NAME
kqueue
, kevent
— kernel event notification
mechanism
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
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. Similarly, kqueues cannot be passed across
UNIX-domain sockets.
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 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.
EV_SET
()
is a macro which is provided for ease of initializing a kevent
structure.
The kevent structure is defined as:
struct kevent { u_int ident; /* identifier for this event */ short filter; /* filter for event */ u_short flags; /* action flags for kqueue */ u_int fflags; /* filter flag value */ int 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 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 theNOTE_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 ofNOTE_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 inEV_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 ofEV_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_* function is called, the event will be registered with the specified kqueue, and the ident argument set to thestruct 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 withNOTE_FORK
set in the fflags field, while the child process will return withNOTE_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
() andsigaction
() 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 asSIG_IGN
. Event notification happens after normal signal delivery processing. data returns the number of times the signal has occurred since the last call tokevent
(). This filter automatically sets theEV_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 tokevent
(). This filter automatically sets theEV_CLEAR
flag internally.
RETURN VALUES
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.
ERRORS
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.
SEE ALSO
poll(2), read(2), select(2), sigaction(2), write(2), signal(3)
HISTORY
The kqueue
() and
kevent
() functions first appeared in
FreeBSD 4.1.
AUTHORS
The kqueue
() system and this manual page
were written by Jonathan Lemon
⟨jlemon@FreeBSD.org⟩.
BUGS
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.