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kqueue(2)                   BSD System Calls Manual                  kqueue(2)


NAME

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


LIBRARY

     Standard C Library (libc, -lc)


SYNOPSIS

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

     int
     kqueue(void);

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

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

     int
     kevent_qos(int kq, const struct kevent_qos_s *changelist, int nchanges,
         struct kevent_qos_s *eventlist, int nevents, void *data_out,
         size_t *data_available, unsigned int flags);

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

     EV_SET64(&kev, ident, filter, flags, fflags, data, udata, ext[0],
         ext[1]);

     EV_SET_QOS(&kev, ident, filter, flags, qos, udata, fflags, xflags, data,
         ext[0], ext[1], ext[2], ext[3]);


DESCRIPTION

     The kqueue() system call allocates a kqueue file descriptor.  This file
     descriptor 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, and optional udata value)
     tuple.  It specifies the interesting conditions to be notified about for
     that tuple. An (ident, filter, and optional udata value) tuple can only
     appear once in a given kqueue.  Subsequent attempts to register the same
     tuple for a given kqueue will result in the replacement of the conditions
     being watched, not an addition.  Whether the udata value is considered as
     part of the tuple is controlled by the EV_UDATA_SPECIFIC flag on the
     kevent.

     The filter identified in a kevent is executed upon the initial registra-
     tion 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,() kevent64() and kevent_qos() system calls are used to regis-
     ter events with the queue, and return any pending events to the user.
     The changelist argument is a pointer to an array of kevent, kevent64_s or
     kevent_qos_s structures, as defined in <sys/event.h>.  All changes con-
     tained 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 out kevent, kevent64_s or
     kevent_qos_s structures.  The nevents argument determines the size of
     eventlist.  If the KEVENT_FLAG_STACK_EVENTS flag is provided on the sys-
     tem call, the eventlist array is filled in in stack order (starting in
     the highest available index) instead of typical array order.  The
     out_data argument provides space for extra out data provided by specific
     filters.  The data_available argument's contents specified the space
     available in the data pool on input, and contains the amount still
     remaining on output.  If the KEVENT_FLAG_STACK_DATA flag is specified on
     the system call, the data is allocated from the pool in stack order
     instead of typical heap order.  If timeout is a non-NULL pointer, it
     specifies a maximum interval to wait for an event, which will be inter-
     preted as a struct timespec.  If timeout is a NULL pointer, both kevent()
     and kevent64() wait indefinitely.  To effect a poll, the flags argument
     to kevent64() or kevent_qos() can include the KEVENT_FLAG_IMMEDIATE value
     to indicate an immediate timeout.  Alternatively, 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.

     The EV_SET() macro is provided for ease of initializing a kevent struc-
     ture. Similarly, EV_SET64() initializes a kevent64_s structure and
     EV_SET_QOS() initializes a kevent_qos_s structure.

     The kevent, kevent64_s and kevent_qos_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 */
     };

     struct kevent_qos_s {
             uint64_t        ident;          /* identifier for this event */
             int16_t         filter;         /* filter for event */
             uint16_t        flags;          /* general flags */
             uint32_t        qos;            /* quality of service when servicing event */
             uint64_t        udata;          /* opaque user data identifier */
             uint32_t        fflags;         /* filter-specific flags */
             uint32_t        xflags;         /* extra filter-specific flags */
             int64_t         data;           /* filter-specific data */
             uint64_t        ext[4];         /* filter-specific extensions */
     };

     ----

     The fields of struct kevent, struct kevent64_s and struct kevent_qos_s
     are:

     ident      Value used to identify the source of the 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.
                It can optionally be part of the uniquing decision of the
                kevent system

     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.

     In addition, struct kevent_qos_s contains:

     xflags     Extra filter-specific flags.

     ext[4]     The QoS variant provides twice as many extension values for
                filter-specific uses.

     ----

     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 automati-
                    cally enables it, unless overridden by the EV_DISABLE
                    flag.

     EV_ENABLE      Permit kevent,() kevent64() and kevent_qos() to return the
                    event if it is triggered.

     EV_DISABLE     Disable the event so kevent,() kevent64() and kevent_qos()
                    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 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_OOBAND      Read filter on socket may set this flag to indicate the
                    presence of out of band data on the descriptor.

     EV_ERROR       See RETURN VALUES below.

