KQUEUE(2) | System Calls Manual | KQUEUE(2) |
kqueue
, kevent
,
EV_SET
—
#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). 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 { 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
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.
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_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
unlink
() was called on the file referenced by
the descriptor.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
fork
().NOTE_EXEC
NOTE_TRACK
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
On return, fflags contains the events which triggered the filter.
EVFILT_SIGNAL
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
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.
kqueue
() function fails if:
ENOMEM
]EMFILE
]ENFILE
]The kevent
() function fails if:
EACCES
]EFAULT
]EBADF
]EINTR
]EINVAL
]ENOENT
]ENOMEM
]ESRCH
]kqueue
() and kevent
()
functions first appeared in FreeBSD 4.1.
kqueue
() system and this manual page were written by
Jonathan Lemon
<jlemon@FreeBSD.org>.
The timeout value is limited to 24 hours; longer timeouts will be silently reinterpreted as 24 hours.
January 13, 2018 | OpenBSD-current |