NAME

kqueue, kqueue1, kevent — kernel event notification mechanism

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <sys/event.h>
#include <sys/time.h>

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);

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 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.

The queue is not inherited by a child created with fork(2).

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:

Filters

Filters are identified by a number. There are two types of filters; pre-defined filters which are described below, and third-party filters that may be added with kfilter_register(9) by kernel sub-systems, third-party device drivers, or loadable kernel modules.

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:

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.

EVFILT_WRITE

Takes a descriptor as the identifier, and returns whenever it is possible to write to the descriptor. For sockets, pipes, fifos, and ttys, 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 of EV_CLEAR. Note that this filter is not supported for vnodes.

For sockets, the low water mark and socket error handling is identical to the EVFILT_READ case.

EVFILT_AIO

This is not implemented in NetBSD.

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:

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:

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 current process. This coexists with the 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

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 to kevent(). This filter automatically sets the EV_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.

EXAMPLES

The following example program monitors a file (provided to it as the first argument) and prints information about some common events it receives notifications for:

#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); 
        } 
}

ERRORS

The kqueue() function fails if:

The kevent() function fails if:

HISTORY

The kqueue() and kevent() functions first appeared in FreeBSD 4.1, and then in NetBSD 2.0. The kqueue1() function first appeared in NetBSD 6.0.

Manual browser: kevent(2)