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KQUEUE(2) System Calls Manual KQUEUE(2)
NAME
kqueue, kqueue1, kevent, EV_SET - kernel event notification mechanism
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/event.h>
#include <sys/time.h>
int
kqueue(void);
int
kqueue1(int flags);
int
kevent(int kq, const struct kevent *changelist, size_t nchanges,
struct kevent *eventlist, size_t nevents,
const struct timespec *timeout);
EV_SET(&kev, ident, filter, flags, fflags, data, udata);
DESCRIPTION
The kqueue() system call provides a generic method of notifying the user
when an event happens or a condition holds, based on the results of small
pieces of kernel code termed filters. A kevent is identified by the
(ident, filter) pair; there may only be one unique kevent per kqueue.
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.
The kqueue() system call creates a new kernel event queue and returns a
descriptor.
The kqueue1() system call also allows to set the following flags on the
returned file descriptor:
O_CLOEXEC Set the close on exec property.
O_NONBLOCK Set non-blocking I/O.
O_NOSIGPIPE Return EPIPE instead of raising SIGPIPE.
The queue is not inherited by a child created with fork(2).
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 <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(3) 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. This macro does not evaluate its parameters multiple times.
The kevent structure is defined as:
struct kevent {
uintptr_t ident; /* identifier for this event */
uint32_t filter; /* filter for event */
uint32_t flags; /* action flags for kqueue */
uint32_t fflags; /* filter flag value */
int64_t data; /* filter data value */
void *udata; /* opaque user data identifier */
uint64_t ext[4]; /* extensions */
};
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.
There are pre-defined system filters (which are
described below), and other filters may be added by
kernel subsystems as necessary.
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.
ext Extended data passed to and from kernel. The ext[0] and
ext[1] members use is defined by the filter. If the
filter does not use them, the members are copied
unchanged. The ext[2] and ext[3] members are always
passed through the kernel as-is, making additional
context available to application.
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 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. Note that if this flag is
encountered and there is no remaining space in
eventlist to hold the EV_ERROR event, then
subsequent changes will not get processed.
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.
Filters
Filters are identified by a number. There are two types of filters; pre-
defined filters which are described below, and third-party filters that
may be added with kfilter_register(9) by kernel sub-systems, third-party
device drivers, or loadable kernel modules.
As a third-party filter is referenced by a well-known name instead of a
statically assigned number, two ioctl(2)s are supported on the file
descriptor returned by kqueue() to map a filter name to a filter number,
and vice-versa (passing arguments in a structure described below):
KFILTER_BYFILTER Map filter to name, which is of size len.
KFILTER_BYNAME Map name to filter. len is ignored.
The following structure is used to pass arguments in and out of the
ioctl(2):
struct kfilter_mapping {
char *name; /* name to lookup or return */
size_t len; /* length of name */
uint32_t filter; /* filter to lookup or return */
};
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.
The predefined system filters are:
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 (i.e.,
the number of connections ready to be accepted with
accept(2).)
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.
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, fifos, and ttys, 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_EMPTY Takes a descriptor as the identifier, and returns whenever
there is no remaining data in the write buffer. This is
currently implemented only for sockets. It's primary
purpose is to provide visibility to an application that
all previously written data has been acknowledged by the
receiver at the TCP layer.
EVFILT_AIO This is not implemented in NetBSD.
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_ATTRIB The file referenced by the descriptor
had its attributes changed.
NOTE_CLOSE A file descriptor without write access
referencing the file was closed.
NOTE_CLOSE_WRITE A file descriptor with write access
referencing the file was closed.
NOTE_DELETE unlink(2) was called on the file
referenced by the descriptor.
NOTE_EXTEND The file referenced by the descriptor
was extended.
NOTE_LINK The link count on the file changed.
NOTE_OPEN The file referenced by the descriptor
was opened.
NOTE_READ A read occurred on the file referenced
by the descriptor.
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.
NOTE_WRITE A write occurred on the file referenced
by the descriptor.
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 code of
the process is stored in data.
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_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 current
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 units
described below, or, if NOTE_ABSTIME is set in fflags,
specifies the absolute time at which the timer should
fire. The timer will repeat unless EV_ONESHOT is set in
flags or NOTE_ABSTIME is set in fflags. On return, data
contains the number of times the timeout has expired since
the last call to kevent(). This filter automatically sets
EV_CLEAR in flags for periodic timers. Timers created
with NOTE_ABSTIME remain activated on the kqueue once the
absolute time has passed unless EV_CLEAR or EV_ONESHOT are
also specified. CLOCK_REALTIME is the reference clock for
timers created with NOTE_ABSTIME.
The filter accepts the following flags in the fflags
argument:
NOTE_SECONDS The timer value in data is expressed in
seconds.
NOTE_MSECONDS The timer value in data is expressed in
milliseconds.
NOTE_USECONDS The timer value in data is expressed in
microseconds.
NOTE_NSECONDS The timer value in data is expressed in
nanoseconds.
NOTE_ABSTIME The timer value is an absolute time; see
discussion above.
Note that NOTE_SECONDS, NOTE_MSECONDS, NOTE_USECONDS, and
NOTE_NSECONDS are mutually exclusive; behavior is
undefined if more than one are specified. If a timer
value unit is not specified, the default is NOTE_MSECONDS.
EVFILT_FS Establishes a file system monitor. Currently it only
monitors file system mount and unmount actions.
