KQUEUE(2)

NAME
kqueue, kevent - kernel event notification mechanism

LIBRARY
libc

SYNOPSIS
#include <sys/time.h>
#include <sys/types.h>
#include <sys/event.h>
int
kqueue(void);
int
kevent(int kq, int nchanges, struct kevent **changelist, int nevents, struct kevent *eventlist, struct timespec *timeout);

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. An 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 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, and if the filter indicates 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). However, you can perform a rfork() and share the descriptor table, which will allow sharing of the kqueue between two processes.

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 pointers to kevent structures, as defined in <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. 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 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:

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. The pre-defined system filters are described below.

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_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_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.

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
    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() 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. 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. 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 the 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 and fifos, 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.

EVFILT_AIO
    A kevent structure is initialized, with ident containing the descriptor of the kqueue that the event should be attached to. The address of the kevent structure is then placed in the aio_lio_opcode field of the AIO request, and the aio_* function is then called. The event will be registered with the specified kqueue, and the ident argument set to the struct aiocb returned by the aio_* function. The filter returns under the same conditions as aio_error.

    NOTE: this interface is unstable and subject to change.

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() 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.

    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.

    NOTE_FORK
        The process has called fork().

    NOTE_EXEC
        The process has executed a new process via execve(2) or similar call.

    NOTE_TRACK
        Follow a process across fork() 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 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 kqueue(). 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.

ERRORS
The kqueue() function fails if:

[ENOMEM]
    The kernel failed to allocate enough memory for the kernel queue.

[EMFILE]
    The per-process descriptor table is full.

[ENFILE]
    The system file table is full.

The kevent() function fails if:

[EACCESS]
    The process does not have permission to register a filter.

[EFAULT]
    There was an error reading or writing the kevent structure.

[EBADF]
    The specified descriptor is invalid.

[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.

[ENOMEM]
    No memory was available to register the event.

[ESRCH]
    The specified process to attach to does not exist.

SEE ALSO
aio_error(2), aio_read(2), aio_return(2), poll(2), read(2), select(2), signal(3), sigaction(2), write(2)

HISTORY
The kqueue() and kevent() functions first appeared in FreeBSD 5.0.

AUTHORS
The kqueue() system and this manual page were written by Jonathan Lemon <jlemon@FreeBSD.org>.

BUGS
It is currently not possible to watch a vnode that resides on anything but a UFS file system.