EPOLL_CTL(2)

NAME
epoll_ctl - control interface for an epoll descriptor

SYNOPSIS
#include <sys/epoll.h>

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

DESCRIPTION
Control an epoll descriptor, epfd, by requesting that the operation op be performed on the target file descriptor, fd. The event 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 set composed using the following available event types:

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

EPOLLOUT
    The associated file is available for write(2) operations.

EPOLLRDHUP (since kernel 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 urgent data available for read(2) operations.

EPOLLERR
    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.

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.

EPOLLET
    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 kernel 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(2) with EPOLL_CTL_MOD to re-enable the file descriptor with a new event mask.

The epoll interface supports all file descriptors that support poll(2). Valid values for the op parameter are :

    EPOLL_CTL_ADD
        Add the target file descriptor fd to the epoll descriptor epfd and associate the event event with the internal file linked to fd.

    EPOLL_CTL_MOD
        Change the event event associated with the target file descriptor fd.

    EPOLL_CTL_DEL
        Remove the target file descriptor fd from the epoll file descriptor, epfd. The event is ignored and can be NULL (but see BUGS below).

RETURN VALUE
When successful, epoll_ctl(2) returns zero. When an error occurs, epoll_ctl(2) returns -1 and errno is set appropriately.

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 in epfd.

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.

ENOENT
    op was EPOLL_CTL_MOD or EPOLL_CTL_DEL, and fd is not in epfd.

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

EPERM
    The target file fd does not support epoll.

CONFORMING TO
epoll_ctl(2) is Linux specific, and was introduced in kernel 2.5.44.

BUGS
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 kernel 2.6.9, event can be specified as NULL when using EPOLL_CTL_DEL.

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