IABSD.fr/src/lib/libc/sys/kqueue.2

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• Author : jan
  Date : 2021-11-21 23:44:55
  Hash : 0215088d
  Message : improve legibility of structs in several manpages General uses tabs for general indentation and 4 spaces on tight spots. Also uses extra space to align pointers and non-pointers as we do this on certain places in our source. with improvements from schwarze@ OK schwarze@

• lib/libc/sys/kqueue.2

• .\"	$OpenBSD: kqueue.2,v 1.45 2021/11/21 23:44:55 jan Exp $
  .\"
  .\" Copyright (c) 2000 Jonathan Lemon
  .\" All rights reserved.
  .\"
  .\" Redistribution and use in source and binary forms, with or without
  .\" modification, are permitted provided that the following conditions
  .\" are met:
  .\" 1. Redistributions of source code must retain the above copyright
  .\"    notice, this list of conditions and the following disclaimer.
  .\" 2. Redistributions in binary form must reproduce the above copyright
  .\"    notice, this list of conditions and the following disclaimer in the
  .\"    documentation and/or other materials provided with the distribution.
  .\"
  .\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND
  .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  .\" ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
  .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  .\" SUCH DAMAGE.
  .\"
  .\" $FreeBSD: src/lib/libc/sys/kqueue.2,v 1.18 2001/02/14 08:48:35 guido Exp $
  .\"
  .Dd $Mdocdate: November 21 2021 $
  .Dt KQUEUE 2
  .Os
  .Sh NAME
  .Nm kqueue ,
  .Nm kevent ,
  .Nm EV_SET
  .Nd kernel event notification mechanism
  .Sh SYNOPSIS
  .In sys/types.h
  .In sys/event.h
  .In sys/time.h
  .Ft int
  .Fn kqueue "void"
  .Ft int
  .Fn kevent "int kq" "const struct kevent *changelist" "int nchanges" "struct kevent *eventlist" "int nevents" "const struct timespec *timeout"
  .Fn EV_SET "&kev" ident filter flags fflags data udata
  .Sh DESCRIPTION
  .Fn 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
  .Dq filters .
  A kevent is identified by the (ident, filter) pair; there may only
  be one unique kevent per kqueue.
  .Pp
  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.
  .Pp
  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.
  .Pp
  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
  .Vt struct kevent .
  Calling
  .Xr close 2
  on a file descriptor will remove any kevents that reference the descriptor.
  .Pp
  .Fn kqueue
  creates a new kernel event queue and returns a descriptor.
  The queue is not inherited by a child created with
  .Xr fork 2 .
  Similarly, kqueues cannot be passed across UNIX-domain sockets.
  .Pp
  .Fn kevent
  is used to register events with the queue, and return any pending
  events to the user.
  .Fa changelist
  is a pointer to an array of
  .Vt kevent
  structures, as defined in
  .In sys/event.h .
  All changes contained in the
  .Fa changelist
  are applied before any pending events are read from the queue.
  .Fa nchanges
  gives the size of
  .Fa changelist .
  .Fa eventlist
  is a pointer to an array of
  .Vt kevent
  structures.
  .Fa nevents
  determines the size of
  .Fa eventlist .
  When
  .Fa nevents
  is zero,
  .Fn kevent
  will return immediately even if there is a
  .Fa timeout
  specified unlike
  .Xr select 2 .
  If
  .Fa timeout
  is not
  .Dv NULL ,
  it specifies a maximum interval to wait
  for an event, which will be interpreted as a
  .Vt struct timespec .
  If
  .Fa timeout
  is
  .Dv NULL ,
  .Fn kevent
  waits indefinitely.
  To effect a poll, the
  .Fa timeout
  argument should not be
  .Dv NULL ,
  pointing to a zero-valued
  .Vt struct timespec .
  The same array may be used for the
  .Fa changelist
  and
  .Fa eventlist .
