KQUEUE(2) System Calls Manual KQUEUE(2)

kqueue, kevent, kevent64 and kevent_qoskernel event notification mechanism

Standard C Library (libc, -lc)

#include <sys/types.h>
#include <sys/event.h>
#include <sys/time.h>

int
kqueue(void);

int
kevent(int kq, const struct kevent *changelist, int nchanges, struct kevent *eventlist, int nevents, const struct timespec *timeout);

int
kevent64(int kq, const struct kevent64_s *changelist, int nchanges, struct kevent64_s *eventlist, int nevents, unsigned int flags, const struct timespec *timeout);

int
kevent_qos(int kq, const struct kevent_qos_s *changelist, int nchanges, struct kevent_qos_s *eventlist, int nevents, void *data_out, size_t *data_available, unsigned int flags);

EV_SET(&kev, ident, filter, flags, fflags, data, udata);

EV_SET64(&kev, ident, filter, flags, fflags, data, udata, ext[0], ext[1]);

EV_SET_QOS(&kev, ident, filter, flags, qos, udata, fflags, xflags, data, ext[0], ext[1], ext[2], ext[3]);

The () system call allocates a kqueue file descriptor. This file descriptor provides a generic method of notifying the user when a kernel event (kevent) happens or a condition holds, based on the results of small pieces of kernel code termed filters.

A kevent is identified by an (ident, filter, and optional udata value) tuple. It specifies the interesting conditions to be notified about for that tuple. An (ident, filter, and optional udata value) tuple can only appear once in a given kqueue. Subsequent attempts to register the same tuple for a given kqueue will result in the replacement of the conditions being watched, not an addition. Whether the udata value is considered as part of the tuple is controlled by the EV_UDATA_SPECIFIC flag on the kevent.

The filter identified in a kevent is executed upon the initial registration of that event 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 () on a file descriptor will remove any kevents that reference the descriptor.

The () system call creates a new kernel event queue and returns a descriptor. The queue is not inherited by a child created with fork(2).

The () kevent64() and () system calls are 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, kevent64_s or kevent_qos_s structures, as defined 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 out kevent, kevent64_s or kevent_qos_s structures. The nevents argument determines the size of eventlist.

The data_out argument provides space for extra out data provided by specific filters. The data_available argument's contents specified the space available in the data pool on input, and contains the amount still remaining on output. If the KEVENT_FLAG_STACK_DATA flag is specified on the system call, the data is allocated from the pool in stack order instead of typical heap order.

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, both () and kevent64() wait indefinitely. To effect a poll, the flags argument to kevent64() or kevent_qos() can include the KEVENT_FLAG_IMMEDIATE value to indicate an immediate timeout. Alternatively, 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 () macro is provided for ease of initializing a kevent structure. Similarly, () initializes a kevent64_s structure and () initializes a kevent_qos_s structure.

The kevent, kevent64_s and kevent_qos_s structures are defined as:

struct kevent {
	uintptr_t 	ident;		/* identifier for this event */
	int16_t		filter;		/* filter for event */
	uint16_t	flags;		/* general flags */
	uint32_t	fflags;		/* filter-specific flags */
	intptr_t  	data;		/* filter-specific data */
	void		*udata;		/* opaque user data identifier */
};

struct kevent64_s {
        uint64_t        ident;          /* identifier for this event */
	int16_t         filter;         /* filter for event */
	uint16_t        flags;          /* general flags */
	uint32_t        fflags;         /* filter-specific flags */
	int64_t         data;           /* filter-specific data */
	uint64_t        udata;          /* opaque user data identifier */
	uint64_t        ext[2];         /* filter-specific extensions */
};

struct kevent_qos_s {
        uint64_t        ident;          /* identifier for this event */
	int16_t         filter;         /* filter for event */
	uint16_t        flags;          /* general flags */
	uint32_t        qos;            /* quality of service when servicing event */
	uint64_t        udata;          /* opaque user data identifier */
	uint32_t        fflags;         /* filter-specific flags */
	uint32_t        xflags;         /* extra filter-specific flags */
	int64_t         data;           /* filter-specific data */
	uint64_t        ext[4];         /* filter-specific extensions */
};

----

The fields of struct kevent, struct kevent64_s and struct kevent_qos_s are:

ident
Value used to identify the source of the 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. It can optionally be part of the uniquing decision of the kevent system

In addition, struct kevent64_s contains:

ext[2]
This field stores extensions for the event's filter. What type of extension depends on what type of filter is being used.

In addition, struct kevent_qos_s contains:

xflags
Extra filter-specific flags.
ext[4]
The QoS variant provides twice as many extension values for filter-specific uses.

----

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 () kevent64() and () to return the event if it is triggered.
EV_DISABLE
Disable the event so kevent,() kevent64() and kevent_qos() 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_RECEIPT
This flag is useful for making bulk changes to a kqueue without draining any pending events. When passed as input, it forces EV_ERROR to always be returned. When a filter is successfully added, the data field will be zero.
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_OOBAND
Read filter on socket may set this flag to indicate the presence of out of band data on the descriptor.
EV_ERROR
See RETURN VALUES below.

----

The predefined system filters are listed below. Arguments may be passed to and from the filter via the data, fflags and optionally xflags fields in the kevent, kevent64_s or kevent_qos_s structure.

