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

     kqueue, kevent â< kernel event notification mechanism

     Standard C Library (libc, -lc)

     #include <sys/event.h>


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

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

     The kqueue() system call 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() 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).
     However, if rfork(2) is called without the RFFDG flag, then the
     descriptor table is shared, which will allow sharing of the kqueue
     between two processes.

     The kevent() system call is 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 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
     structures.  The nevents argument determines the size of eventlist.  When
     nevents is zero, kevent() will return immediately even if there is a
     timeout specified unlike select(2).  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 structure.  The same array
     may be used for the changelist and eventlist.

     The EV_SET() macro is provided for ease of initializing a kevent

     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 */
             int64_t   data;         /* filter data value */
             void      *udata;       /* opaque user data identifier */
             uint64_t  ext[4];       /* extensions */

     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

     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.

     ext        Extended data passed to and from kernel.  The ext[0] and
                ext[1] members use is defined by the filter.  If the filter
                does not use them, the members are copied unchanged.  The
                ext[2] and ext[3] members are always passed through the kernel
                as-is, making additional context available to application.

     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.  Note that
                  if this flag is encountered and there is no remaining space
                  in eventlist to hold the EV_ERROR event, then subsequent
                  changes will not get processed.

     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

     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

     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 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
                             pending 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.

                             This behavior is different from poll(2), where
                             read events are triggered for regular files
                             unconditionally.  This event can be triggered
                             unconditionally by setting the NOTE_FILE_POLL
                             flag in fflags.

                         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 will be cleared by the
                             filter when a new writer connects, at which point
                             the filter will resume waiting for data to become
                             available before returning.

                         BPF devices
                             Returns when the BPF buffer is full, the BPF
                             timeout has expired, or when the BPF has
                             â<immediate modeâ< enabled and there is any data to
                             read; data contains the number of bytes

                             Returns when the counter is greater than 0; data
                             contains the counter value, which must be cast to

     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 will be cleared when a
                         new reader connects.  Note that this filter is not
                         supported for vnodes or BPF devices.

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

                         For eventfds, data will contain the maximum value
                         that can be added to the counter without blocking.

     EVFILT_EMPTY        Takes a descriptor as the identifier, and returns
                         whenever there is no remaining data in the write

     EVFILT_AIO          Events for this filter are not registered with
                         kevent() directly but are registered via the
                         aio_sigevent member of an asynchronous I/O request
                         when it is scheduled via an asynchronous I/O system
                         call such as aio_read().  The filter returns under
                         the same conditions as aio_error().  For more details
                         on this filter see sigevent(3) and aio(4).

     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_ATTRIB          The file referenced by the
                                              descriptor had its attributes

                         NOTE_CLOSE           A file descriptor referencing
                                              the monitored file, was closed.
                                              The closed file descriptor did
                                              not have write access.

                         NOTE_CLOSE_WRITE     A file descriptor referencing
                                              the monitored file, was closed.
                                              The closed file descriptor had
                                              write access.

                                              This note, as well as
                                              NOTE_CLOSE, are not activated
                                              when files are closed forcibly
                                              by unmount(2) or revoke(2).
                                              Instead, NOTE_REVOKE is sent for
                                              such events.

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

                         NOTE_EXTEND          For regular file, the file
                                              referenced by the descriptor was

                                              For directory, reports that a
                                              directory entry was added or
                                              removed, as the result of rename
                                              operation.  The NOTE_EXTEND
                                              event is not reported when a
                                              name is changed inside the

                         NOTE_LINK            The link count on the file
                                              changed.  In particular, the
                                              NOTE_LINK event is reported if a
                                              subdirectory was created or
                                              deleted inside the directory
                                              referenced by the descriptor.

                         NOTE_OPEN            The file referenced by the
                                              descriptor was opened.

                         NOTE_READ            A read occurred on the file
                                              referenced by the descriptor.

                         NOTE_RENAME          The file referenced by the
                                              descriptor was renamed.

                         NOTE_REVOKE          Access to the file was revoked
                                              via revoke(2) or the underlying
                                              file system was unmounted.

                         NOTE_WRITE           A write occurred on the file
                                              referenced by the descriptor.

                         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
                                           status will be stored in data.

                         NOTE_FORK         The process has called fork().

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

                         NOTE_TRACK        Follow a process across fork()
                                           calls.  The parent process
                                           registers a new kevent to monitor
                                           the child process using the same
                                           fflags as the original event.  The
                                           child process will signal an event
                                           with NOTE_CHILD set in fflags and
                                           the parent PID in data.

                                           If the parent process fails to
                                           register a new kevent (usually due
                                           to resource limitations), it will
                                           signal an event with NOTE_TRACKERR
                                           set in fflags, and the child
                                           process will not signal a
                                           NOTE_CHILD event.

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

     EVFILT_PROCDESC     Takes the process descriptor created by pdfork(2) to
                         monitor as the identifier and the events to watch for
                         in fflags, and returns when the associated process
                         performs one or more of the requested events.  The
                         events to monitor are:

                         NOTE_EXIT     The process has exited.  The exit
                                       status will be stored in data.