  .Pp
  .Fn EV_SET
  is a macro which is provided for ease of initializing a
  .Vt kevent
  structure.
  .Pp
  The
  .Vt kevent
  structure is defined as:
  .Bd -literal
  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 */
  };
  .Ed
  .Pp
  The fields of
  .Vt struct kevent
  are:
  .Bl -tag -width XXXfilter
  .It Fa ident
  Value used to identify this event.
  The exact interpretation is determined by the attached filter,
  but often is a file descriptor.
  .It Fa filter
  Identifies the kernel filter used to process this event.
  The pre-defined system filters are described below.
  .It Fa flags
  Actions to perform on the event.
  .It Fa fflags
  Filter-specific flags.
  .It Fa data
  Filter-specific data value.
  .It Fa udata
  Opaque user-defined value passed through the kernel unchanged.
  .El
  .Pp
  The
  .Fa flags
  field can contain the following values:
  .Bl -tag -width XXXEV_ONESHOT
  .It Dv 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
  .Dv EV_DISABLE
  flag.
  .It Dv EV_ENABLE
  Permit
  .Fn kevent
  to return the event if it is triggered.
  .It Dv EV_DISABLE
  Disable the event so
  .Fn kevent
  will not return it.
  The filter itself is not disabled.
  .It Dv EV_DISPATCH
  Disable the event source immediately after delivery of an event.
  See
  .Dv EV_DISABLE
  above.
  .It Dv 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.
  .It Dv EV_RECEIPT
  Causes
  .Fn kevent
  to return with
  .Dv EV_ERROR
  set without draining any pending events after updating events in the kqueue.
  When a filter is successfully added the
  .Fa data
  field will be zero.
  This flag is useful for making bulk changes to a kqueue.
  .It Dv 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.
  .It Dv 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.
  .It Dv EV_EOF
  Filters may set this flag to indicate filter-specific EOF condition.
  .It Dv EV_ERROR
  See
  .Sx RETURN VALUES
  below.
  .El
  .Pp
  The predefined system filters are listed below.
  Arguments may be passed to and from the filter via the
  .Fa fflags
  and
  .Fa data
  fields in the
  .Vt kevent
  structure.
  .Bl -tag -width EVFILT_SIGNAL
  .It Dv 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.
  .Bl -tag -width 2n
  .It Sockets
  Sockets which have previously been passed to
  .Xr listen 2
  return when there is an incoming connection pending.
  .Fa data
  contains the size of the listen backlog.
  .Pp
  Other socket descriptors return when there is data to be read,
  subject to the
  .Dv 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
  .Dv NOTE_LOWAT
  flag in
  .Fa fflags ,
  and specifying the new low water mark in
  .Fa data .
  On return,
  .Fa data
  contains the number of bytes in the socket buffer.
  .Pp
  If the read direction of the socket has shutdown, then the filter
  also sets
  .Dv EV_EOF
  in
  .Fa flags ,
  and returns the socket error (if any) in
  .Fa fflags .
  It is possible for EOF to be returned (indicating the connection is gone)
  while there is still data pending in the socket buffer.
  .It Vnodes
  Returns when the file pointer is not at the end of file.
  .Fa data
  contains the offset from current position to end of file,
  and may be negative.
  If
  .Dv NOTE_EOF
  is set in
  .Fa fflags ,
  .Fn 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
  .Dv NOTE_EOF
  in
  .Fa fflags
  on return.
  .It "FIFOs, Pipes"
  Returns when there is data to read;
  .Fa data
  contains the number of bytes available.
  .Pp
  When the last writer disconnects, the filter will set
  .Dv EV_EOF
  in
  .Fa flags .
  This may be cleared by passing in
  .Dv EV_CLEAR ,
  at which point the filter will resume waiting for data to become
  available before returning.
  .It "BPF devices"
  Returns when the BPF buffer is full, the BPF timeout has expired, or
  when the BPF has
  .Dq immediate mode
  enabled and there is any data to read;
  .Fa data
  contains the number of bytes available.
