.BR poll (2).

.SH BUGS

In kernel versions before 2.6.9, the

.B EPOLL_CTL_DEL

operation required a non-null pointer in

.IR event ,

even though this argument is ignored.

Since Linux 2.6.9,

.I event

can be specified as NULL

when using

.BR EPOLL_CTL_DEL .

Applications that need to be portable to kernels before 2.6.9

should specify a non-null pointer in

.IR event .

.PP

If

.B EPOLLWAKEUP

is specified in

.IR flags ,

but the caller does not have the

.BR CAP_BLOCK_SUSPEND

capability, then the

.B EPOLLWAKEUP

flag is

.IR "silently ignored" .

This unfortunate behavior is necessary because no validity

checks were performed on the

.IR flags

argument in the original implementation, and the addition of the

.B EPOLLWAKEUP

with a check that caused the call to fail if the caller did not have the

.B CAP_BLOCK_SUSPEND

capability caused a breakage in at least one existing user-space

application that happened to randomly (and uselessly) specify this bit.

.\" commit a8159414d7e3af7233e7a5a82d1c5d85379bd75c (behavior change)

.\" https://lwn.net/Articles/520198/

A robust application should therefore double check that it has the

.B CAP_BLOCK_SUSPEND

capability if attempting to use the

.B EPOLLWAKEUP

flag.

.SH SEE ALSO

.BR epoll_create (2),

.BR epoll_wait (2),

.BR poll (2),

.BR epoll (7)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I 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/.