                         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, except for the SIGCHLD signal,
                         which, if ignored, will not be recorded by the
                         filter.  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 moment to
                         fire the timer (for NOTE_ABSTIME) or the timeout
                         period.  The timer will be periodic unless EV_ONESHOT
                         or NOTE_ABSTIME is specified.  On return, data
                         contains the number of times the timeout has expired
                         since the last call to kevent().  For non-monotonic
                         timers, this filter automatically sets the EV_CLEAR
                         flag internally.

                         The filter accepts the following flags in the fflags

                         NOTE_SECONDS      data is in seconds.

                         NOTE_MSECONDS     data is in milliseconds.

                         NOTE_USECONDS     data is in microseconds.

                         NOTE_NSECONDS     data is in nanoseconds.

                         NOTE_ABSTIME      The specified expiration time is

                         If fflags is not set, the default is milliseconds.
                         On return, fflags contains the events which triggered
                         the filter.

                         If an existing timer is re-added, the existing timer
                         will be effectively canceled (throwing away any
                         undelivered record of previous timer expiration) and
                         re-started using the new parameters contained in data
                         and fflags.

                         There is a system wide limit on the number of timers
                         which is controlled by the kern.kq_calloutmax sysctl.

     EVFILT_USER         Establishes a user event identified by ident which is
                         not associated with any kernel mechanism but is
                         triggered by user level code.  The lower 24 bits of
                         the fflags may be used for user defined flags and
                         manipulated using the following:

                         NOTE_FFNOP          Ignore the input fflags.

                         NOTE_FFAND          Bitwise AND fflags.

                         NOTE_FFOR           Bitwise OR fflags.

                         NOTE_FFCOPY         Copy fflags.

                         NOTE_FFCTRLMASK     Control mask for fflags.

                         NOTE_FFLAGSMASK     User defined flag mask for

                         A user event is triggered for output with the

                         NOTE_TRIGGER        Cause the event to be triggered.

                         On return, fflags contains the users defined flags in
                         the lower 24 bits.

     If nevents is non-zero, i.e., the function is potentially blocking, the
     call is a cancellation point.  Otherwise, i.e., if nevents is zero, the
     call is not cancellable.  Cancellation can only occur before any changes
     are made to the kqueue, or when the call was blocked and no changes to
     the queue were requested.

     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() system call 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.

     #include <sys/event.h>
     #include <err.h>
     #include <fcntl.h>
     #include <stdio.h>
     #include <stdlib.h>
     #include <string.h>

     main(int argc, char **argv)
         struct kevent event;    /* Event we want to monitor */
         struct kevent tevent;   /* Event triggered */
         int kq, fd, ret;

         if (argc != 2)
             err(EXIT_FAILURE, "Usage: %s path\n", argv[0]);
         fd = open(argv[1], O_RDONLY);
         if (fd == -1)
             err(EXIT_FAILURE, "Failed to open '%s'", argv[1]);

         /* Create kqueue. */
         kq = kqueue();
         if (kq == -1)
             err(EXIT_FAILURE, "kqueue() failed");

         /* Initialize kevent structure. */
             0, NULL);
         /* Attach event to the kqueue. */
         ret = kevent(kq, &event, 1, NULL, 0, NULL);
         if (ret == -1)
             err(EXIT_FAILURE, "kevent register");
         if (event.flags & EV_ERROR)
             errx(EXIT_FAILURE, "Event error: %s", strerror(event.data));

         for (;;) {
             /* Sleep until something happens. */
             ret = kevent(kq, NULL, 0, &tevent, 1, NULL);
             if (ret == -1) {
                 err(EXIT_FAILURE, "kevent wait");
             } else if (ret > 0) {
                 printf("Something was written in '%s'\n", argv[1]);

     The kqueue() system call fails if:

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

     [ENOMEM]           The RLIMIT_KQUEUES rlimit (see getrlimit(2)) for the
                        current user would be exceeded.

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

     [ENFILE]           The system file table is full.

     The kevent() system call fails if:

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

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

     [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

     [EINTR]            A cancellation request was delivered to the thread,
                        but not yet handled.

     [EINVAL]           The specified time limit or filter is invalid.

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

     [ENOMEM]           No memory was available to register the event or, in
                        the special case of a timer, the maximum number of
                        timers has been exceeded.  This maximum is
                        configurable via the kern.kq_calloutmax sysctl.

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

     When kevent() call fails with EINTR error, all changes in the changelist
     have been applied.

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

     Jonathan Lemon, â<Kqueue: A Generic and Scalable Event Notification
     Facilityâ<, Proceedings of the FREENIX Track: 2001 USENIX Annual Technical
     Conference, USENIX Association, June 25-30, 2001.

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

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

     The timeout value is limited to 24 hours; longer timeouts will be
     silently reinterpreted as 24 hours.

     In versions older than FreeBSD 12.0, <sys/event.h> failed to parse
     without including <sys/types.h> manually.

FreeBSD 13.0-RELEASE            October 8, 2020           FreeBSD 13.0-RELEASE