KQUEUE(2) BSD Programmer's Manual KQUEUE(2)
kqueue, kevent - 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 execut-
ed 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.
kqueue() 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.
kevent() is used to register events with the queue, and return any pend-
ing 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 a non-null pointer, it
specifies a maximum interval to wait for an event, which will be inter-
preted 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.
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 */
intptr_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:
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_DELETE Removes the event from the kqueue. Events which are at-
tached to file descriptors are automatically deleted on
the last close of the descriptor.
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 tran-
sitions 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 sock-
et buffer. This may be overridden with a per-filter
low water mark at the time the filter is added by set-
ting 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 condi-
tion 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 wait-
ing 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" en-
abled 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_AIO The sigevent portion of the AIO request is filled in, with
sigev_notify_kqueue containing the descriptor of the
kqueue that the event should be attached to, sigev_value
containing the udata value, and sigev_notify set to
SIGEV_KEVENT. When the aio_* function is called, the event
will be registered with the specified kqueue, and the
ident argument set to the struct aiocb returned by the
aio_* function. The filter returns under the same condi-
tions as aio_error.
Alternatively, a kevent structure may be initialized, with
ident containing the descriptor of the kqueue, and the ad-
dress of the kevent structure placed in the aio_lio_opcode
field of the AIO request. However, this approach will not
work on architectures with 64-bit pointers, and should be
considered deprecated.
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_DELETE unlink() 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 via
revoke(2) or the underlying filesystem 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 in fflags, and returns when the pro-
cess 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.
NOTE_FORK The process has called fork().
NOTE_EXEC The process has executed a new process
via execve(2) or similar call.
NOTE_TRACK Follow a process across fork() 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.
NOTE_TRACKERR This flag is returned if the system was
unable to attach an event to the child
process, usually due to resource limita-
tions.
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() facili-
ties, 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 hap-
pens 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.
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:
[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.
The kevent() function 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 be-
fore 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.
poll(2), read(2), select(2), sigaction(2), write(2), signal(3)
The kqueue() and kevent() functions first appeared in FreeBSD 4.1.
The kqueue() system and this manual page were written by Jonathan Lemon
<jlemon@FreeBSD.org>.
It is currently not possible to watch FIFOs, AIO, or a vnode that resides
on anything but a UFS filesystem.
The timeout value is limited to 24 hours; longer timeouts will be silent-
ly reinterpreted as 24 hours.
MirBSD #10-current April 14, 2000 5