KQUEUE(2) BSD System Calls Manual KQUEUE(2)NAME
kqueue, kqueue1, kevent — 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);
DESCRIPTIONkqueue() 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, fil‐
ter) 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 exe‐
cuted 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.
kqueue() creates a new kernel event queue and returns a descriptor.
The kqueue1() also allows to set the following flags on the returned file
descriptor:
O_CLOEXEC Set the close on exec property.
O_NONBLOCK Sets non-blocking I/O.
O_NOSIGPIPE Return EPIPE instead of raising SIGPIPE.
The queue is not inherited by a child created with fork(2).
kevent() is used to register events with the queue, and return any pend‐
ing 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.
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
structure. 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 */
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 */
intptr_t udata; /* opaque user data identifier */
};
The fields of struct kevent are:
ident Value used to identify this event. The exact interpre‐
tation 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 ker‐
nel 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.
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 trig‐
gered.
EV_DISABLE Disable the event so kevent() 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_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-spe‐
cific 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 */
};
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 fil‐
ter 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 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-fil‐
ter 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 con‐
tains 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 buf‐
fer.
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 wait‐
ing for data to become available before returning.
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_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_DELETE unlink() 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_FORK The process has called fork().
NOTE_EXEC The process has executed a new process
via execve(2) or similar call.
NOTE_TRACK Follow a process across fork() 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 limita‐
tions.
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() and sigaction()
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 notifica‐
tion happens after normal signal delivery processing.
data returns the number of times the signal has occurred
since the last call to kevent(). This filter automati‐
cally 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 mil‐
liseconds. 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.
RETURN VALUESkqueue() 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.
kevent() 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.
[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 ALSOioctl(2), poll(2), read(2), select(2), sigaction(2), write(2), signal(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.
BSD November 24, 2012 BSD
