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KQUEUE(2)                   BSD System Calls Manual                  KQUEUE(2)

     kqueue, kevent, and kevent64 -- kernel event notification mechanism

     Standard C Library (libc, -lc)

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


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

     kevent64(int kq, const struct kevent64_s *changelist, int nchanges, struct kevent64_s *eventlist,
         int nevents, unsigned int flags, const struct timespec *timeout);

     EV_SET(&kev, ident, filter, flags, fflags, data, udata);

     EV_SET64(&kev, ident, filter, flags, fflags, data, udata, ext[_], ext[1]);

     The kqueue() system call provides a generic method of notifying the user when a kernel event (kevent)
     happens or a condition holds, based on the results of small pieces of kernel code termed filters.  A
     kevent is identified by an (ident, filter) pair and specifies the interesting conditions to be notified
     about for that pair.  An (ident, filter) pair can only appear once in a given kqueue.  Subsequent
     attempts to register the same pair for a given kqueue will result in the replacement of the conditions
     being watched, not an addition.

     The filter identified in a kevent is executed upon the initial registration of that event 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.

     The kqueue() system call creates a new kernel event queue and returns a descriptor.  The queue is not
     inherited by a child created with fork(2).

     The kevent() and kevent64() system calls are used to register events with the queue, and return any
     pending events to the user.  The changelist argument is a pointer to an array of kevent or kevent64_s
     structures, as defined in <sys/event.h>.  All changes contained in the changelist are applied before
     any pending events are read from the queue.  The nchanges argument gives the size of changelist.  The
     eventlist argument is a pointer to an array of kevent or kevent64_s structures.  The nevents argument
     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, both
     kevent() and kevent64() wait 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

     The EV_SET() macro is provided for ease of initializing a kevent structure. Similarly, EV_SET64() ini-tializes initializes
     tializes a kevent64_s structure.

     The kevent and kevent64_s structures are defined as:

     struct kevent {
             uintptr_t       ident;          /* identifier for this event */
             int16_t         filter;         /* filter for event */
             uint16_t        flags;          /* general flags */
             uint32_t        fflags;         /* filter-specific flags */
             intptr_t        data;           /* filter-specific data */
             void            *udata;         /* opaque user data identifier */

     struct kevent64_s {
             uint64_t        ident;          /* identifier for this event */
             int16_t         filter;         /* filter for event */
             uint16_t        flags;          /* general flags */
             uint32_t        fflags;         /* filter-specific flags */
             int64_t         data;           /* filter-specific data */
             uint64_t        udata;          /* opaque user data identifier */
             uint64_t        ext[2];         /* filter-specific extensions */

     ----The ---The

     The fields of struct kevent and struct kevent64_s 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.

     In addition, struct kevent64_s contains:

     ext[2]     This field stores extensions for the event's filter. What type of extension depends on what
                type of filter is being used.

     ----The ---The

     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() and kevent64() to return the event if it is triggered.

     EV_DISABLE     Disable the event so kevent() and kevent64() 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_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 fil-ters filters
                    ters which report state transitions instead of the current state.  Note that some fil-ters filters
                    ters 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 ---The

     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 or kevent64_s structure.

     EVFILT_READ      Takes a file descriptor as the identifier, and returns whenever there is data avail-able available
                      able 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, 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 of protocol data available to read.

                          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 pend-ing pending
                          ing in the socket buffer.

                          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 avail-able. 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 file 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.

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

     EVFILT_AIO       This filter is currently unsupported.

     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    The unlink() system call was called on the file referenced by the

                      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 process has exited and its exit status is in filter specific data.
                                   Valid only on child processes and to be used along with NOTE_EXIT.

                      NOTE_FORK    The process created a child process via fork(2) or similar call.

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

                      NOTE_SIGNAL  The process was sent a signal. Status can be checked via waitpid(2) or
                                   similar call.

                      NOTE_REAP    The process was reaped by the parent via wait(2) or similar call. Depre-cated, Deprecated,
                                   cated, use NOTE_EXIT.

                      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 generated for the process.  This coexists with the signal() and sigaction() facili-ties, facilities,
                      ties, and has a lower precedence.  Only signals sent to the process, not to a particu-lar particular
                      lar thread, will trigger the filter. The filter will record all attempts to deliver a
                      signal to a process, even if the signal has been marked as SIG_IGN.  Event notifica-tion notification
                      tion happens before normal signal delivery processing.  data returns the number of
                      times the signal has been generated since the last call to kevent().  This filter
                      automatically sets the EV_CLEAR flag internally.

     EVFILT_MACHPORT  Takes the name of a mach port, or port set, in ident and waits until a message is
                      received on the port or port set. When a message is recieved, the size of the message
                      is returned in data and if fflags is set to MACH_RCV_MSG, a pointer to the message is
                      returned in ext[0].

     EVFILT_TIMER     Establishes an interval timer with the data timer identified by ident.  When adding a
                      timer, data specifies the timeout period and fflags can be set to one of the follow-ing: following:

                      NOTE_SECONDS   data is in seconds

                      NOTE_USECONDS  data is in microseconds

                      NOTE_NSECONDS  data is in nanoseconds

                      NOTE_ABSOLUTE  data is an absolute timeout

                      NOTE_CRITICAL  system makes a best effort to fire this timer as scheduled.

                                     system has extra leeway to coalesce this timer.

                      NOTE_LEEWAY    ext[1] holds user-supplied slop in deadline for timer coalescing.

                      If fflags is not set, the default is 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() or kevent64().  This filter automatically sets
                      the EV_CLEAR flag internally.

     ----In ---In

     In the ext[2] field of the kevent64_s struture, ext[_] is only used with the EVFILT_MACHPORT filter.
     With other filters, ext[_] is passed through kevent64() much like udata.  ext[1] can always be used
     like udata.  For the use of ext[0], see the EVFILT_MACHPORT filter above.

     The kqueue() system call 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.

     The kevent() and kevent64() system calls return 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() and kevent64() return 0.

     The kqueue() system call 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() and kevent64() system calls fail if:

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

     [EFAULT]           There was an error reading or writing the kevent or kevent64_s 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.

     [ENOENT]           The event could not be found to be modified or deleted.

     [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), read(2), select(2), sigaction(2), write(2), signal(3)

     The kqueue() and kevent() system calls first appeared in FreeBSD 4.1.

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

     Not all filesystem types support kqueue-style notifications.  And even some that do, like some remote
     filesystems, may only support a subset of the notification semantics described here.

BSD                            October 21, 2008                            BSD

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