| KQUEUE(2) | NetBSD System Calls Manual | KQUEUE(2) |
int
kqueue(void);
int
kqueue1(int flags);
int
kevent(int kq, const struct kevent *changelist, size_t nchanges, struct kevent *eventlist, size_t nevents, const struct timespec *timeout);
EV_SET(&kev, ident, filter, flags, fflags, data, udata);
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 kqueue1() also allows to set the following flags on the returned file descriptor:
| O_CLOEXEC Set the close on exec property. |
| O_NONBLOCK Sets non-blocking I/O. |
| O_NOSIGPIPE Return EPIPE instead of raising SIGPIPE. |
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. 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 */
uint32_t filter; /* filter for event */
uint32_t flags; /* action flags for kqueue */
uint32_t fflags; /* filter flag value */
int64_t data; /* filter data value */
intptr_t udata; /* opaque user data identifier */
};
The fields of struct kevent are:
The flags field can contain the following values:
As a third-party filter is referenced by a well-known name instead of a statically assigned number, two ioctl(2)s are supported on the file descriptor returned by kqueue() to map a filter name to a filter number, and vice-versa (passing arguments in a structure described below):
The following structure is used to pass arguments in and out of the ioctl(2):
struct kfilter_mapping {
char *name; /* name to lookup or return */
size_t len; /* length of name */
uint32_t filter; /* filter to lookup or return */
};
Arguments may be passed to and from the filter via the fflags and data fields in the kevent structure.
The predefined system filters are:
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.
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.
On return, fflags contains the events which triggered the filter.
On return, fflags contains the events which triggered the filter.
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.
#include <sys/types.h>
#include <sys/event.h>
#include <sys/time.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <fcntl.h>
#include <err.h>
int
main(int argc, char *argv[])
{
int fd, kq, nev;
struct kevent ev;
static const struct timespec tout = { 1, 0 };
if ((fd = open(argv[1], O_RDONLY)) == -1)
err(1, "Cannot open `%s'", argv[1]);
if ((kq = kqueue()) == -1)
err(1, "Cannot create kqueue");
EV_SET(&ev, fd, EVFILT_VNODE, EV_ADD | EV_ENABLE | EV_CLEAR,
NOTE_DELETE|NOTE_WRITE|NOTE_EXTEND|NOTE_ATTRIB|NOTE_LINK|
NOTE_RENAME|NOTE_REVOKE, 0, 0);
if (kevent(kq, &ev, 1, NULL, 0, &tout) == -1)
err(1, "kevent");
for (;;) {
nev = kevent(kq, NULL, 0, &ev, 1, &tout);
if (nev == -1)
err(1, "kevent");
if (nev == 0)
continue;
if (ev.fflags & NOTE_DELETE) {
printf("deleted ");
ev.fflags &= ~NOTE_DELETE;
}
if (ev.fflags & NOTE_WRITE) {
printf("written ");
ev.fflags &= ~NOTE_WRITE;
}
if (ev.fflags & NOTE_EXTEND) {
printf("extended ");
ev.fflags &= ~NOTE_EXTEND;
}
if (ev.fflags & NOTE_ATTRIB) {
printf("chmod/chown/utimes ");
ev.fflags &= ~NOTE_ATTRIB;
}
if (ev.fflags & NOTE_LINK) {
printf("hardlinked ");
ev.fflags &= ~NOTE_LINK;
}
if (ev.fflags & NOTE_RENAME) {
printf("renamed ");
ev.fflags &= ~NOTE_RENAME;
}
if (ev.fflags & NOTE_REVOKE) {
printf("revoked ");
ev.fflags &= ~NOTE_REVOKE;
}
printf("\n");
if (ev.fflags)
warnx("unknown event 0x%x\n", ev.fflags);
}
}
The kevent() function fails if:
Jonathan Lemon, Kqueue: A Generic and Scalable Event Notification Facility, Proceedings of the FREENIX Track: 2001 USENIX Annual Technical Conference, USENIX Association, June 25-30, 2001, http://www.usenix.org/event/usenix01/freenix01/full_papers/lemon/lemon.pdf.
| January 23, 2012 | NetBSD 5.99 |