EPOLL_CTL

Section: Linux Programmer's Manual (2)
Updated: 2009-01-17
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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 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 re-arm 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 on with this epoll instance.
ENOMEM: 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.
EPERM: The target file fd does not support epoll.

CONFORMING TO

epoll_ctl() is Linux-specific, and was introduced in kernel 2.5.44.

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.

COLOPHON

This page is part of release 3.22 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/.

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Time: 05:33:03 GMT, December 24, 2015