EVFILT_USER Establishes a user event identified by ident which is not
associated with any kernel mechanism but is triggered by
user level code. The lower 24 bits of the fflags may be
used for user defined flags and manipulated using the
following:
NOTE_FFNOP Ignore the input fflags.
NOTE_FFAND Bitwise AND fflags.
NOTE_FFOR Bitwise OR fflags.
NOTE_FFCOPY Copy fflags.
NOTE_FFCTRLMASK Control mask for fflags.
NOTE_FFLAGSMASK User defined flag mask for fflags.
A user event is triggered for output with the following:
NOTE_TRIGGER Cause the event to be triggered.
On return, fflags contains the users defined flags in the
lower 24 bits.
CANCELLATION BEHAVIOUR
If nevents is non-zero, i.e., the function is potentially blocking, the
call is a cancellation point. Otherwise, i.e., if nevents is zero, the
call is not cancellable. Cancellation can only occur before any changes
are made to the kqueue, or when the call was blocked and no changes to
the queue were requested.
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 is 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.
EXAMPLES
The following example program monitors a file (provided to it as the
first argument) and prints information about some common events it
receives notifications for:
#include <sys/types.h>
#include <sys/event.h>
#include <sys/time.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <fcntl.h>
#include <err.h>
int
main(int argc, char *argv[])
{
int fd, kq, nev;
struct kevent ev;
static const struct timespec tout = { 1, 0 };
if ((fd = open(argv[1], O_RDONLY)) == -1)
err(1, "Cannot open `%s'", argv[1]);
if ((kq = kqueue()) == -1)
err(1, "Cannot create kqueue");
EV_SET(&ev, fd, EVFILT_VNODE, EV_ADD | EV_ENABLE | EV_CLEAR,
NOTE_DELETE|NOTE_WRITE|NOTE_EXTEND|NOTE_ATTRIB|NOTE_LINK|
NOTE_RENAME|NOTE_REVOKE, 0, 0);
if (kevent(kq, &ev, 1, NULL, 0, &tout) == -1)
err(1, "kevent");
for (;;) {
nev = kevent(kq, NULL, 0, &ev, 1, &tout);
if (nev == -1)
err(1, "kevent");
if (nev == 0)
continue;
if (ev.fflags & NOTE_DELETE) {
printf("deleted ");
ev.fflags &= ~NOTE_DELETE;
}
if (ev.fflags & NOTE_WRITE) {
printf("written ");
ev.fflags &= ~NOTE_WRITE;
}
if (ev.fflags & NOTE_EXTEND) {
printf("extended ");
ev.fflags &= ~NOTE_EXTEND;
}
if (ev.fflags & NOTE_ATTRIB) {
printf("chmod/chown/utimes ");
ev.fflags &= ~NOTE_ATTRIB;
}
if (ev.fflags & NOTE_LINK) {
printf("hardlinked ");
ev.fflags &= ~NOTE_LINK;
}
if (ev.fflags & NOTE_RENAME) {
printf("renamed ");
ev.fflags &= ~NOTE_RENAME;
}
if (ev.fflags & NOTE_REVOKE) {
printf("revoked ");
ev.fflags &= ~NOTE_REVOKE;
}
printf("\n");
if (ev.fflags)
warnx("unknown event 0x%x\n", ev.fflags);
}
}
ERRORS
The kqueue() function fails if:
[EMFILE] The per-process descriptor table is full.
[ENFILE] The system file table is full.
[ENOMEM] The kernel failed to allocate enough memory for the
kernel queue.
The kevent() function fails if:
[EACCES] The process does not have permission to register a
filter.
[EBADF] The specified descriptor is invalid.
[EFAULT] There was an error reading or writing the kevent
structure.
[EINTR] A signal was delivered before the timeout expired and
before any events were placed on the kqueue for
return. All changes contained in the changelist are
applied before returning this error.
[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.
[EOPNOTSUPP] This type of file descriptor is not supported for
kevent() operations.
[ESRCH] The specified process to attach to does not exist.
SEE ALSO
fork(2), ioctl(2), listen(2), poll(2), read(2), select(2), sigaction(2),
unlink(2), write(2), signal(3), timespec(3), kfilter_register(9),
knote(9)
Jonathan Lemon, "Kqueue: A Generic and Scalable Event Notification
Facility", Proceedings of the FREENIX Track: 2001 USENIX Annual Technical
Conference, USENIX Association,
http://www.usenix.org/event/usenix01/freenix01/full_papers/lemon/lemon.pdf,
June 25-30, 2001.
HISTORY
The kqueue() and kevent() functions first appeared in FreeBSD 4.1, and
then in NetBSD 2.0. The kqueue1() function first appeared in NetBSD 6.0.
The EV_SET() macro was protected from evaluating multiple times the first
argument in NetBSD 8.0.
The udata type was changed from intptr_t to void * in NetBSD 10.0.
Support for NOTE_SECONDS, NOTE_MSECONDS, NOTE_USECONDS, NOTE_NSECONDS,
and NOTE_ABSTIME filter flags for EVFILT_TIMER was added in NetBSD 10.0.
Support for NOTE_OPEN, NOTE_CLOSE, NOTE_CLOSE_WRITE, and NOTE_READ filter
flags for EVFILT_VNODE was added in NetBSD 10.0.
Support for EVFILT_EMPTY was added in NetBSD 10.0.
NetBSD 10.99 June 19, 2023 NetBSD 10.99