KQUEUE(2) FreeBSD System Calls Manual KQUEUE(2)
NAME
kqueue, kevent - kernel event notification mechanism
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#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);
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() 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:
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.
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.
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.
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() 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.
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.
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.
Fifos, Pipes
Returns when the 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 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.
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.
Eventfds
Returns when the counter is greater than 0; data
contains the counter value, which must be cast to
uint64_t.
Kqueues
Returns when pending events are present on the
queue; data contains the number of events
available.
EVFILT_WRITE Takes a 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 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.
For eventfds, data will contain the maximum value
that can be added to the counter without blocking.
EVFILT_EMPTY Takes a descriptor as the identifier, and returns
whenever there is no remaining data in the write
buffer.
EVFILT_AIO Events for this filter are not registered with
kevent() 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_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 referencing
the monitored file, was closed.
The closed file descriptor did
not have write access.
NOTE_CLOSE_WRITE A file descriptor referencing
the monitored file, was closed.
The closed file descriptor had
write access.
This 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_DELETE The unlink() system call was
called on the file referenced by
the descriptor.
NOTE_EXTEND For regular file, the file
referenced by the descriptor was
extended.
For 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_LINK The link count on the file
changed. In particular, the
NOTE_LINK event is reported if a
subdirectory was created or
deleted inside the directory
referenced by the descriptor.
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
status will be stored in data in
the same format as the status
returned by wait(2).
NOTE_FORK The process has called fork().
NOTE_EXEC The process has executed a new
process via execve(2) or a similar
call.
NOTE_TRACK Follow a process across fork()
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_PROCDESC Takes the process descriptor created by pdfork(2) to
monitor as the identifier and the events to watch for
in fflags, and returns when the associated process
performs one or more of the requested events. The
events to monitor are:
NOTE_EXIT The process has exited. The exit
status will be stored in data.
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
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, 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_TIMER Establishes an arbitrary timer identified by ident.
When adding a timer, data specifies the moment to
fire the timer (for NOTE_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_SECONDS data is in seconds.
NOTE_MSECONDS data is in milliseconds.
NOTE_USECONDS data is in microseconds.
NOTE_NSECONDS data is in nanoseconds.
NOTE_ABSTIME The specified expiration time is
absolute.
If fflags is not set, the default is milliseconds.
On return, fflags contains the events which triggered
the filter.
Periodic timers with a specified timeout of 0 will be
silently adjusted to timeout after 1 of the time
units specified by the requested precision in fflags.
If an absolute time is specified that has already
passed, then it is treated as if the current time
were specified and the event will fire as soon as
possible.
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_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 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
#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]);
}
}
}
ERRORS
The kqueue() system call fails if:
[ENOMEM] The kernel failed to allocate enough memory for the
kernel queue.
[ENOMEM] The RLIMIT_KQUEUES rlimit (see getrlimit(2)) for the
current user would be exceeded.
[EMFILE] The per-process descriptor table is full.
[ENFILE] The system file table is full.
The kevent() system call fails if:
[EACCES] The process does not have permission to register a
filter.
[EFAULT] There was an error reading or writing the kevent
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.
[EINTR] A cancellation request was delivered to the thread,
but not yet handled.
[EINVAL] The specified time limit or filter is invalid.
[EINVAL] The specified length of the event or change lists is
negative.
[ENOENT] The event could not be found to be modified or
deleted.
[ENOMEM] No memory was available to register the event or, in
the special case of a timer, the maximum number of
timers has been exceeded. This maximum is
configurable via the kern.kq_calloutmax sysctl.
[ESRCH] The specified process to attach to does not exist.
When kevent() call fails with EINTR error, all changes in the changelist
have been applied.
SEE ALSO
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.
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
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.
FreeBSD 13.1-RELEASE-p6 January 20, 2022 FreeBSD 13.1-RELEASE-p6
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