KQUEUE(2)                 FreeBSD System Calls Manual                KQUEUE(2)

     kqueue, kevent - kernel event notification mechanism

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


     kevent(int kq, int nchanges, struct kevent **changelist, int nevents,
             struct kevent *eventlist, struct timespec *timeout)

     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, fil-
     ter) 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 when-
     ever an event is passed to the filter for evaluation.  If the filter de-
     termines 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

     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 de-
     scriptor 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 shar-
     ing of the kqueue between two processes.

     kevent() is used to register events with the queue, and return any pend-
     ing 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 de-

     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 automatical-
                    ly 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 at-
                    tached 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 re-
                    set.  This is useful for filters which report state tran-
                    sitions instead of the current state.  Note that some fil-
                    ters may automatically set this flag internally.

     EV_EOF         Filters may set this flag to indicate filter-specific EOF

     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

       Takes a descriptor as the identifier, and returns whenever there is da-
       ta available to read.  The behavior of the filter is slightly different
       depending on the descriptor type.

         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, sub-
         ject 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 al-
         so sets EV_EOF in flags. It is possible for EOF to be returned (indi-
         cating the connection is gone) while there is still data pending in
         the socket buffer.

         Returns when the file pointer is not at the end of file.  data con-
         tains the offset from current position to end of file, and may be

       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 re-

       Takes a descriptor as the identifier, and returns whenever it is possi-
       ble 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.

       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 reg-
       istered 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.

       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 de-

       NOTE_WRITE     A write occurred on the file referenced by the descrip-

       NOTE_EXTEND    The file referenced by the descriptor was extended.

       NOTE_ATTRIB    The file referenced by the descriptor had its attributes

       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.

       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 pro-

       cess, 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 pro-
                        cess 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 at-
                        tach an event to the child process, usually due to re-
                        source limitations.

       On return, fflags contains the events which triggered the filter.

       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.

     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.

     The kqueue() function fails if:

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

     [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 be-
                   fore 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.

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

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

     The kqueue() system and this manual page were written by Jonathan Lemon

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

BSD                             April 14, 2000                               5