epoll_ctl - control interface for an epoll file descriptor
Standard C library (
libc,
-lc)
#include <sys/epoll.h>
int epoll_ctl(int epfd, int op, int fd, struct epoll_event *_Nullable event);
This system call is used to add, modify, or remove entries in the interest list
of 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
- Add an entry to the interest list of the epoll file
descriptor, epfd. The entry includes the file descriptor,
fd, a reference to the corresponding open file description (see
epoll(7) and open(2)), and the settings specified in
event.
- EPOLL_CTL_MOD
- Change the settings associated with fd in the
interest list to the new settings specified in event.
- EPOLL_CTL_DEL
- Remove (deregister) the target file descriptor fd
from the interest list. The event argument 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 described in
epoll_event(3type).
The
data member of the
epoll_event structure specifies data that
the kernel should save and then return (via
epoll_wait(2)) when this
file descriptor becomes ready.
The
events member of the
epoll_event structure is a bit mask
composed by ORing together zero or more event types, returned by
epoll_wait(2), and input flags, which affect its behaviour, but aren't
returned. The available event types are:
- 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 when calling
epoll_ctl().
- 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 when calling
epoll_ctl().
- 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.
And the available input flags are:
- EPOLLET
- Requests edge-triggered notification for the associated
file descriptor. The default behavior for epoll is level-triggered.
See epoll(7) for more detailed information about edge-triggered and
level-triggered notification.
-
EPOLLONESHOT (since Linux 2.6.2)
- Requests one-shot notification for the associated file
descriptor. This means that after an event notified for the file
descriptor by epoll_wait(2), the file descriptor is disabled in the
interest list 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 the error EINVAL.
-
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.
When successful,
epoll_ctl() returns zero. When an error occurs,
epoll_ctl() returns -1 and
errno is set to indicate the error.
- 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 or a nesting depth of epoll instances
greater than 5.
- 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.
epoll_ctl() was added to in Linux 2.6. Library support is provided in
glibc 2.3.2.
epoll_ctl() is Linux-specific.
The
epoll interface supports all file descriptors that support
poll(2).
Before Linux 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 Linux 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.
epoll_create(2),
epoll_wait(2),
poll(2),
epoll(7)