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The filter is executed upon the initial registration of a kevent in order to detect whether a preexisting condition is present, and is also executed whenever an event is passed to the filter for evaluation. If the filter determines that the condition should be reported, then the kevent is placed on the kqueue for the user to retrieve.

The filter is also run when the user attempts to retrieve the kevent from the kqueue. If the filter indicates that the condition that triggered the event no longer holds, the kevent is removed from the kqueue and is not returned.

Multiple events which trigger the filter do not result in multiple kevents being placed on the kqueue; instead, the filter will aggregate the events into a single struct kevent. Calling close on a file descriptor will remove any kevents that reference the descriptor.

The kqueue system call creates a new kernel event queue and returns a descriptor. The queue is not inherited by a child created with fork(2). However, if rfork(2) is called without the RFFDG flag, then the descriptor table is shared, which will allow sharing of the kqueue between two processes.

The kevent system call is used to register events with the queue, and return any pending events to the user. The changelist argument is a pointer to an array of kevent structures, as defined in
.In sys/event.h . All changes contained in the changelist are applied before any pending events are read from the queue. The nchanges argument gives the size of changelist. The eventlist argument is a pointer to an array of kevent structures. The nevents argument determines the size of eventlist. When nevents is zero, kevent will return immediately even if there is a timeout specified unlike select(2). If timeout is a non-NULL pointer, it specifies a maximum interval to wait for an event, which will be interpreted as a struct timespec. If timeout is a NULL pointer, kevent waits indefinitely. To effect a poll, the timeout argument should be non-NULL, pointing to a zero-valued timespec structure. The same array may be used for the changelist and eventlist.

The EV_SET macro is provided for ease of initializing a kevent structure.

The kevent structure is defined as:
struct kevent {
uintptr_t ident; /* identifier for this event */
shortfilter; /* filter for event */
u_short flags; /* action flags for kqueue */
u_intfflags; /* filter flag value */
intptr_t data; /* filter data value */
void *udata; /* opaque user data identifier */
};

The fields of struct kevent are:

ident	Value used to identify this event. The exact interpretation is determined by the attached filter, but often is a file descriptor.
filter	Identifies the kernel filter used to process this event. The pre-defined system filters are described below.
flags	Actions to perform on the event.
fflags	Filter-specific flags.
data	Filter-specific data value.
udata	Opaque user-defined value passed through the kernel unchanged.

The flags field can contain the following values:

EV_ADD	Adds the event to the kqueue. Re-adding an existing event will modify the parameters of the original event, and not result in a duplicate entry. Adding an event automatically enables it, unless overridden by the EV_DISABLE flag.
EV_ENABLE	Permit `kevent` to return the event if it is triggered.
EV_DISABLE	Disable the event so `kevent` will not return it. The filter itself is not disabled.
EV_DELETE	Removes the event from the kqueue. Events which are attached to file descriptors are automatically deleted on the last close of the descriptor.
EV_ONESHOT	Causes the event to return only the first occurrence of the filter being triggered. After the user retrieves the event from the kqueue, it is deleted.
EV_CLEAR	After the event is retrieved by the user, its state is reset. This is useful for filters which report state transitions instead of the current state. Note that some filters may automatically set this flag internally.
EV_EOF	Filters may set this flag to indicate filter-specific EOF condition.
EV_ERROR	See RETURN VALUES below.

The predefined system filters are listed below. Arguments may be passed to and from the filter via the fflags and data fields in the kevent structure.

EVFILT_READ

Takes a descriptor as the identifier, and returns whenever there is data available to read. The behavior of the filter is slightly different depending on the descriptor type.

Sockets
	Sockets which have previously been passed to `listen` return when there is an incoming connection pending. data contains the size of the listen backlog. Other socket descriptors return when there is data to be read, subject to the `SO_RCVLOWAT` value of the socket buffer. This may be overridden with a per-filter low water mark at the time the filter is added by setting the NOTE_LOWAT flag in fflags, and specifying the new low water mark in data. On return, data contains the number of bytes of protocol data available to read. If the read direction of the socket has shutdown, then the filter also sets EV_EOF in flags, and returns the socket error (if any) in fflags. It is possible for EOF to be returned (indicating the connection is gone) while there is still data pending in the socket buffer.
Vnodes
	Returns when the file pointer is not at the end of file. data contains the offset from current position to end of file, and may be negative.
"Fifos, Pipes"
	Returns when the there is data to read; data contains the number of bytes available. When the last writer disconnects, the filter will set EV_EOF in flags. This may be cleared by passing in EV_CLEAR, at which point the filter will resume waiting for data to become available before returning.
"BPF devices"
	Returns when the BPF buffer is full, the BPF timeout has expired, or when the BPF has "immediate mode" enabled and there is any data to read; data contains the number of bytes available.