  .El
  .It Dv 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 in
  .Fa fflags .
  Currently, a filter can monitor the reception of out-of-band data
  on a socket or pseudo terminal with
  .Dv NOTE_OOB .
  .It Dv EVFILT_WRITE
  Takes a descriptor as the identifier, and returns whenever
  it is possible to write to the descriptor.
  For sockets, pipes, and FIFOs,
  .Fa data
  will contain the amount of space remaining in the write buffer.
  The filter will set
  .Dv EV_EOF
  when the reader disconnects, and for the FIFO case,
  this may be cleared by use of
  .Dv EV_CLEAR .
  Note that this filter is not supported for vnodes or BPF devices.
  .Pp
  For sockets, the low water mark and socket error handling is
  identical to the
  .Dv EVFILT_READ
  case.
  .\".It Dv EVFILT_AIO
  .\"The sigevent portion of the AIO request is filled in, with
  .\".Va sigev_notify_kqueue
  .\"containing the descriptor of the kqueue that the event should
  .\"be attached to,
  .\".Va sigev_value
  .\"containing the udata value, and
  .\".Va sigev_notify
  .\"set to
  .\".Dv SIGEV_KEVENT .
  .\"When the aio_* function is called, the event will be registered
  .\"with the specified kqueue, and the
  .\".Va ident
  .\"argument set to the
  .\".Li struct aiocb
  .\"returned by the aio_* function.
  .\"The filter returns under the same conditions as aio_error.
  .\".Pp
  .\"Alternatively, a kevent structure may be initialized, with
  .\".Va ident
  .\"containing the descriptor of the kqueue, and the
  .\"address of the kevent structure placed in the
  .\".Va 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.
  .It Dv EVFILT_VNODE
  Takes a file descriptor as the identifier and the events to watch for in
  .Fa fflags ,
  and returns when one or more of the requested events occurs on the descriptor.
  The events to monitor are:
  .Bl -tag -width XXNOTE_RENAME
  .It Dv NOTE_DELETE
  .Xr unlink 2
  was called on the file referenced by the descriptor.
  .It Dv NOTE_WRITE
  A write occurred on the file referenced by the descriptor.
  .It Dv NOTE_EXTEND
  The file referenced by the descriptor was extended.
  .It Dv NOTE_TRUNCATE
  The file referenced by the descriptor was truncated.
  .It Dv NOTE_ATTRIB
  The file referenced by the descriptor had its attributes changed.
  .It Dv NOTE_LINK
  The link count on the file changed.
  .It Dv NOTE_RENAME
  The file referenced by the descriptor was renamed.
  .It Dv NOTE_REVOKE
  Access to the file was revoked via
  .Xr revoke 2
  or the underlying file system was unmounted.
  .El
  .Pp
  On return,
  .Fa fflags
  contains the events which triggered the filter.
  .It Dv EVFILT_PROC
  Takes the process ID to monitor as the identifier and the events to watch for
  in
  .Fa 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:
  .Bl -tag -width XXNOTE_TRACKERR
  .It Dv NOTE_EXIT
  The process has exited.
  The exit status will be stored in
  .Fa data
  in the same format as the status set by
  .Xr wait 2 .
  .It Dv NOTE_FORK
  The process has called
  .Xr fork 2 .
  .It Dv NOTE_EXEC
  The process has executed a new process via
  .Xr execve 2
  or similar call.
  .It Dv NOTE_TRACK
  Follow a process across
  .Xr fork 2
  calls.
  The parent process will return with
  .Dv NOTE_FORK
  set in the
  .Fa fflags
  field, while the child process will return with
  .Dv NOTE_CHILD
  set in
  .Fa fflags
  and the parent PID in
  .Fa data .
  .It Dv NOTE_TRACKERR
  This flag is returned if the system was unable to attach an event to
  the child process, usually due to resource limitations.
