kqueue, kevent - kernel event notification mechanism
#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);
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() 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).
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.
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 a nonnull 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.
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 */
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_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 filterspecific 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 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.
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.
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_* function is
called, the
event will be registered with the specified
kqueue, and
the ident argument set to the struct aiocb
returned by the
aio_* function. The filter returns under the
same conditions
as aio_error.
Alternatively, a kevent structure may be initialized, with
ident containing the descriptor of the
kqueue, and the address
of the kevent structure placed in the
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.
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() 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.
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_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() 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.
kqueue() 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.
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.
The kqueue() function 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() 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.
poll(2), read(2), select(2), sigaction(2), write(2), signal(3)
The kqueue() and kevent() functions first appeared in FreeBSD 4.1.
The kqueue() system and this manual page were written by
Jonathan Lemon
<[email protected]>.
It is currently not possible to watch FIFOs, AIO, or a vnode
that resides
on anything but a UFS file system.
The timeout value is limited to 24 hours; longer timeouts
will be silently
reinterpreted as 24 hours.
OpenBSD 3.6 April 14, 2000
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