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.\" Copyright (c) 2000 Jonathan Lemon
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.\" $FreeBSD: src/lib/libc/sys/kqueue.2,v 1.32 2002/12/19 09:40:25 ru Exp $
.\"
.Dd April 14, 2000
.Dt KQUEUE 2
.Os
.Sh NAME
.Nm kqueue ,
.Nm kevent
.Nd kernel event notification mechanism
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/types.h
.In sys/event.h
.In sys/time.h
.Ft int
.Fn kqueue "void"
.Ft int
.Fn kevent "int kq" "const struct kevent *changelist" "int nchanges" "struct kevent *eventlist" "int nevents" "const struct timespec *timeout"
.Fn EV_SET "&kev" ident filter flags fflags data udata
.Sh DESCRIPTION
The
.Fn 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 identified by an (ident, filter) pair and specifies
the interesting conditions 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 conditions being watched,
not an addition.
.Pp
The filter identified in a kevent is executed upon the initial
registration 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.
.Pp
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.
.Pp
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
.Fn close
on a file descriptor will remove any kevents that reference the descriptor.
.Pp
The
.Fn kqueue
system call
creates a new kernel event queue and returns a descriptor.
The queue is not inherited by a child created with
.Xr fork 2 .
.Pp
The
.Fn 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
.Va kevent
structures, as defined in
.Aq Pa 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 .
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,
.Fn kevent
waits indefinitely.  To effect a poll, the
.Fa timeout
argument should be non-NULL, pointing to a zero-valued
.Va timespec
structure.  The same array may be used for the
.Fa changelist
and
.Fa eventlist .
.Pp
The
.Fn EV_SET
macro is provided for ease of initializing a
kevent structure.
.Pp
The
.Va kevent
structure is defined as:
.Bd -literal
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 */
};
.Ed
.Pp
The fields of
.Fa struct kevent
are:
.Bl -tag -width XXXfilter
.It ident
Value used to identify this event.
The exact interpretation is determined by the attached filter,
but often is a file descriptor.
.It filter
Identifies the kernel filter used to process this event.  The pre-defined
system filters are described below.
.It flags
Actions to perform on the event.
.It fflags
Filter-specific flags.
.It data
Filter-specific data value.
.It udata
Opaque user-defined value passed through the kernel unchanged.
.El
.Pp
The
.Va flags
field can contain the following values:
.Bl -tag -width XXXEV_ONESHOT
.It 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.
.It EV_ENABLE
Permit
.Fn kevent
to return the event if it is triggered.
.It EV_DISABLE
Disable the event so
.Fn kevent
will not return it.  The filter itself is not disabled.
.It 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.
.It 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.
.It 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.
.It EV_EOF
Filters may set this flag to indicate filter-specific EOF condition.
.It EV_ERROR
See
.Sx RETURN VALUES
below.
.El
.Pp
The predefined system filters are listed below.
Arguments may be passed to and from the filter via the
.Va fflags
and
.Va data
fields in the kevent structure.
.Bl -tag -width EVFILT_SIGNAL
.It 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 descriptor type.
.Pp
.Bl -tag -width 2n
.It Sockets
Sockets which have previously been passed to
.Fn listen
return when there is an incoming connection pending.
.Va data
contains the size of the listen backlog.
.Pp
Other socket descriptors return when there is data to be read,
subject to the
.Dv 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
.Va fflags ,
and specifying the new low water mark in
.Va data .
On return,
.Va data
contains the number of bytes of protocol data available to read.
.Pp
If the read direction of the socket has shutdown, then the filter
also sets EV_EOF in
.Va flags ,
and returns the socket error (if any) in
.Va fflags .
It is possible for EOF to be returned (indicating the connection is gone)
while there is still data pending in the socket buffer.
.It Vnodes
Returns when the file pointer is not at the end of file.
.Va data
contains the offset from current position to end of file,
and may be negative.
.It "Fifos, Pipes"
Returns when the there is data to read;
.Va data
contains the number of bytes available.
.Pp
When the last writer disconnects, the filter will set EV_EOF in
.Va 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.
.El
.It 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,
.Va 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.
.Pp
For sockets, the low water mark and socket error handling is
identical to the EVFILT_READ case.
.It EVFILT_AIO
This filter is currently unsupported.
.\"The sigevent portion of the AIO request is filled in, with
.\".Va sigev_notify_kqueue
.\"containing the descriptor of the kqueue that the event should
.\"be attached to,
.\".Va sigev_value
.\"containing the udata value, and
.\".Va sigev_notify
.\"set to SIGEV_KEVENT.
.\"When the
.\".Fn aio_*
.\"system call is made, the event will be registered
.\"with the specified kqueue, and the
.\".Va ident
.\"argument set to the
.\".Fa struct aiocb
.\"returned by the
.\".Fn aio_*
.\"system call.
