EPOLL_CTL(3C) Standard C Library Functions EPOLL_CTL(3C)


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(5) instance specified by epfd, associate the
file descriptor specified by fd with the event specified by
event.


EPOLL_CTL_DEL
For the epoll(5) 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(5) 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(5) 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(5) 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(5) instance.


NOTES


The epoll(5) 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(5) for
compatibility details and restrictions.


SEE ALSO


epoll_create(3C), epoll_wait(3C), port_create(3C), port_associate(3C),
port_get(3C), epoll(5)


June 29, 2020 EPOLL_CTL(3C)