This system call performs control operations on the epoll 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; it and can be null
on Linux 2.6.9 or later.
The event argument describes the object linked to the file descriptor
fd. 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.
Description
This system call performs control operations on the epoll 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; it and can be null on Linux 2.6.9 or later.
The event argument describes the object linked to the file descriptor fd. 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.