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 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 */
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 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.
- 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.
- 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 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_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_AIO
- The sigevent portion of the AIO request is filled in, with
sigev_notify_kqueue containing the
descriptor of the kqueue that the event should be attached to,
sigev_notify_kevent_flags containing the
kevent flags which should be EV_ONESHOT, EV_CLEAR or EV_DISPATCH,
sigev_value containing the udata value,
and sigev_notify set to SIGEV_KEVENT.
When the
aio_*
() system call is made,
the event will be registered with the specified kqueue, and the
ident argument set to the
struct aiocb returned by the
aio_*
() system call. The filter returns
under the same conditions as aio_error.
- 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 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.
- 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_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,
won't 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 timeout period in
milliseconds. 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. There is a system wide limit on the
number of timers which is controlled by the
kern.kq_calloutmax sysctl.
On return, fflags contains the events which
triggered the filter.
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.
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 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.
SEE ALSO¶
aio_error(2),
aio_read(2),
aio_return(2),
poll(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¶
The
timeout value is limited to 24 hours;
longer timeouts will be silently reinterpreted as 24 hours.