KQUEUE(2)

KQUEUE(2) | System Calls Manual | KQUEUE(2)

# NAME

kqueue, kqueue1, kevent, EV_SET — 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(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 kqueue1() function also allows to set the following flags on the returned file descriptor:

`O_CLOEXEC` Set the close on exec property.
`O_NONBLOCK` Set 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(3) structure. The same array may be used for the changelist and eventlist.
EV_SET() is a static inline function which is provided for ease of initializing a kevent structure. Its original form was a preprocessor macro, which caused bugs in third party code.
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:

ident
  Value used to identify this event. The exact interpretation is determined by the attached filter, but often is a file descriptor.
filter
  Identifies the kernel filter used to process this event. There are pre-defined system filters (which are described below), and other filters may be added by kernel subsystems as necessary.
flags
  Actions to perform on the event.
fflags
  Filter-specific flags.
data
  Filter-specific data value.
udata
  Opaque user-defined value passed through the kernel unchanged.

The flags field can contain the following values:

EV_ADD
  Adds the event to the kqueue. Re-adding an existing event will modify the parameters of the original event, and not result in a duplicate entry. Adding an event automatically enables it, unless overridden by the EV_DISABLE flag.
EV_ENABLE
  Permit kevent() to return the event if it is triggered.
EV_DISABLE
  Disable the event so kevent() will not return it. The filter itself is not disabled.
EV_DISPATCH
  Disable the event source immediately after delivery of an event. See `EV_DISABLE` above.
EV_DELETE
  Removes the event from the kqueue. Events which are attached to file descriptors are automatically deleted on the last close of the descriptor.
EV_RECEIPT
  This flag is useful for making bulk changes to a kqueue without draining any pending events. When passed as input, it forces `EV_ERROR` to always be returned. When a filter is successfully added the data field will be zero.
EV_ONESHOT
  Causes the event to return only the first occurrence of the filter being triggered. After the user retrieves the event from the kqueue, it is deleted.
EV_CLEAR
  After the event is retrieved by the user, its state is reset. This is useful for filters which report state transitions instead of the current state. Note that some filters may automatically set this flag internally.
EV_EOF
  Filters may set this flag to indicate filter-specific EOF condition.
EV_ERROR
  See RETURN VALUES below.

## 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):

KFILTER_BYFILTER
  Map filter to name, which is of size len.
KFILTER_BYNAME
  Map name to filter. len is ignored.

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.
  Sockets
    Sockets which have previously been passed to listen(2) return when there is an incoming connection pending. data contains the size of the listen backlog (i.e., the number of connections ready to be accepted with accept(2).)
    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.
  Vnodes
    Returns when the file pointer is not at the end of file. data contains the offset from current position to end of file, and may be negative.
  Fifos, Pipes
    Returns when there is data to read; data contains the number of bytes available.
    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
  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:
  NOTE_DELETE
    unlink(2) was called on the file referenced by the descriptor.
  NOTE_WRITE
    A write occurred on the file referenced by the descriptor.
  NOTE_EXTEND
    The file referenced by the descriptor was extended.
  NOTE_ATTRIB
    The file referenced by the descriptor had its attributes changed.
  NOTE_LINK
    The link count on the file changed.
  NOTE_RENAME
    The file referenced by the descriptor was renamed.
  NOTE_REVOKE
    Access to the file was revoked via revoke(2) or the underlying fileystem was unmounted.
  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:
  NOTE_EXIT
    The process has exited. The exit code of the process is stored in data.
  NOTE_FORK
    The process has called fork(2).
  NOTE_EXEC
    The process has executed a new process via execve(2) or similar call.
  NOTE_TRACK
    Follow a process across fork(2) calls. The parent process will return with NOTE_TRACK 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
    This flag is returned if the system was unable to attach an event to the child process, usually due to resource limitations.
  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(3) and sigaction(2) 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` is 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:

[`EMFILE`]
  The per-process descriptor table is full.
[`ENFILE`]
  The system file table is full.
[`ENOMEM`]
  The kernel failed to allocate enough memory for the kernel queue.

The kevent() function fails if:

[`EACCES`]
  The process does not have permission to register a filter.
[`EBADF`]
  The specified descriptor is invalid.
[`EFAULT`]
  There was an error reading or writing the kevent structure.
[`EINTR`]
  A signal was delivered before the timeout expired and before any events were placed on the kqueue for return.
[`EINVAL`]
  The specified time limit or filter is invalid.
[`ENOENT`]
  The event could not be found to be modified or deleted.
[`ENOMEM`]
  No memory was available to register the event.
[`EOPNOTSUPP`]
  This type of file descriptor is not supported for kevent() operations.
[`ESRCH`]
  The specified process to attach to does not exist.

# SEE ALSO

fork(2), ioctl(2), listen(2), poll(2), read(2), select(2), sigaction(2), unlink(2), write(2), signal(3), timespec(3), kfilter_register(9), knote(9)
Jonathan Lemon, Kqueue: A Generic and Scalable Event Notification Facility, Proceedings of the FREENIX Track: 2001 USENIX Annual Technical Conference, USENIX Association, http://www.usenix.org/event/usenix01/freenix01/full_papers/lemon/lemon.pdf, June 25-30, 2001.

# 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.
EV_SET() was converted from a macro to a static inline function in NetBSD 8.0.

December 8, 2015 | NetBSD 8.3