EPOLL_CTL(2) Linux Programmer's Manual EPOLL_CTL(2)NAMEepoll_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 opera‐
tion 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 descrip‐
tor 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 sim‐
ple code to detect peer shutdown when using Edge Triggered moni‐
toring.)
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 neces‐
sary 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 neces‐
sary 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 inter‐
face.
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.
CONFORMING TOepoll_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.
SEE ALSOepoll_create(2), epoll_wait(2), poll(2), epoll(7)COLOPHON
This page is part of release 3.27 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/.
Linux 2010-08-29 EPOLL_CTL(2)
