NAME

kqueue,kqueue1,kevent,EV_SET —kernel event notification mechanism

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);

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

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

int
kqueue1(int flags);

DESCRIPTION

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 singlestruct kevent. Callingclose(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 withfork(2). Similarly, kqueues cannot be passed across UNIX-domain sockets.

Thekqueue1() function is identical tokqueue() except that the close-on-exec flag on the new file descriptor is determined by theO_CLOEXEC flag in theflags argument.

kevent() is used to register events with the queue, and return any pending events to the user.changelist is a pointer to an array ofkevent structures, as defined in<sys/event.h>. All changes contained in thechangelist are applied before any pending events are read from the queue.nchanges gives the size ofchangelist.eventlist is a pointer to an array ofkevent structures.nevents determines the size ofeventlist. Whennevents is zero,kevent() will return immediately even if there is atimeout specified, unlikeselect(2). Iftimeout is notNULL, it specifies a maximum interval to wait for an event, which will be interpreted as astruct timespec. Iftimeout isNULL,kevent() waits indefinitely. To effect a poll, thetimeout argument should not beNULL, pointing to a zero-valuedstruct timespec. The same array may be used for thechangelist andeventlist.

EV_SET() is a macro which is provided for ease of initializing akevent structure.

Thekevent 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 ofstruct 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.

Theflags 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 theEV_DISABLE flag.

EV_ENABLE

Permitkevent() to return the event if it is triggered.

EV_DISABLE

Disable the event sokevent() will not return it. The filter itself is not disabled.

EV_DISPATCH

Disable the event source immediately after delivery of an event. SeeEV_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

Causeskevent() to return withEV_ERROR set without draining any pending events after updating events in the kqueue. When a filter is successfully added, thedata field will be zero. This flag is useful for making bulk changes to a kqueue.

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

SeeRETURN VALUES below.

The predefined system filters are listed below. Arguments may be passed to and from the filter via thefflags anddata fields in thekevent 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

Sockets which have previously been passed tolisten(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 theSO_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 theNOTE_LOWAT flag infflags, and specifying the new low water mark indata. 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 setsEV_EOF inflags, and returns the socket error (if any) infflags. 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. IfNOTE_EOF is set infflags,kevent() will also return when the file pointer is at the end of file. The end of file condition is indicated by the presence ofNOTE_EOF infflags 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 setEV_EOF inflags. This may be cleared by passing inEV_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.

EVFILT_EXCEPT

Takes a descriptor as the identifier, and returns whenever one of the specified exceptional conditions has occurred on the descriptor. Conditions are specified infflags. Currently, a filter can monitor the reception of out-of-band data on a socket or pseudo terminal withNOTE_OOB.

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 setEV_EOF when the reader disconnects, and for the FIFO case, this may be cleared by use ofEV_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 theEVFILT_READ case.

EVFILT_VNODE

Takes a file descriptor as the identifier and the events to watch for infflags, and returns when one or more of the requested events occurs on the descriptor. The events to monitor are:

NOTE_DELETE

unlink(2) 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_TRUNCATE

The file referenced by the descriptor was truncated.

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 viarevoke(2) or the underlying file system was unmounted.

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 infflags, 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 indata in the same format as the status set bywait(2).

NOTE_FORK

The process has calledfork(2).

NOTE_EXEC

The process has executed a new process viaexecve(2) or similar call.

NOTE_TRACK

Follow a process acrossfork(2) calls. The parent process will return withNOTE_FORK set in thefflags field, while the child process will return withNOTE_CHILD set infflags and the parent PID indata.

NOTE_TRACKERR

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.

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 thesignal(3) andsigaction(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 asSIG_IGN. Event notification happens after normal signal delivery processing.data returns the number of times the signal has occurred since the last call tokevent(). This filter automatically sets theEV_CLEAR flag internally.

EVFILT_TIMER

Establishes an arbitrary timer identified byident. When adding a timer,data specifies the timeout period in units described below or, ifNOTE_ABSTIME is set infflags, the absolute time at which the timer should fire. The timer will repeat unlessEV_ONESHOT is set inflags orNOTE_ABSTIME is set infflags. On return,data contains the number of times the timeout has expired since the last call tokevent(). This filter automatically setsEV_CLEAR inflags for periodic timers. Timers created withNOTE_ABSTIME remain activated on the kqueue once the absolute time has passed unlessEV_CLEAR orEV_ONESHOT are also specified.

The filter accepts the following flags in thefflags argument:

NOTE_SECONDS

The timer value indata is expressed in seconds.

NOTE_MSECONDS

The timer value indata is expressed in milliseconds.

NOTE_USECONDS

The timer value indata is expressed in microseconds.

NOTE_NSECONDS

The timer value indata is expressed in nanoseconds.

NOTE_ABSTIME

The timer value is an absolute time withCLOCK_REALTIME as the reference clock.

Note thatNOTE_SECONDS,NOTE_MSECONDS,NOTE_USECONDS, andNOTE_NSECONDS are mutually exclusive; behavior is undefined if more than one are specified. If a timer value unit is not specified, the default isNOTE_MSECONDS.

If an existing timer is re-added, the existing timer and related pending events will be cancelled. The timer will be re-started using the timeout perioddata.

EVFILT_DEVICE

Takes a descriptor as the identifier and the events to watch for infflags, and returns when one or more of the requested events occur on the descriptor. The events to monitor are:

NOTE_CHANGE

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.

EVFILT_USER

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

NOTE_FFNOP

Ignore the inputfflags.

NOTE_FFAND

Bitwise ANDfflags.

NOTE_FFOR

Bitwise ORfflags.

NOTE_FFCOPY

Copyfflags.

NOTE_FFCTRLMASK

Control mask forfflags.

NOTE_FFLAGSMASK

User defined flag mask forfflags.

A user event is triggered for output with the following:

NOTE_TRIGGER

Cause the event to be triggered.

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

RETURN VALUES

kqueue() andkqueue1() create 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 anderrno set.

kevent() returns the number of events placed in theeventlist, up to the value given bynevents. If an error occurs while processing an element of thechangelist and there is enough room in theeventlist, then the event will be placed in theeventlist withEV_ERROR set inflags and the system error indata. Otherwise, -1 will be returned, anderrno will be set to indicate the error condition. If the time limit expires, thenkevent() returns 0.

ERRORS

Thekqueue() andkqueue1() functions fail 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.

In addition,kqueue1() fails if:

[EINVAL]

flags is invalid.

Thekevent() function fails if:

[EACCES]

The process does not have permission to register a filter.

[EFAULT]

There was an error reading or writing thekevent 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

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

HISTORY

Thekqueue() andkevent() functions first appeared inFreeBSD 4.1 and have been available sinceOpenBSD 2.9.

AUTHORS

Thekqueue() system and this manual page were written byJonathan Lemon <jlemon@FreeBSD.org>.