DESCRIPTION

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(2) on a file descriptor will remove any kevents that reference the descriptor.

kqueue() creates a new kernel event queue and returns a descriptor. The queue is not inherited by a child created with fork(2). Similarly, kqueues cannot be passed across UNIX-domain sockets.

kevent() is used to register events with the queue, and return any pending events to the user. changelist is a pointer to an array of kevent structures, as defined in <sys/event.h>. All changes contained in the changelist are applied before any pending events are read from the queue. nchanges gives the size of changelist. eventlist is a pointer to an array of kevent structures. nevents 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 not NULL, it specifies a maximum interval to wait for an event, which will be interpreted as a struct timespec. If timeout is NULL, kevent() waits indefinitely. To effect a poll, the timeout argument should not be NULL, pointing to a zero-valued struct timespec. The same array may be used for the changelist and eventlist.

EV_SET() is a macro which is provided for ease of initializing a kevent structure.

The kevent structure is defined as:

struct kevent {
	uintptr_t  ident;	/* identifier for this event */
	short	   filter;	/* filter for event */
	u_short	   flags;	/* action flags for kqueue */
	u_int	   fflags;	/* filter flag value */
	int64_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_DISPATCH

Disable the event source immediately after delivery of an event. See EV_DISABLE above.

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_RECEIPT

Causes kevent() to return with EV_ERROR set without draining any pending events after updating events in the kqueue. When a filter is successfully added the data field will be zero. This flag is useful for making bulk changes to a kqueue.

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(2) 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 in the socket buffer.

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. If NOTE_EOF is set in fflags, kevent() will also return when the file pointer is at the end of file. The end of file condition is indicated by the presence of NOTE_EOF in fflags on return.

FIFOs, Pipes

Returns when 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_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

unlink(2) 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_TRUNCATE

The file referenced by the descriptor was truncated.

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. The exit status will be stored in data in the same format as the status set by wait(2).

NOTE_FORK

The process has called fork(2).

NOTE_EXEC

The process has executed a new process via execve(2) or similar call.

NOTE_TRACK

Follow a process across fork(2) calls. The parent process will return with NOTE_FORK 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(3) and sigaction(2) 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_DEVICE

Takes a descriptor as the identifier and the events to watch for in fflags, and returns when one or more of the requested events occur on the descriptor. The events to monitor are:

NOTE_CHANGE

A device change event has occurred, e.g. an HDMI cable has been plugged in to a port.

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