| KQUEUE(2) | System Calls Manual | KQUEUE(2) |
kqueue, kevent
— 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);
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:
The flags field can contain the following values:
kevent()
to return the event if it is triggered.kevent() will not return it.
The filter itself is not disabled.EV_DISABLE above.EV_ERROR to always be returned. When a filter is
successfully added the data field will be zero.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.
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.
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.
For sockets, the low water mark and socket error handling is identical to the EVFILT_READ case.
unlink()
system call was called on the file referenced by the descriptor.On return, fflags contains the events which triggered the filter.
signal()
and
sigaction()
facilities, and has a higher precedence. The filter will record all
attempts to deliver a signal to a process, even if the signal has been
marked as SIG_IGN. 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.If fflags is not set, the default is
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.
EVFILT_USERNOTE_FFNOPNOTE_FFANDNOTE_FFORNOTE_COPYNOTE_FFCTRLMASKNOTE_FFLAGSMASKA user event is triggered for output with the following:
NOTE_TRIGGEROn return, fflags contains the users defined flags in the lower 24 bits.
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.
The kqueue() system call fails if:
ENOMEM]EMFILE]ENFILE]The kevent() system call fails if:
EACCES]EFAULT]EBADF]EINTR]EINVAL]ENOENT]ENOMEM]ESRCH]aio_error(2), aio_read(2), aio_return(2), poll(2), read(2), select(2), sigaction(2), write(2), signal(3)
The kqueue() and
kevent() system calls first appeared in
FreeBSD 4.1.
The kqueue() system and this manual page
were written by Jonathan Lemon
⟨jlemon@FreeBSD.org⟩.
| September 17, 2010 | Debian |