KQUEUE(2) BSD System Calls Manual KQUEUE(2)NAME
kqueue, kevent — 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);
EV_SET(&kev, ident, filter, flags, fflags, data, udata);
DESCRIPTION
The kqueue() system call provides a generic method of notifying the user
when an 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 is 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() 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. 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 struc‐
ture.
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 */
intptr_t data; /* filter data value */
void *udata; /* opaque user data identifier */
};
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.
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() 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_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-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 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 descrip‐
tor 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-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 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 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 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 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 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 handling
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_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 process.
This coexists with the signal() and sigaction() facili‐
ties, 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 This filter is currently unsupported.
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.
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() 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.
[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 April 14, 2000 BSD
