#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
Fa changelist
argument
is a pointer to an array of
kevent
structures, as defined in
In sys/event.h .
All changes contained in the
Fa changelist
are applied before any pending events are read from the queue.
The
Fa nchanges
argument
gives the size of
Fa changelist .
The
Fa eventlist
argument
is a pointer to an array of kevent structures.
The
Fa nevents
argument
determines the size of
Fa eventlist .
When
Fa nevents
is zero,
kevent ();
will return immediately even if there is a
Fa timeout
specified unlike
select(2).
If
Fa 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
Fa timeout
is a NULL pointer,
kevent ();
waits indefinitely.
To effect a poll, the
Fa timeout
argument should be non-NULL, pointing to a zero-valued
timespec
structure.
The same array may be used for the
Fa changelist
and
Fa 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
Fa 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_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
Sx 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_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
Fa 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 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 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 ();
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.
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.
EVFILT_NETDEV
Takes a descriptor to a network interface as the identifier, and the events to watch for in
fflags
It returns, when one or more of the requested events occur on the descriptor.
The events to monitor are:
NOTE_LINKUP
The link is up.
NOTE_LINKDOWN
The link is down.
NOTE_LINKINV
The link state is invalid.
On return,
fflags
contains the events which triggered the filter.
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
Fa eventlist ,
up to the value given by
Fa nevents .
If an error occurs while processing an element of the
Fa changelist
and there is enough room in the
Fa eventlist ,
then the event will be placed in the
Fa 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:
Bq Er ENOMEM
The kernel failed to allocate enough memory for the kernel queue.
Bq Er EMFILE
The per-process descriptor table is full.
Bq Er ENFILE
The system file table is full.
The
kevent ();
system call fails if:
Bq Er EACCES
The process does not have permission to register a filter.
Bq Er EFAULT
There was an error reading or writing the
kevent
structure.
Bq Er EBADF
The specified descriptor is invalid.
Bq Er EINTR
A signal was delivered before the timeout expired and before any
events were placed on the kqueue for return.
Bq Er EINVAL
The specified time limit or filter is invalid.
Bq Er ENOENT
The event could not be found to be modified or deleted.
Bq Er 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.
Bq Er ESRCH
The specified process to attach to does not exist.
The
kqueue ();
and
kevent ();
system calls first appeared in
Fx 4.1 .
AUTHORS
The
kqueue ();
system and this manual page were written by
An Jonathan Lemon Aq jlemon@FreeBSD.org .
BUGS
The
EVFILT_NETDEV
filter is currently only implemented for devices that use the
miibus(4)
driver for LINKUP and LINKDOWN operations.
Therefore, it will not work with many non-ethernet devices.
The
Fa timeout
value is limited to 24 hours; longer timeouts will be silently
reinterpreted as 24 hours.
