KQUEUE(2) | System Calls Manual | KQUEUE(2) |
kqueue
, kevent
,
EV_SET
— 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(2) 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).
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 not NULL
, it
specifies a maximum interval to wait for an event, which will be interpreted
as a struct timespec. If timeout
is NULL
, kevent
() waits
indefinitely. To effect a poll, the timeout argument
should not be NULL
, pointing to a zero-valued
struct timespec. 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 */ int64_t data; /* filter data value */ void *udata; /* opaque user data identifier */ };
The fields of struct kevent are:
The flags field can contain the following values:
EV_ADD
EV_DISABLE
flag.EV_ENABLE
kevent
()
to return the event if it is triggered.EV_DISABLE
kevent
() will not return it.
The filter itself is not disabled.EV_DISPATCH
EV_DISABLE
above.EV_DELETE
EV_RECEIPT
kevent
() to return with
EV_ERROR
set without draining any pending events
after updating events in the kqueue. When a filter is successfully added
the data field will be zero. This flag is useful for
making bulk changes to a kqueue.EV_ONESHOT
EV_CLEAR
EV_EOF
EV_ERROR
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
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.
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.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_EXCEPT
NOTE_OOB
.EVFILT_WRITE
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_VNODE
NOTE_DELETE
NOTE_WRITE
NOTE_EXTEND
NOTE_TRUNCATE
NOTE_ATTRIB
NOTE_LINK
NOTE_RENAME
NOTE_REVOKE
On return, fflags contains the events which triggered the filter.
EVFILT_PROC
NOTE_EXIT
NOTE_FORK
NOTE_EXEC
NOTE_TRACK
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
On return, fflags contains the events which triggered the filter.
EVFILT_SIGNAL
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
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.EVFILT_DEVICE
NOTE_CHANGE
On return, fflags contains the events which triggered the filter.
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
]EMFILE
]ENFILE
]The kevent
() function fails if:
EACCES
]EFAULT
]EBADF
]EINTR
]EINVAL
]ENOENT
]ENOMEM
]ESRCH
]poll(2), read(2), select(2), sigaction(2), wait(2), write(2), signal(3)
The kqueue
() and
kevent
() functions first appeared in
FreeBSD 4.1 and have been available since
OpenBSD 2.9.
The kqueue
() system and this manual page
were written by Jonathan Lemon
<jlemon@FreeBSD.org>.
November 14, 2020 | OpenBSD-current |