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DESCRIPTION

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 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.

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

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.

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.

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 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.

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 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.

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 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 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 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 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 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 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 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 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.

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

int
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. */
    EV_SET(&event, fd, EVFILT_VNODE, EV_ADD | EV_CLEAR, NOTE_WRITE,
	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]);
	}
    }
}