     ----

     The predefined system filters are listed below.  Arguments may be passed
     to and from the filter via the data, fflags and optionally xflags fields
     in the kevent, kevent64_s or kevent_qos_s structure.

     EVFILT_READ      Takes a file 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 back-
                          log.

                          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.  The
                          derived per filter low water mark value is, however,
                          bounded by socket receive buffer's high and low
                          water mark values.  On return, data contains the
                          number of bytes of protocol data available to read.

                          The presence of EV_OOBAND in flags, indicates the
                          presence of out of band data on the socket data
                          equal to the potential number of OOB bytes availble
                          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 con-
                          nection 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 posi-
                          tion 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 return-
                          ing.

                      Device nodes
                          Returns when there is data to read from the device;
                          data contains the number of bytes available.  If the
                          device does not support returning number of bytes,
                          it will not allow the filter to be attached.  How-
                          ever, if the NOTE_LOWAT flag is specified and the
                          data field contains 1 on input, those devices will
                          attach - but cannot be relied upon to provide an
                          accurate count of bytes to be read on output.

     EVFILT_EXCEPT    Takes a descriptor as the identifier, and returns when-
                      ever one of the specified exceptional conditions has
                      occurred on the descriptor. Conditions are specified in
                      fflags.  Currently, this filter can be used to monitor
                      the arrival of out-of-band data on a socket descriptor
                      using the filter flag NOTE_OOB.

                      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.

     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 han-
                      dling 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 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.

                      NOTE_FUNLOCK   The file was unlocked by calling flock(2)
                                     or close(2)

                      On return, fflags contains the filter-specific flags
                      which are associated with the triggered events seen by
                      this 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_EXITSTATUS
                                   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. Deprecated, 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() facilities, and has a lower precedence.
                      Only signals sent to the process, not to a particular
                      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 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 enqueued on the port or port
                      set. When a message is detected, but not directly
                      received by the kevent call, the name of the specific
                      port where the message is enqueued is returned in data.
                      If fflags contains MACH_RCV_MSG, the ext[0] and ext[1]
                      flags are assumed to contain a pointer to the buffer
                      where the message is to be received and the size of the
                      receive buffer, respectively.  If MACH_RCV_MSG is
                      specifed, yet the buffer size in ext[1] is zero, The
                      space for the buffer may be carved out of the data_out
                      area provided to kevent_qos() if there is enough space
                      remaining there.

     EVFILT_TIMER     Establishes an interval timer identified by ident where
                      data specifies the timeout period (in milliseconds).

                      fflags can include one of the following flags to specify
                      a different unit:

                      NOTE_SECONDS   data is in seconds

                      NOTE_USECONDS  data is in microseconds

                      NOTE_NSECONDS  data is in nanoseconds

                      NOTE_MACHTIME  data is in Mach absolute time units

                      fflags can also include NOTE_ABSOLUTE, which establishes
                      an EV_ONESHOT timer with an absolute deadline instead of
                      an interval.  The absolute deadline is expressed in
                      terms of gettimeofday(2).  With NOTE_MACHTIME, the dead-
                      line is expressed in terms of mach_absolute_time().

                      The timer can be coalesced with other timers to save
                      power. The following flags can be set in fflags to mod-
                      ify this behavior:

                      NOTE_CRITICAL    override default power-saving tech-
                                       niques to more strictly respect the
                                       leeway value

                      NOTE_BACKGROUND  apply more power-saving techniques to
                                       coalesce this timer with other timers

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

                      The timer will be periodic unless EV_ONESHOT is speci-
                      fied.  On return, data contains the number of times the
                      timeout has expired since the last arming or last deliv-
                      ery of the timer event.

                      This filter automatically sets the EV_CLEAR flag.

     ----

     In the ext[2] field of the kevent64_s struture, ext[0] is only used with
     the EVFILT_MACHPORT filter.  With other filters, ext[0] 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.


RETURN VALUES

     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(), kevent64() and kevent_qos() 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.  Oth-
     erwise, -1 will be returned, and errno will be set to indicate the error
     condition.  If the time limit expires, then kevent(), kevent64() and
     kevent_qos() return 0.


ERRORS

     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.


SEE ALSO

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


HISTORY

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


AUTHORS

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


BUGS

     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

Mac OS X 10.13.1 - Generated Mon Nov 6 15:55:51 CST 2017
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