EVFILT_READ
Takes a file 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 () 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. The derived per filter low water mark value is, however, bounded by socket receive buffer's high and low water mark values. On return, data contains the number of bytes of protocol data available to read.

The presence of EV_OOBAND in flags, indicates the presence of out of band data on the socket data equal to the potential number of OOB bytes availble 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 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.

Device nodes
Returns when there is data to read from the device; data contains the number of bytes available. If the device does not support returning number of bytes, it will not allow the filter to be attached. However, if the NOTE_LOWAT flag is specified and the data field contains 1 on input, those devices will attach - but cannot be relied upon to provide an accurate count of bytes to be read on output.
Takes a descriptor as the identifier, and returns whenever one of the specified exceptional conditions has occurred on the descriptor. Conditions are specified in fflags. Currently, this filter can be used to monitor the arrival of out-of-band data on a socket descriptor using the filter flag NOTE_OOB.

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.

EVFILT_WRITE
Takes a file 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.

For sockets, the low water mark and socket error handling is identical to the EVFILT_READ case.

EVFILT_AIO
This filter is currently unsupported.
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:
The () 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 fileystem was unmounted.
NOTE_FUNLOCK
The file was unlocked by calling flock(2) or close(2)
NOTE_LEASE_DOWNGRADE
A lease break to downgrade the lease to read lease is requested on the file referenced by the descriptor.
NOTE_LEASE_RELEASE
A lease break to release the lease is requested on the file or directory referenced by the descriptor.

On return, fflags contains the filter-specific flags which are associated with the triggered events seen by this 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_EXITSTATUS
The process has exited and its exit status is in filter specific data. Valid only on child processes and to be used along with NOTE_EXIT.
NOTE_FORK
The process created a child process via fork(2) or similar call.
NOTE_EXEC
The process executed a new process via execve(2) or similar call.
NOTE_SIGNAL
The process was sent a signal. Status can be checked via waitpid(2) or similar call.
NOTE_REAP
The process was reaped by the parent via wait(2) or similar call. Deprecated, use NOTE_EXIT.

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 generated for the process. This coexists with the () and () facilities, and has a lower precedence. Only signals sent to the process, not to a particular thread, will trigger the filter. 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 before normal signal delivery processing. data returns the number of times the signal has been generated since the last call to kevent(). This filter automatically sets the EV_CLEAR flag internally.
EVFILT_MACHPORT
Takes the name of a mach port, or port set, in ident and waits until a message is enqueued on the port or port set. When a message is detected, but not directly received by the kevent call, the name of the specific port where the message is enqueued is returned in data. If fflags contains MACH_RCV_MSG, the ext[0] and ext[1] flags are assumed to contain a pointer to the buffer where the message is to be received and the size of the receive buffer, respectively. If MACH_RCV_MSG is specifed, yet the buffer size in ext[1] is zero, The space for the buffer may be carved out of the data_out area provided to kevent_qos() if there is enough space remaining there.
EVFILT_TIMER
Establishes an interval timer identified by ident where data specifies the timeout period (in milliseconds).

fflags can include one of the following flags to specify a different unit:

NOTE_SECONDS
data is in seconds
NOTE_USECONDS
data is in microseconds
NOTE_NSECONDS
data is in nanoseconds
NOTE_MACHTIME
data is in Mach absolute time units

fflags can also include NOTE_ABSOLUTE, which establishes an EV_ONESHOT timer with an absolute deadline instead of an interval. The absolute deadline is expressed in terms of gettimeofday(2). With NOTE_MACHTIME, the deadline is expressed in terms of ().

The timer can be coalesced with other timers to save power. The following flags can be set in fflags to modify this behavior:

NOTE_CRITICAL
override default power-saving techniques to more strictly respect the leeway value
NOTE_BACKGROUND
apply more power-saving techniques to coalesce this timer with other timers
NOTE_LEEWAY
ext[1] holds user-supplied slop in deadline for timer coalescing.

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 arming or last delivery of the timer event.

This filter automatically sets the EV_CLEAR flag.

----

In the ext[2] field of the kevent64_s struture, ext[0] is only used with the EVFILT_MACHPORT filter. With other filters, ext[0] is passed through () much like udata. ext[1] can always be used like udata. For the use of ext[0], see the EVFILT_MACHPORT filter above.

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(), kevent64() and kevent_qos() system calls return 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(), kevent64() and kevent_qos() return 0.

The kqueue() system call fails if:

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

The kevent() and kevent64() system calls fail if:

[]
The process does not have permission to register a filter.
[]
There was an error reading or writing the kevent or kevent64_s structure.
[]
The specified descriptor is invalid.
[]
A signal was delivered before the timeout expired and before any events were placed on the kqueue for return.
[]
The specified time limit or filter is invalid.
[]
The event could not be found to be modified or deleted.
[]
No memory was available to register the event.
[]
The specified process to attach to does not exist.

aio_error(2), aio_read(2), aio_return(2), read(2), select(2), sigaction(2), write(2), signal(3)

The kqueue() and kevent() system calls first appeared in FreeBSD 4.1.

The kqueue() system and this manual page were written by Jonathan Lemon ⟨jlemon@FreeBSD.org⟩.

Not all filesystem types support kqueue-style notifications. And even some that do, like some remote filesystems, may only support a subset of the notification semantics described here.

October 21, 2008 macOS 14.5