Tribblix: manual page: epoll

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(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