epoll_ctl - control interface for an epoll file descriptor


Standard C library (libc, -lc)


#include <sys/epoll.h>
int epoll_ctl(int epfd, int op, int fd,
 struct epoll_event *_Nullable event);


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:


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.


Change the settings associated with fd in the interest list to the new settings specified in event.


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:


The associated file is available for read(2) operations.


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.)


There is an exceptional condition on the file descriptor. See the discussion of POLLPRI in poll(2).


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().


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:


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 epfdfd 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.


When successful, epoll_ctl() returns zero. When an error occurs, epoll_ctl() returns -1 and errno is set to indicate the error.



epfd or fd is not a valid file descriptor.


op was EPOLL_CTL_ADD, and the supplied file descriptor fd is already registered with this epoll instance.


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.


An invalid event type was specified along with EPOLLEXCLUSIVE in events.


op was EPOLL_CTL_MOD and events included EPOLLEXCLUSIVE.


op was EPOLL_CTL_MOD and the EPOLLEXCLUSIVE flag has previously been applied to this epfdfd pair.


EPOLLEXCLUSIVE was specified in event and fd refers to an epoll instance.


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.


op was EPOLL_CTL_MOD or EPOLL_CTL_DEL, and fd is not registered with this epoll instance.


There was insufficient memory to handle the requested op control operation.


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.


The target file fd does not support epoll. This error can occur if fd refers to, for example, a regular file or a directory.




Linux 2.6, glibc 2.3.2.


The epoll interface supports all file descriptors that support poll(2).


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.


epoll_create(2), epoll_wait(2), poll(2), epoll(7)