.\"The filter returns under the same conditions as aio_error.
.\".Pp
.\"Alternatively, a kevent structure may be initialized, with
.\".Va ident
.\"containing the descriptor of the kqueue, and the
.\"address of the kevent structure placed in the
.\".Va aio_lio_opcode
.\"field of the AIO request.  However, this approach will not work on
.\"architectures with 64-bit pointers, and should be considered deprecated.
.It EVFILT_VNODE
Takes a file descriptor as the identifier and the events to watch for in
.Va fflags ,
and returns when one or more of the requested events occurs on the descriptor.
The events to monitor are:
.Bl -tag -width XXNOTE_RENAME
.It NOTE_DELETE
The
.Fn unlink
system call
was called on the file referenced by the descriptor.
.It NOTE_WRITE
A write occurred on the file referenced by the descriptor.
.It NOTE_EXTEND
The file referenced by the descriptor was extended.
.It NOTE_ATTRIB
The file referenced by the descriptor had its attributes changed.
.It NOTE_LINK
The link count on the file changed.
.It NOTE_RENAME
The file referenced by the descriptor was renamed.
.It NOTE_REVOKE
Access to the file was revoked via
.Xr revoke 2
or the underlying fileystem was unmounted.
.El
.Pp
On return,
.Va fflags
contains the events which triggered the filter.
.It EVFILT_PROC
Takes the process ID to monitor as the identifier and the events to watch for
in
.Va 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:
.Bl -tag -width XXNOTE_TRACKERR
.It NOTE_EXIT
The process has exited.
.It NOTE_FORK
The process has called
.Fn fork .
.It NOTE_EXEC
The process has executed a new process via
.Xr execve 2
or similar call.
.It NOTE_TRACK
Follow a process across
.Fn fork
calls.  The parent process will return with NOTE_TRACK set in the
.Va fflags
field, while the child process will return with NOTE_CHILD set in
.Va fflags
and the parent PID in
.Va data .
.It NOTE_TRACKERR
This flag is returned if the system was unable to attach an event to
the child process, usually due to resource limitations.
.El
.Pp
On return,
.Va fflags
contains the events which triggered the filter.
.It 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
.Fn signal
and
.Fn 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.
.Va data
returns the number of times the signal has occurred since the last call to
.Fn kevent .
This filter automatically sets the EV_CLEAR flag internally.
.It EVFILT_TIMER
This filter is currently unsupported.
.\"Establishes an arbitrary timer identified by
.\".Va ident .
.\"When adding a timer,
.\".Va data
.\"specifies the timeout period in milliseconds.
.\"The timer will be periodic unless EV_ONESHOT is specified.
.\"On return,
.\".Va data
.\"contains the number of times the timeout has expired since the last call to
.\".Fn kevent .
.\"This filter automatically sets the EV_CLEAR flag internally.
.El
.Sh RETURN VALUES
The
.Fn 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.
.Pp
The
.Fn 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
.Dv EV_ERROR
set in
.Va flags
and the system error in
.Va data .
Otherwise,
.Dv -1
will be returned, and
.Dv errno
will be set to indicate the error condition.
If the time limit expires, then
.Fn kevent
returns 0.
.Sh ERRORS
The
.Fn kqueue
system call fails if:
.Bl -tag -width Er
.It Bq Er ENOMEM
The kernel failed to allocate enough memory for the kernel queue.
.It Bq Er EMFILE
The per-process descriptor table is full.
.It Bq Er ENFILE
The system file table is full.
.El
.Pp
The
.Fn kevent
system call fails if:
.Bl -tag -width Er
.It Bq Er EACCES
The process does not have permission to register a filter.
.It Bq Er EFAULT
There was an error reading or writing the
.Va kevent
structure.
.It Bq Er EBADF
The specified descriptor is invalid.
.It Bq Er EINTR
A signal was delivered before the timeout expired and before any
events were placed on the kqueue for return.
.It Bq Er EINVAL
The specified time limit or filter is invalid.
.It Bq Er ENOENT
The event could not be found to be modified or deleted.
.It Bq Er ENOMEM
No memory was available to register the event.
.It Bq Er ESRCH
The specified process to attach to does not exist.
.El
.Sh SEE ALSO
.Xr aio_error 2 ,
.Xr aio_read 2 ,
.Xr aio_return 2 ,
.Xr read 2 ,
.Xr select 2 ,
.Xr sigaction 2 ,
.Xr write 2 ,
.Xr signal 3
.Sh HISTORY
The
.Fn kqueue
and
.Fn kevent
system calls first appeared in
.Fx 4.1 .
.Sh AUTHORS
The
.Fn kqueue
system and this manual page were written by
.An Jonathan Lemon Aq jlemon@FreeBSD.org .
.Sh 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.