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KQUEUE(2) System Calls Manual KQUEUE(2)

kqueue, kevent, EV_SET —
kernel event notification mechanism

#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);

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

kqueue() 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(2) on a file descriptor will remove any kevents that reference the descriptor.

kqueue() creates a new kernel event queue and returns a descriptor. The queue is not inherited by a child created with fork(2). Similarly, kqueues cannot be passed across UNIX-domain sockets.

kevent() is used to register events with the queue, and return any pending events to the user. changelist 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. nchanges gives the size of changelist. eventlist is a pointer to an array of kevent structures. nevents 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 not NULL, it specifies a maximum interval to wait for an event, which will be interpreted as a struct timespec. If timeout is NULL, kevent() waits indefinitely. To effect a poll, the timeout argument should not be NULL, pointing to a zero-valued struct timespec. The same array may be used for the changelist and eventlist.

EV_SET() is a macro which 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 */
};

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.

The flags field can contain the following values:

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.
Permit kevent() to return the event if it is triggered.
Disable the event so kevent() will not return it. The filter itself is not disabled.
Disable the event source immediately after delivery of an event. See EV_DISABLE above.
Removes the event from the kqueue. Events which are attached to file descriptors are automatically deleted on the last close of the descriptor.
Causes kevent() to return with EV_ERROR set without draining any pending events after updating events in the kqueue. When a filter is successfully added the data field will be zero. This flag is useful for making bulk changes to a kqueue.
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.
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.
Filters may set this flag to indicate filter-specific EOF condition.
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.

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
Sockets which have previously been passed to listen(2) 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 in the socket buffer.

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.

Vnodes
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. If NOTE_EOF is set in fflags, kevent() will also return when the file pointer is at the end of file. The end of file condition is indicated by the presence of NOTE_EOF in fflags on return.
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 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.
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 may be cleared by use of EV_CLEAR. 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.

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:
unlink(2) was called on the file referenced by the descriptor.
A write occurred on the file referenced by the descriptor.
The file referenced by the descriptor was extended.
The file referenced by the descriptor was truncated.
The file referenced by the descriptor had its attributes changed.
The link count on the file changed.
The file referenced by the descriptor was renamed.
Access to the file was revoked via revoke(2) or the underlying file system was unmounted.

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

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:
The process has exited. The exit status will be stored in data in the same format as the status set by wait(2).
The process has called fork(2).
The process has executed a new process via execve(2) or similar call.
Follow a process across fork(2) calls. The parent process will return with NOTE_FORK set in the fflags field, while the child process will return with NOTE_CHILD set in fflags and the parent PID in data.
This flag is returned if the system was unable to attach an event to the child process, usually due to resource limitations.

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

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(3) and sigaction(2) 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. 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.
Establishes an arbitrary timer identified by ident. When adding a timer, data specifies the timeout period in 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(). This filter automatically sets the EV_CLEAR flag internally.
Takes a descriptor as the identifier and the events to watch for in fflags, and returns when one or more of the requested events occur on the descriptor. The events to monitor are:
A device change event has occurred, e.g. an HDMI cable has been plugged in to a port.

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

kqueue() 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.

kevent() 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.

The kqueue() function fails if:
[]
The kernel failed to allocate enough memory for the kernel queue.
[]
The per-process descriptor table is full.
[]
The system file table is full.

The kevent() function fails if:

[]
The process does not have permission to register a filter.
[]
There was an error reading or writing the kevent structure.
[]
The specified descriptor is invalid.
[]
A signal was delivered before the timeout expired and before any events were placed on the kqueue for return.
[]
The specified time limit or filter is invalid.
[]
The event could not be found to be modified or deleted.
[]
No memory was available to register the event.
[]
The specified process to attach to does not exist.

poll(2), read(2), select(2), sigaction(2), wait(2), write(2), signal(3)

The kqueue() and kevent() functions first appeared in FreeBSD 4.1 and have been available since OpenBSD 2.9.

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

It is currently not possible to watch FIFOs or AIO that reside on anything but a UFS file system. Watching a vnode is possible on UFS, NFS and MS-DOS file systems.
July 1, 2019 OpenBSD-current