EVFILT_WRITE
	Takes a descriptor as the identifier, and returns whenever it is possible to write to the descriptor. For sockets, pipes and fifos, data will contain the amount of space remaining in the write buffer. The filter will set EV_EOF when the reader disconnects, and for the fifo case, this may be cleared by use of EV_CLEAR. Note that this filter is not supported for vnodes or BPF devices. For sockets, the low water mark and socket error handling is identical to the EVFILT_READ case.
EVFILT_AIO
	The sigevent portion of the AIO request is filled in, with sigev_notify_kqueue containing the descriptor of the kqueue that the event should be attached to, sigev_value containing the udata value, and sigev_notify set to SIGEV_KEVENT. When the `aio_` system call is made, the event will be registered with the specified kqueue, and the ident* argument set to the struct aiocb returned by the `aio_*` system call. The filter returns under the same conditions as aio_error.
EVFILT_VNODE
	Takes a file descriptor as the identifier and the events to watch for in fflags, and returns when one or more of the requested events occurs on the descriptor. The events to monitor are:

NOTE_DELETE	The `unlink` system call was called on the file referenced by the descriptor.
NOTE_WRITE	A write occurred on the file referenced by the descriptor.
NOTE_EXTEND	The file referenced by the descriptor was extended.
NOTE_ATTRIB	The file referenced by the descriptor had its attributes changed.
NOTE_LINK	The link count on the file changed.
NOTE_RENAME	The file referenced by the descriptor was renamed.
NOTE_REVOKE	Access to the file was revoked via revoke(2) or the underlying file system was unmounted.

On return, fflags contains the events which triggered the filter.

EVFILT_PROC

Takes the process ID to monitor as the identifier and the events to watch for in fflags, and returns when the process performs one or more of the requested events. If a process can normally see another process, it can attach an event to it. The events to monitor are:

NOTE_EXIT	The process has exited.
NOTE_FORK	The process has called `fork`.
NOTE_EXEC	The process has executed a new process via execve(2) or similar call.
NOTE_TRACK	Follow a process across `fork` calls. The parent process will return with NOTE_TRACK set in the fflags field, while the child process will return with NOTE_CHILD set in fflags and the parent PID in data.
NOTE_TRACKERR	This flag is returned if the system was unable to attach an event to the child process, usually due to resource limitations.

On return, fflags contains the events which triggered the filter.

EVFILT_SIGNAL	Takes the signal number to monitor as the identifier and returns when the given signal is delivered to the process. This coexists with the `signal` and `sigaction` facilities, and has a lower precedence. The filter will record all attempts to deliver a signal to a process, even if the signal has been marked as SIG_IGN. Event notification happens after normal signal delivery processing. data returns the number of times the signal has occurred since the last call to `kevent`. This filter automatically sets the EV_CLEAR flag internally.
EVFILT_TIMER	Establishes an arbitrary timer identified by ident. When adding a timer, data specifies the timeout period in milliseconds. The timer will be periodic unless EV_ONESHOT is specified. On return, data contains the number of times the timeout has expired since the last call to `kevent`. This filter automatically sets the EV_CLEAR flag internally.
`EVFILT_NETDEV`	Takes a descriptor to a network interface as the identifier, and the events to watch for in fflags. It returns, when one or more of the requested events occur on the descriptor. The events to monitor are:
`NOTE_LINKUP`	The link is up.
`NOTE_LINKDOWN`	The link is down.
`NOTE_LINKINV`	The link state is invalid.

On return, fflags contains the events which triggered the filter.

RETURN VALUES

The kqueue system call creates a new kernel event queue and returns a file descriptor. If there was an error creating the kernel event queue, a value of -1 is returned and errno set.

The kevent system call returns the number of events placed in the eventlist, up to the value given by nevents. If an error occurs while processing an element of the changelist and there is enough room in the eventlist, then the event will be placed in the eventlist with EV_ERROR set in flags and the system error in data. Otherwise, -1 will be returned, and errno will be set to indicate the error condition. If the time limit expires, then kevent returns 0.

ERRORS

The kqueue system call fails if:

[`ENOMEM`]
	The kernel failed to allocate enough memory for the kernel queue.
[`EMFILE`]
	The per-process descriptor table is full.
[`ENFILE`]
	The system file table is full.

The kevent system call fails if:

[`EACCES`]
	The process does not have permission to register a filter.
[`EFAULT`]
	There was an error reading or writing the kevent structure.
[`EBADF`]
	The specified descriptor is invalid.
[`EINTR`]
	A signal was delivered before the timeout expired and before any events were placed on the kqueue for return.
[`EINVAL`]
	The specified time limit or filter is invalid.
[`ENOENT`]
	The event could not be found to be modified or deleted.
[`ENOMEM`]
	No memory was available to register the event.
[`ESRCH`]
	The specified process to attach to does not exist.

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