epoll_ctl - control interface for an epoll file descriptor
#include <sys/epoll.h>
int epoll_ctl(int epfd, int op, int
fd, struct epoll_event *event);
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.
When successful, epoll_ctl() returns zero. When an error
occurs, epoll_ctl() returns -1 and errno is set
appropriately.
- 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.
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.
The epoll interface supports all file descriptors that
support poll(2).
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.
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