KQUEUE(2) FreeBSD System Calls Manual KQUEUE(2)
NAME
kqueue, kqueue1, kevent, EV_SET - kernel event notification mechanism
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);
#include <sys/types.h>
#include <sys/event.h>
#include <sys/time.h>
#include <fcntl.h>
int
kqueue1(int flags);
DESCRIPTION
kqueue() 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.
kqueue() creates a new kernel event queue and returns a descriptor. The
queue is not inherited by a child created with fork(2). Similarly,
kqueues cannot be passed across UNIX-domain sockets.
The kqueue1() function is identical to kqueue() except that the close-on-
exec flag on the new file descriptor is determined by the O_CLOEXEC flag
in the flags argument.
kevent() is used to register events with the queue, and return any
pending 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.
When nevents is zero, kevent() will return immediately even if there is a
timeout specified, unlike select(2). If timeout is not NULL, it
specifies a maximum interval to wait for an event, which will be
interpreted as a struct timespec. If timeout is NULL, kevent() waits
indefinitely. To effect a poll, the timeout argument should not be NULL,
pointing to a zero-valued struct timespec. 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 */
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 */
};
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
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 Causes kevent() to return with EV_ERROR set without
draining any pending events after updating events in the
kqueue. When a filter is successfully added, the data
field will be zero. This flag is useful for making bulk
changes to a kqueue.
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(2)
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 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. If NOTE_EOF is
set in fflags, kevent() will also return when the file
pointer is at the end of file. The end of file
condition is indicated by the presence of NOTE_EOF in
fflags on return.
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_EXCEPT Takes a descriptor as the identifier, and returns whenever
one of the specified exceptional conditions has occurred
on the descriptor. Conditions are specified in fflags.
Currently, a filter can monitor the reception of out-of-
band data on a socket or pseudo terminal with NOTE_OOB.
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
may be cleared by use of EV_CLEAR. 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_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(2) 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_TRUNCATE The file referenced by the descriptor was
truncated.
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 file system 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. The exit status
will be stored in data in the same format
as the status set by wait(2).
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_FORK 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 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, 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.
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 with
CLOCK_REALTIME as the reference clock.
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.
If an existing timer is re-added, the existing timer and
related pending events will be cancelled. The timer will
be re-started using the timeout period data.
EVFILT_DEVICE Takes a descriptor as the identifier and the events to
watch for in fflags, and returns when one or more of the
requested events occur on the descriptor. The events to
monitor are:
NOTE_CHANGE A device change event has occurred, e.g. an
HDMI cable has been plugged in to a port.
On return, fflags contains the events which triggered the
filter.
RETURN VALUES
kqueue() and kqueue1() create 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.
ERRORS
The kqueue() and kqueue1() functions fail 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.
In addition, kqueue1() fails if:
[EINVAL] flags is invalid.
The kevent() function 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 ALSO
clock_gettime(2), poll(2), read(2), select(2), sigaction(2), wait(2),
write(2), signal(3)
HISTORY
The kqueue() and kevent() functions first appeared in FreeBSD 4.1 and
have been available since OpenBSD 2.9.
AUTHORS
The kqueue() system and this manual page were written by Jonathan Lemon
<
[email protected]>.
FreeBSD 14.1-RELEASE-p8 August 20, 2023 FreeBSD 14.1-RELEASE-p8