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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
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.
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_COPY 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.