| KQUEUE(2) | System Calls Manual | KQUEUE(2) |
kqueue, kevent
— kernel event notification mechanism
Standard C Library (libc, -lc)
#include
<sys/event.h>
int
kqueue(void);
int
kevent(int
kq, const struct kevent
*changelist, int
nchanges, struct kevent
*eventlist, int
nevents, const struct
timespec *timeout);
EV_SET(kev,
ident,
filter,
flags,
fflags,
data,
udata);
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() 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).
However, if rfork(2) is called without the
RFFDG flag, then the descriptor table is shared,
which will allow sharing of the kqueue between two processes.
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 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.
The kevent structure is defined as:
struct kevent {
uintptr_t ident; /* identifier for this event */
short filter; /* filter for event */
u_short flags; /* action flags for kqueue */
u_int 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:
The flags field can contain the following values:
EV_ADDEV_ENABLEkevent()
to return the event if it is triggered.EV_DISABLEkevent() will not return it.
The filter itself is not disabled.EV_DISPATCHEV_DISABLE above.EV_DELETEEV_RECEIPTEV_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_ONESHOTEV_CLEAREV_EOFEV_ERRORThe 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.
EVFILT_READlisten()
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. On return, data
contains the number of bytes of protocol 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 connection is gone) while there is still
data pending in the socket buffer.
This behavior is different from poll(2),
where read events are triggered for regular files unconditionally.
This event can be triggered unconditionally by setting the
NOTE_FILE_POLL flag in
fflags.
When the last writer disconnects, the filter will set
EV_EOF in flags. This
will be cleared by the filter when a new writer connects, at which
point the filter will resume waiting for data to become available
before returning.
EVFILT_WRITEEV_EOF when
the reader disconnects, and for the fifo case, this will be cleared when a
new reader connects. Note that this filter is not supported for vnodes or
BPF devices.
For sockets, the low water mark and socket error handling is
identical to the EVFILT_READ case.
EVFILT_EMPTYEVFILT_AIOkevent()
directly but are registered via the aio_sigevent
member of an asynchronous I/O request when it is scheduled via an
asynchronous I/O system call such as
aio_read().
The filter returns under the same conditions as
aio_error().
For more details on this filter see sigevent(3) and
aio(4).EVFILT_VNODENOTE_ATTRIBNOTE_CLOSENOTE_CLOSE_WRITEThis note, as well as NOTE_CLOSE,
are not activated when files are closed forcibly by
unmount(2) or revoke(2).
Instead, NOTE_REVOKE is sent for such
events.
NOTE_DELETEunlink()
system call was called on the file referenced by the descriptor.NOTE_EXTENDFor directory, reports that a directory entry was added or
removed, as the result of rename operation. The
NOTE_EXTEND event is not reported when a
name is changed inside the directory.
NOTE_LINKNOTE_LINK event is reported if a subdirectory
was created or deleted inside the directory referenced by the
descriptor.NOTE_OPENNOTE_READNOTE_RENAMENOTE_REVOKENOTE_WRITEOn return, fflags contains the events which triggered the filter.
EVFILT_PROCNOTE_EXITNOTE_FORKfork().NOTE_EXECNOTE_TRACKfork() calls. The
parent process registers a new kevent to monitor the child process
using the same fflags as the original event. The
child process will signal an event with
NOTE_CHILD set in fflags
and the parent PID in data.
If the parent process fails to register a new kevent
(usually due to resource limitations), it will signal an event with
NOTE_TRACKERR set in
fflags, and the child process will not signal
a NOTE_CHILD event.
On return, fflags contains the events which triggered the filter.
EVFILT_PROCDESCNOTE_EXITOn return, fflags contains the events which triggered the filter.
EVFILT_SIGNALsignal()
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, except for the
SIGCHLD signal, which, if ignored, will not be
recorded by the filter. 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_TIMERNOTE_ABSTIME) or the timeout
period. The timer will be periodic unless
EV_ONESHOT or NOTE_ABSTIME
is specified. On return, data contains the number of
times the timeout has expired since the last call to
kevent(). For non-monotonic timers, this filter
automatically sets the EV_CLEAR flag internally.
The filter accepts the following flags in the fflags argument:
NOTE_SECONDSNOTE_MSECONDSNOTE_USECONDSNOTE_NSECONDSNOTE_ABSTIMEIf fflags is not set, the default is milliseconds. On return, fflags contains the events which triggered the filter.
If an existing timer is re-added, the existing timer will be effectively canceled (throwing away any undelivered record of previous timer expiration) and re-started using the new parameters contained in data and fflags.
There is a system wide limit on the number of timers which is controlled by the kern.kq_calloutmax sysctl.
EVFILT_USERNOTE_FFNOPNOTE_FFANDNOTE_FFORNOTE_FFCOPYNOTE_FFCTRLMASKNOTE_FFLAGSMASKA user event is triggered for output with the following:
NOTE_TRIGGEROn return, fflags contains the users defined flags in the lower 24 bits.
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.
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() 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.
#include <sys/event.h>
#include <err.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int
main(int argc, char **argv)
{
struct kevent event; /* Event we want to monitor */
struct kevent tevent; /* Event triggered */
int kq, fd, ret;
if (argc != 2)
err(EXIT_FAILURE, "Usage: %s path\n", argv[0]);
fd = open(argv[1], O_RDONLY);
if (fd == -1)
err(EXIT_FAILURE, "Failed to open '%s'", argv[1]);
/* Create kqueue. */
kq = kqueue();
if (kq == -1)
err(EXIT_FAILURE, "kqueue() failed");
/* Initialize kevent structure. */
EV_SET(&event, fd, EVFILT_VNODE, EV_ADD | EV_CLEAR, NOTE_WRITE,
0, NULL);
/* Attach event to the kqueue. */
ret = kevent(kq, &event, 1, NULL, 0, NULL);
if (ret == -1)
err(EXIT_FAILURE, "kevent register");
if (event.flags & EV_ERROR)
errx(EXIT_FAILURE, "Event error: %s", strerror(event.data));
for (;;) {
/* Sleep until something happens. */
ret = kevent(kq, NULL, 0, &tevent, 1, NULL);
if (ret == -1) {
err(EXIT_FAILURE, "kevent wait");
} else if (ret > 0) {
printf("Something was written in '%s'\n", argv[1]);
}
}
}
The kqueue() system call fails if:
ENOMEM]ENOMEM]RLIMIT_KQUEUES rlimit (see
getrlimit(2)) for the current user would be
exceeded.EMFILE]ENFILE]The kevent() system call fails if:
EACCES]EFAULT]EBADF]EINTR]EINTR]EINVAL]ENOENT]ENOMEM]ESRCH]When kevent() call fails with
EINTR error, all changes in the
changelist have been applied.
aio_error(2), aio_read(2), aio_return(2), poll(2), read(2), select(2), sigaction(2), write(2), pthread_setcancelstate(3), signal(3)
Jonathan Lemon, Kqueue: A Generic and Scalable Event Notification Facility, Proceedings of the FREENIX Track: 2001 USENIX Annual Technical Conference, USENIX Association, June 25-30, 2001.
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>.
The timeout value is limited to 24 hours; longer timeouts will be silently reinterpreted as 24 hours.
In versions older than FreeBSD 12.0,
<sys/event.h> failed to
parse without including
<sys/types.h> manually.
| May 1, 2020 | Debian |