  .El
  .Pp
  On return,
  .Fa fflags
  contains the events which triggered the filter.
  .It Dv 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
  .Xr signal 3
  and
  .Xr 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
  .Dv SIG_IGN .
  Event notification happens after normal signal delivery processing.
  .Fa data
  returns the number of times the signal has occurred since the last call to
  .Fn kevent .
  This filter automatically sets the
  .Dv EV_CLEAR
  flag internally.
  .It Dv EVFILT_TIMER
  Establishes an arbitrary timer identified by
  .Fa ident .
  When adding a timer,
  .Fa data
  specifies the timeout period in milliseconds.
  The timer will be periodic unless
  .Dv EV_ONESHOT
  is specified.
  On return,
  .Fa data
  contains the number of times the timeout has expired since the last call to
  .Fn kevent .
  This filter automatically sets the
  .Dv EV_CLEAR
  flag internally.
  .Pp
  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 period
  .Fa data .
  .It Dv EVFILT_DEVICE
  Takes a descriptor as the identifier and the events to watch for in
  .Fa fflags ,
  and returns when one or more of the requested events occur on the
  descriptor.
  The events to monitor are:
  .Bl -tag -width XXNOTE_CHANGE
  .It Dv NOTE_CHANGE
  A device change event has occurred, e.g. an HDMI cable has been plugged in to a port.
  .El
  .Pp
  On return,
  .Fa fflags
  contains the events which triggered the filter.
  .El
  .Sh RETURN VALUES
  .Fn 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
  .Va errno
  set.
  .Pp
  .Fn kevent
  returns the number of events placed in the
  .Fa eventlist ,
  up to the value given by
  .Fa nevents .
  If an error occurs while processing an element of the
  .Fa changelist
  and there is enough room in the
  .Fa eventlist ,
  then the event will be placed in the
  .Fa eventlist
  with
  .Dv EV_ERROR
  set in
  .Fa flags
  and the system error in
  .Fa data .
  Otherwise, -1 will be returned, and
  .Va errno
  will be set to indicate the error condition.
  If the time limit expires, then
  .Fn kevent
  returns 0.
  .Sh ERRORS
  The
  .Fn kqueue
  function fails if:
  .Bl -tag -width Er
  .It Bq Er ENOMEM
  The kernel failed to allocate enough memory for the kernel queue.
  .It Bq Er EMFILE
  The per-process descriptor table is full.
  .It Bq Er ENFILE
  The system file table is full.
  .El
  .Pp
  The
  .Fn kevent
  function fails if:
  .Bl -tag -width Er
  .It Bq Er EACCES
  The process does not have permission to register a filter.
  .It Bq Er EFAULT
  There was an error reading or writing the
  .Vt kevent
  structure.
  .It Bq Er EBADF
  The specified descriptor is invalid.
  .It Bq Er EINTR
  A signal was delivered before the timeout expired and before any
  events were placed on the kqueue for return.
  .It Bq Er EINVAL
  The specified time limit or filter is invalid.
  .It Bq Er ENOENT
  The event could not be found to be modified or deleted.
  .It Bq Er ENOMEM
  No memory was available to register the event.
  .It Bq Er ESRCH
  The specified process to attach to does not exist.
  .El
  .Sh SEE ALSO
  .Xr poll 2 ,
  .Xr read 2 ,
  .Xr select 2 ,
  .Xr sigaction 2 ,
  .Xr wait 2 ,
  .Xr write 2 ,
  .Xr signal 3
  .Sh HISTORY
  The
  .Fn kqueue
  and
  .Fn kevent
  functions first appeared in
  .Fx 4.1
  and have been available since
  .Ox 2.9 .
  .Sh AUTHORS
  The
  .Fn kqueue
  system and this manual page were written by
  .An Jonathan Lemon Aq Mt jlemon@FreeBSD.org .