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KQUEUE(9) Kernel Developer's Manual KQUEUE(9)


kqueue_add_filteropts, kqueue_del_filteropts, kqfd_register, knote_fdclose, knlist_init, knlist_init_mtx, knlist_init_rw_reader, knlist_add, knlist_remove, knlist_remove_inevent, knlist_empty, knlist_clear, knlist_delete, knlist_destroy, KNOTE_LOCKED, KNOTE_UNLOCKEDevent delivery subsystem


#include <sys/event.h>

kqueue_add_filteropts(int filt, struct filterops *filtops);

kqueue_del_filteropts(int filt);

kqfd_register(int fd, struct kevent *kev, struct thread *td, int waitok);

knote_fdclose(struct thread *td, int fd);

knlist_init(struct knlist *knl, void *lock, void (*kl_lock)(void *), void (*kl_unlock)(void *), int (*kl_locked)(void *));

knlist_init_mtx(struct knlist *knl, struct mtx *lock);

knlist_init_rw_reader(struct knlist *knl, struct rwlock *lock);

knlist_add(struct knlist *knl, struct knote *kn, int islocked);

knlist_remove(struct knlist *knl, struct knote *kn, int islocked);

knlist_remove_inevent(struct knlist *knl, struct knote *kn);

knlist_empty(struct knlist *knl);

knlist_clear(struct knlist *knl, int islocked);

knlist_delete(struct knlist *knl, struct thread *td, int islocked);

knlist_destroy(struct knlist *knl);

KNOTE_LOCKED(struct knlist *knl, long hint);

KNOTE_UNLOCKED(struct knlist *knl, long hint);


The functions () and () allow for the addition and removal of a filter type. The filter is statically defined by the EVFILT_* macros. The function kqueue_add_filteropts() will make filt available. The struct filterops has the following members:

If f_isfd is set, ident in struct kevent is taken to be a file descriptor. In this case, the knote passed into f_attach will have the kn_fp member initialized to the struct file * that represents the file descriptor.
The f_attach function will be called when attaching a knote to the object. The method should call () to add the knote to the list that was initialized with knlist_init(). The call to knlist_add() is only necessary if the object can have multiple knotes associated with it. If there is no knlist to call knlist_add() with, the function f_attach must clear the KN_DETACHED bit of kn_status in the knote. The function shall return 0 on success, or appropriate error for the failure, such as when the object is being destroyed, or does not exist. During f_attach, it is valid to change the kn_fops pointer to a different pointer. This will change the f_event and f_detach functions called when processing the knote.
The f_detach function will be called to detach the knote if the knote has not already been detached by a call to (), () or knlist_delete(). The list lock will not be held when this function is called.
The f_event function will be called to update the status of the knote. If the function returns 0, it will be assumed that the object is not ready (or no longer ready) to be woken up. The hint argument will be 0 when scanning knotes to see which are triggered. Otherwise, the hint argument will be the value passed to either KNOTE_LOCKED or KNOTE_UNLOCKED. The kn_data value should be updated as necessary to reflect the current value, such as number of bytes available for reading, or buffer space available for writing. If the note needs to be removed, knlist_remove_inevent() must be called. The function knlist_remove_inevent() will remove the note from the list, the f_detach function will not be called and the knote will not be returned as an event.

Locks be acquired in f_event. If a lock is required in f_event, it must be obtained in the kl_lock function of the knlist that the knote was added to.

The function () will register the kevent on the kqueue file descriptor fd. If it is safe to sleep, waitok should be set.

The function () is used to delete all knotes associated with fd. Once returned, there will no longer be any knotes associated with the fd. The knotes removed will never be returned from a kevent(2) call, so if userland uses the knote to track resources, they will be leaked. The () lock must be held over the call to knote_fdclose() so that file descriptors cannot be added or removed.

The () family of functions are for managing knotes associated with an object. A knlist is not required, but is commonly used. If used, the knlist must be initialized with either knlist_init(), knlist_init_mtx() or knlist_init_rw_reader(). The knlist structure may be embedded into the object structure. The lock will be held over f_event calls.

For the () function, if lock is NULL, a shared global lock will be used and the remaining arguments must be NULL. The function pointers kl_lock, kl_unlock and kl_locked will be used to manipulate the argument lock. If any of the function pointers are NULL, a function operating on MTX_DEF style mutex(9) locks will be used instead.

The function () may be used to initialize a knlist when lock is a MTX_DEF style mutex(9) lock.

The function () may be used to initialize a knlist when lock is a rwlock(9) read lock. Lock is acquired via () function.

The function () returns true when there are no knotes on the list. The function requires that the lock be held when called.

The function () removes all knotes from the list. The islocked argument declares if the lock has been acquired. All knotes will have EV_ONESHOT set so that the knote will be returned and removed during the next scan. The f_detach function will be called when the knote is deleted during the next scan. This function must not be used when f_isfd is set in struct filterops, as the td argument of () will be NULL.

The function () removes and deletes all knotes on the list. The function f_detach will not be called, and the knote will not be returned on the next scan. Using this function could leak userland resources if a process uses the knote to track resources.

Both the () and knlist_delete() functions may sleep. They also may release the lock to wait for other knotes to drain.

The () function is used to destroy a knlist. There must be no knotes associated with the knlist (knlist_empty() returns true) and no more knotes may be attached to the object. A knlist may be emptied by calling knlist_clear() or knlist_delete().

The macros () and () are used to notify knotes about events associated with the object. It will iterate over all knotes on the list calling the f_event function associated with the knote. The macro KNOTE_LOCKED() must be used if the lock associated with the knl is held. The function KNOTE_UNLOCKED() will acquire the lock before iterating over the list of knotes.


The function kqueue_add_filteropts() will return zero on success, EINVAL in the case of an invalid filt, or EEXIST if the filter has already been installed.

The function kqueue_del_filteropts() will return zero on success, EINVAL in the case of an invalid filt, or EBUSY if the filter is still in use.

The function kqfd_register() will return zero on success, EBADF if the file descriptor is not a kqueue, or any of the possible values returned by kevent(2).


kevent(2), kqueue(2)


This manual page was written by John-Mark Gurney <>.

March 26, 2012 Debian