NAME

epoll_ctl - control an epoll instance

SYNOPSIS

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

DESCRIPTION

The epoll_ctl() function executes the operation specified by op (as parameterized by event) on the epfd epoll instance. Valid values for op:

EPOLL_CTL_ADD

For the epoll(7) instance specified by epfd, associate the file descriptor specified by fd with the event specified by event.

EPOLL_CTL_DEL

For the epoll(7) instance specified by epfd, remove all event associations for the file descriptor specified by fd. event is ignored, and may be NULL.

EPOLL_CTL_MOD

For the epoll(7) instance specified by epfd, modify the event association for the file descriptor specified by fd to be that specified by event.

The event parameter has the following structure:

typedef union epoll_data {
	void    *ptr;
	int      fd;
	uint32_t u32;
	uint64_t u64;
} epoll_data_t;
struct epoll_event {
	uint32_t     events;
	epoll_data_t data;
};

The data field specifies the datum to be associated with the event and will be returned via epoll_wait(3C). The events field denotes both the desired events (when specified via epoll_ctl()) and the events that have occurred (when returned via epoll_wait(3C)). In either case, the events field is a bitmask constructed by a logical OR operation of any combination of the following event flags:

EPOLLIN

Data other than high priority data may be read without blocking. For streams, this flag is set in the returned events even if the message is of zero length.

EPOLLPRI

Normal data (priority band equals 0) may be read without blocking. For streams, this flag is set in the returned events even if the message is of zero length.

EPOLLOUT

Normal data (priority band equals 0) may be written without blocking.

EPOLLRDNORM

Normal data (priority band equals 0) may be read without blocking. For streams, this flag is set in the returned revents even if the message is of zero length.

EPOLLRDBAND

Data from a non-zero priority band may be read without blocking. For streams, this flag is set in the returned revents even if the message is of zero length.

EPOLLWRNORM

The same as EPOLLOUT.

EPOLLWRBAND

Priority data (priority band > 0) may be written. This event only examines bands that have been written to at least once.

EPOLLMSG

This exists only for backwards binary and source compatibility with Linux; it has no meaning and is ignored.

EPOLLERR

An error has occurred on the device or stream. This flag is only valid in the returned events field.

EPOLLHUP

A hangup has occurred on the stream. This event and EPOLLOUT are mutually exclusive; a stream can never be writable if a hangup has occurred. However, this event and EPOLLIN, EPOLLRDNORM, EPOLLRDBAND, EPOLLRDHUP or EPOLLPRI are not mutually exclusive. This flag is only valid in the events field returned from epoll_wait(3C); it is not used in the events field specified via epoll_ctl().

EPOLLRDHUP

The stream socket peer shutdown the writing half of the connection and no further data will be readable via the socket. This event is not mutually exclusive with EPOLLIN.

EPOLLEXCLUSIVE

This is present for binary compatibility and is effectively a no-op on illumos.

The flag was added to Linux in v4.5 to provide a means to mitigate thundering herd problems when multiple epoll instances contain the same event source. Set on a specified event source during EPOLL_CTL_ADD (and not allowed with EPOLL_CTL_MOD), it indicates that epoll should attempt to limit the scope of pollers woken when a shared target resource changes state. All pollers without the flag set in the event will be notified and one or more of those with it set will be as well.

EPOLLWAKEUP

This exists only for backwards binary and source compatibility with Linux; it has no meaning and is ignored.

EPOLLONESHOT

Sets the specified event to be in one-shot mode, whereby the event association with the epoll(7) instance specified by epfd is removed atomically as the event is returned via epoll_wait(3C). Use of this mode allows for resolution of some of the races inherent in multithreaded use of epoll_wait(3C).

EPOLLET

Sets the specified event to be edge-triggered mode instead of the default mode of level-triggered. In this mode, events will be induced by transitions on an event source rather than the state of the event source. While perhaps superficially appealing, this mode introduces several new potential failure modes for user-level software and should be used with caution.

RETURN VALUES

Upon successful completion, epoll_ctl() returns 0. If an error occurs, -1 is returned and errno is set to indicate the error.

ERRORS

epoll_ctl() will fail if:

EBADF

epfd is not a valid file descriptor.

EFAULT

The memory associated with event was not mapped.

EEXIST

The operation specified was EPOLL_CTL_ADD and the specified file descriptor is already associated with an event for the specified epoll(7) instance.

ENOENT

The operation specified was EPOLL_CTL_MOD or EPOLL_CTL_DEL and the specified file descriptor is not associated with an event for the specified epoll(7) instance.

NOTES

The epoll(7) facility is implemented for purposes of offering compatibility for Linux-borne applications; native applications should continue to prefer using event ports via the port_create(3C), port_associate(3C) and port_get(3C) interfaces. See epoll(7) for compatibility details and restrictions.