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NAME
epoll_ctl - control interface for an epoll file 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 mask composed by ORing together zero or more of 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 an exceptional condition on the file descriptor. See the discussion of
POLLPRI in poll(2).
EPOLLERR
Error condition happened on the associated file descriptor. This event is also
reported for the write end of a pipe when the read end has been closed.
epoll_wait(2) will always report 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.
Note that when reading from a channel such as a pipe or a stream socket, this event
merely indicates that the peer closed its end of the channel. Subsequent reads
from the channel will return 0 (end of file) only after all outstanding data in the
channel has been consumed.
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.
EPOLLWAKEUP (since Linux 3.5)
If EPOLLONESHOT and EPOLLET are clear and the process has the CAP_BLOCK_SUSPEND
capability, ensure that the system does not enter "suspend" or "hibernate" while
this event is pending or being processed. The event is considered as being
"processed" from the time when it is returned by a call to epoll_wait(2) until the
next call to epoll_wait(2) on the same epoll(7) file descriptor, the closure of
that file descriptor, the removal of the event file descriptor with EPOLL_CTL_DEL,
or the clearing of EPOLLWAKEUP for the event file descriptor with EPOLL_CTL_MOD.
See also BUGS.
EPOLLEXCLUSIVE (since Linux 4.5)
Sets an exclusive wakeup mode for the epoll file descriptor that is being attached
to the target file descriptor, fd. When a wakeup event occurs and multiple epoll
file descriptors are attached to the same target file using EPOLLEXCLUSIVE, one or
more of the epoll file descriptors will receive an event with epoll_wait(2). The
default in this scenario (when EPOLLEXCLUSIVE is not set) is for all epoll file
descriptors to receive an event. EPOLLEXCLUSIVE is thus useful for avoiding
thundering herd problems in certain scenarios.
If the same file descriptor is in multiple epoll instances, some with the
EPOLLEXCLUSIVE flag, and others without, then events will be provided to all epoll
instances that did not specify EPOLLEXCLUSIVE, and at least one of the epoll
instances that did specify EPOLLEXCLUSIVE.
The following values may be specified in conjunction with EPOLLEXCLUSIVE: EPOLLIN,
EPOLLOUT, EPOLLWAKEUP, and EPOLLET. EPOLLHUP and EPOLLERR can also be specified,
but this is not required: as usual, these events are always reported if they occur,
regardless of whether they are specified in events. Attempts to specify other
values in events yield an error. EPOLLEXCLUSIVE may be used only in an
EPOLL_CTL_ADD operation; attempts to employ it with EPOLL_CTL_MOD yield an error.
If EPOLLEXCLUSIVE has been set using epoll_ctl(), then a subsequent EPOLL_CTL_MOD
on the same epfd, fd pair yields an error. A call to epoll_ctl() that specifies
EPOLLEXCLUSIVE in events and specifies the target file descriptor fd as an epoll
instance will likewise fail. The error in all of these cases is EINVAL.
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.
EINVAL An invalid event type was specified along with EPOLLEXCLUSIVE in events.
EINVAL op was EPOLL_CTL_MOD and events included EPOLLEXCLUSIVE.
EINVAL op was EPOLL_CTL_MOD and the EPOLLEXCLUSIVE flag has previously been applied to
this epfd, fd pair.
EINVAL EPOLLEXCLUSIVE was specified in event and fd refers to an epoll instance.
ELOOP fd refers to an epoll instance and this EPOLL_CTL_ADD operation would result in a
circular loop of epoll instances monitoring one another.
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. This error can occur if fd refers to,
for example, a regular file or a directory.
VERSIONS
epoll_ctl() was added to the kernel in version 2.6.
CONFORMING TO
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.
If EPOLLWAKEUP is specified in flags, but the caller does not have the CAP_BLOCK_SUSPEND
capability, then the EPOLLWAKEUP flag is silently ignored. This unfortunate behavior is
necessary because no validity checks were performed on the flags argument in the original
implementation, and the addition of the EPOLLWAKEUP with a check that caused the call to
fail if the caller did not have the CAP_BLOCK_SUSPEND capability caused a breakage in at
least one existing user-space application that happened to randomly (and uselessly)
specify this bit. A robust application should therefore double check that it has the
CAP_BLOCK_SUSPEND capability if attempting to use the EPOLLWAKEUP flag.
SEE ALSO
epoll_create(2), epoll_wait(2), poll(2), epoll(7)
COLOPHON
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