epoll_ctl man page

epoll_ctl — control interface for an epoll file descriptor


#include <sys/epoll.h>

int epoll_ctl(int epfd, int op, int fd, struct epoll_event *event);


This system call performs control operations on 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:

Register the target file descriptor fd on the epoll instance referred to by the file descriptor epfd and associate the event event with the internal file linked to fd.
Change the event event associated with the target file descriptor fd.
Remove (deregister) the target file descriptor fd from the epoll instance referred to by epfd. The event 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 defined as:

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 */

The events member is a bit mask composed using the following available event types:

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 urgent data available for read(2) operations.
Error condition happened on the associated file descriptor. epoll_wait(2) will always wait for this event; it is not necessary to set it in events.
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. 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.
Sets the Edge Triggered behavior for the associated file descriptor. The default behavior for epoll is Level Triggered. See epoll(7) for more detailed information about Edge and Level Triggered event distribution architectures.
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 epoll_wait(2) the associated file descriptor is internally disabled 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 an error. 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.

Return Value

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


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.
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.


epoll_ctl() was added to the kernel in version 2.6.

Conforming to

epoll_ctl() is Linux-specific. Library support is provided in glibc starting with version 2.3.2.


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


In kernel versions before 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 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.

See Also

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

Referenced By

epoll(7), epoll_create(2), epoll_wait(2), proc(5), sd_event_add_io(3), syscalls(2).

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