Ubuntu Manpage: epoll_ctl - control interface for an epoll file descriptor

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 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.

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 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.

STANDARDS

       Linux.

HISTORY

       Linux 2.6, glibc 2.3.2.

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.
       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.