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(7) 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 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 interface.
- 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.
VERSIONS¶
epoll_ctl() was added to the kernel in version 2.6.
epoll_ctl() is Linux-specific. Library support is provided in glibc
starting with version 2.3.2.
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 ALSO¶
epoll_create(2),
epoll_wait(2),
poll(2),
epoll(7)
COLOPHON¶
This page is part of release 3.44 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/.