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KQUEUE(2) BSD System Calls Manual KQUEUE(2)
NAME
kqueue, kevent, kevent64 and kevent_qos -- kernel event notification
mechanism
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#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]);
DESCRIPTION
The kqueue() 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 registra-
tion 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 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).
The kevent,() kevent64() and kevent_qos() system calls are used to regis-
ter 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 con-
tained 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. If the KEVENT_FLAG_STACK_EVENTS flag is provided on the sys-
tem call, the eventlist array is filled in in stack order (starting in
the highest available index) instead of typical array order. The
out_data 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 inter-
preted as a struct timespec. If timeout is a NULL pointer, both kevent()
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 EV_SET() macro is provided for ease of initializing a kevent struc-
ture. Similarly, EV_SET64() initializes a kevent64_s structure and
EV_SET_QOS() 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 automati-
cally enables it, unless overridden by the EV_DISABLE
flag.
EV_ENABLE Permit kevent,() kevent64() and kevent_qos() 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
listen() return when there is an incoming connection
pending. data contains the size of the listen back-
log.
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 con-
nection 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 posi-
tion 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 return-
ing.
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. How-
ever, 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.
EVFILT_EXCEPT Takes a descriptor as the identifier, and returns when-
ever 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 han-
dling 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:
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 fileystem was
unmounted.
NOTE_FUNLOCK The file was unlocked by calling flock(2)
or close(2)
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 signal() and
sigaction() 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 notifica-
tion 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 dead-
line is expressed in terms of mach_absolute_time().
The timer can be coalesced with other timers to save
power. The following flags can be set in fflags to mod-
ify this behavior:
NOTE_CRITICAL override default power-saving tech-
niques 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 speci-
fied. On return, data contains the number of times the
timeout has expired since the last arming or last deliv-
ery 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
kevent64() much like udata. ext[1] can always be used like udata. For
the use of ext[0], see the EVFILT_MACHPORT filter above.
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(), 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. Oth-
erwise, -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.
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() and kevent64() system calls fail if:
[EACCES] The process does not have permission to register a
filter.
[EFAULT] There was an error reading or writing the kevent or
kevent64_s 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.
SEE ALSO
aio_error(2), aio_read(2), aio_return(2), read(2), select(2),
sigaction(2), write(2), signal(3)
HISTORY
The kqueue() and kevent() system calls first appeared in FreeBSD 4.1.
AUTHORS
The kqueue() system and this manual page were written by Jonathan Lemon
<jlemon@FreeBSD.org>.
BUGS
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.
BSD October 21, 2008 BSD