KQUEUE(2) BSD System Calls Manual KQUEUE(2)
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 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.
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.
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).
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. 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_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 fil-ters filters
ters which report state transitions instead of the current state. Note that some fil-ters filters
ters 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 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.
Sockets
Sockets which have previously been passed to listen() return when there is an incom-ing incoming
ing 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.
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, 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 This filter is currently unsupported.
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 fileystem
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 created a child process via fork(2) or similar call.
NOTE_EXEC
The process executed a new process via execve(2) or similar call.
NOTE_SIGNAL
The process was sent a signal. Status can be checked via waitpid(2) or similar call.
NOTE_REAP
The process was reaped by the parent via wait(2) or similar call.
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() facili-ties, facilities,
ties, and has a lower precedence. The filter will record all attempts to deliver a sig-nal signal
nal 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 This filter is currently unsupported.
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.
[ESRCH] The specified process to attach to does not exist.
SEE ALSO
aio_error(2), aio_read(2), aio_return(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
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.
BSD April 14, 2000 BSD
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