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.