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
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 :

EPOLL_CTL_ADD
    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.

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

EPOLL_CTL_DEL
    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 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 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 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 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.

RETURN VALUE
When successful, epoll_ctl() returns zero. When an error occurs, epoll_ctl() 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 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.

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.

VERSIONS
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.

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

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 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.

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

COLOPHON
This page is part of release 3.44 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.