mirror of https://github.com/mkerrisk/man-pages
480 lines
11 KiB
Groff
480 lines
11 KiB
Groff
.\" Copyright (C) 2003 Davide Libenzi
|
|
.\" Davide Libenzi <davidel@xmailserver.org>
|
|
.\" and Copyright 2009, 2014, 2016, 2018, 2019 Michael Kerrisk <tk.manpages@gmail.com>
|
|
.\"
|
|
.\" %%%LICENSE_START(GPLv2+_SW_3_PARA)
|
|
.\" This program is free software; you can redistribute it and/or modify
|
|
.\" it under the terms of the GNU General Public License as published by
|
|
.\" the Free Software Foundation; either version 2 of the License, or
|
|
.\" (at your option) any later version.
|
|
.\"
|
|
.\" This program is distributed in the hope that it will be useful,
|
|
.\" but WITHOUT ANY WARRANTY; without even the implied warranty of
|
|
.\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
|
|
.\" GNU General Public License for more details.
|
|
.\"
|
|
.\" You should have received a copy of the GNU General Public
|
|
.\" License along with this manual; if not, see
|
|
.\" <http://www.gnu.org/licenses/>.
|
|
.\" %%%LICENSE_END
|
|
.\"
|
|
.TH EPOLL_CTL 2 2020-04-11 "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 is used to add, modify, or remove
|
|
entries in the interest list of 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
|
|
Add an entry to the interest list of the epoll file descriptor,
|
|
.IR epfd .
|
|
The entry includes the file descriptor,
|
|
.IR fd ,
|
|
a reference to the corresponding open file description (see
|
|
.BR epoll (7)
|
|
and
|
|
.BR open (2)),
|
|
and the settings specified in
|
|
.IR event .
|
|
.TP
|
|
.B EPOLL_CTL_MOD
|
|
Change the settings associated with
|
|
.IR fd
|
|
in the interest list to the new settings specified in
|
|
.IR event .
|
|
.TP
|
|
.B EPOLL_CTL_DEL
|
|
Remove (deregister) the target file descriptor
|
|
.I fd
|
|
from the interest list.
|
|
The
|
|
.I event
|
|
argument 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 data
|
|
member of the
|
|
.I epoll_event
|
|
structure specifies data that the kernel should save and then return (via
|
|
.BR epoll_wait (2))
|
|
when this file descriptor becomes ready.
|
|
.PP
|
|
The
|
|
.I events
|
|
member of the
|
|
.I epoll_event
|
|
structure 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.
|
|
.IP
|
|
.BR epoll_wait (2)
|
|
will always report for this event; it is not necessary to set it in
|
|
.IR events
|
|
when calling
|
|
.BR epoll_ctl ().
|
|
.TP
|
|
.B EPOLLHUP
|
|
Hang up happened on the associated file descriptor.
|
|
.IP
|
|
.BR epoll_wait (2)
|
|
will always wait for this event; it is not necessary to set it in
|
|
.IR events
|
|
when calling
|
|
.BR epoll_ctl ().
|
|
.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
|
|
Requests edge-triggered notification for the associated file descriptor.
|
|
The default behavior for
|
|
.B epoll
|
|
is level-triggered.
|
|
See
|
|
.BR epoll (7)
|
|
for more detailed information about edge-triggered and
|
|
level-triggered notification.
|
|
.IP
|
|
This flag is an input flag for the
|
|
.I event.events
|
|
field when calling
|
|
.BR epoll_ctl ();
|
|
it is never returned by
|
|
.BR epoll_wait (2).
|
|
.TP
|
|
.BR 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
|
|
.BR epoll_wait (2),
|
|
the file descriptor is disabled in the interest list 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.
|
|
.IP
|
|
This flag is an input flag for the
|
|
.I event.events
|
|
field when calling
|
|
.BR epoll_ctl ();
|
|
it is never returned by
|
|
.BR epoll_wait (2).
|
|
.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.
|
|
.IP
|
|
This flag is an input flag for the
|
|
.I event.events
|
|
field when calling
|
|
.BR epoll_ctl ();
|
|
it is never returned by
|
|
.BR epoll_wait (2).
|
|
.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 the error
|
|
.BR EINVAL .
|
|
.IP
|
|
.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 .
|
|
.IP
|
|
The
|
|
.BR EPOLLEXCLUSIVE
|
|
flag is an input flag for the
|
|
.I event.events
|
|
field when calling
|
|
.BR epoll_ctl ();
|
|
it is never returned by
|
|
.BR epoll_wait (2).
|
|
.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)
|