KQUEUE(2) BSD System Calls Manual KQUEUE(2)NAME
kqueue, kevent, and kevent64 — 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);
EV_SET(&kev, ident, filter, flags, fflags, data, udata);
EV_SET64(&kev, ident, filter, flags, fflags, data, udata, ext[0],
ext[1]);
DESCRIPTION
The kqueue() system call 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 iden‐
tified by an (ident, filter) pair and specifies the interesting condi‐
tions to be notified about for that pair. An (ident, filter) pair can
only appear once in a given kqueue. Subsequent attempts to register the
same pair for a given kqueue will result in the replacement of the condi‐
tions being watched, not an addition.
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() and kevent64() 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 or kevent64_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 kevent or kevent64_s structures. The nevents
argument determines the size of eventlist. 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
kevent() and kevent64() wait indefinitely. To effect a poll, the timeout
argument should be non-NULL, pointing to a zero-valued timespec struc‐
ture. 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.
The kevent and kevent64_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 */
};
----
The fields of struct kevent and struct kevent64_s 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.
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.
----
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() and kevent64() to return the event if it
is triggered.
EV_DISABLE Disable the event so kevent() and kevent64() 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_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 or
kevent64_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. On
return, data contains the number of bytes of proto‐
col 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 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 the there is data to read; data con‐
tains 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.
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.
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_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 received on the port or port
set. When a message is recieved, the size of the message
is returned in data and if fflags is set to
MACH_RCV_MSG, a pointer to the message is returned in
ext[0].
EVFILT_TIMER Establishes an interval timer with the data timer iden‐
tified by ident. When adding a timer, data specifies
the timeout period and fflags can be set to one of the
following:
NOTE_SECONDS data is in seconds
NOTE_USECONDS data is in microseconds
NOTE_NSECONDS data is in nanoseconds
NOTE_ABSOLUTE data is an absolute timeout
NOTE_CRITICAL system makes a best effort to fire this
timer as scheduled.
NOTE_BACKGROUND
system has extra leeway to coalesce this
timer.
NOTE_LEEWAY ext[1] holds user-supplied slop in dead‐
line for timer coalescing.
If fflags is not set, the default is 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() or
kevent64(). This filter automatically sets the EV_CLEAR
flag internally.
----
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() and kevent64() 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() and kevent64() 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 ALSOaio_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
