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     kqueue, kevent — 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 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 */
             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 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.

     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

     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

     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 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

                             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

                             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

                         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 “immediate mode” enabled and there is any data to
                             read; data contains the number of bytes available.

     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.

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

     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

     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 changed.

                         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

                                              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

                         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 extended.

                                              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 directory.

                         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

                         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

                         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 call.

                         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

                         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

     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

                         The filter accepts the following flags in the fflags argument:

                         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 absolute.

                         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 fflags.

                         A user event is triggered for output with the following:

                         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(;

         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 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.

     [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 deleted.

     [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, “Kqueue: A Generic and Scalable Event Notification Facility”, Proceedings of
     the FREENIX Track: 2001 USENIX Annual Technical Conference, USENIX Association, June 25-30,


     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.