339 lines
8.1 KiB
Groff
339 lines
8.1 KiB
Groff
.\"
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.\" Copyright (c) 1996 Joerg Wunsch
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.\"
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE DEVELOPERS ``AS IS'' AND ANY EXPRESS OR
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.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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.\" IN NO EVENT SHALL THE DEVELOPERS BE LIABLE FOR ANY DIRECT, INDIRECT,
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.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
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.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
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.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd December 12, 2009
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.Os
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.Dt SLEEP 9
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.Sh NAME
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.Nm msleep ,
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.Nm msleep_spin ,
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.Nm pause ,
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.Nm tsleep ,
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.Nm wakeup
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.Nd wait for events
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.Sh SYNOPSIS
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.In sys/param.h
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.In sys/systm.h
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.In sys/proc.h
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.Ft int
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.Fn msleep "void *chan" "struct mtx *mtx" "int priority" "const char *wmesg" "int timo"
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.Ft int
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.Fn msleep_spin "void *chan" "struct mtx *mtx" "const char *wmesg" "int timo"
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.Ft void
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.Fn pause "const char *wmesg" "int timo"
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.Ft int
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.Fn tsleep "void *chan" "int priority" "const char *wmesg" "int timo"
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.Ft void
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.Fn wakeup "void *chan"
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.Ft void
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.Fn wakeup_one "void *chan"
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.Sh DESCRIPTION
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The functions
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.Fn tsleep ,
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.Fn msleep ,
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.Fn msleep_spin ,
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.Fn pause ,
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.Fn wakeup ,
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and
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.Fn wakeup_one
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handle event-based thread blocking.
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If a thread must wait for an
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external event, it is put to sleep by
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.Fn tsleep ,
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.Fn msleep ,
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.Fn msleep_spin ,
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or
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.Fn pause .
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Threads may also wait using one of the locking primitive sleep routines
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.Xr mtx_sleep 9 ,
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.Xr rw_sleep 9 ,
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or
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.Xr sx_sleep 9 .
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.Pp
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The parameter
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.Fa chan
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is an arbitrary address that uniquely identifies the event on which
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the thread is being put to sleep.
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All threads sleeping on a single
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.Fa chan
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are woken up later by
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.Fn wakeup ,
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often called from inside an interrupt routine, to indicate that the
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resource the thread was blocking on is available now.
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.Pp
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The parameter
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.Fa priority
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specifies a new priority for the thread as well as some optional flags.
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If the new priority is not 0,
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then the thread will be made
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runnable with the specified
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.Fa priority
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when it resumes.
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.Dv PZERO
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should never be used, as it is for compatibility only.
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A new priority of 0 means to use the thread's current priority when
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it is made runnable again.
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.Pp
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If
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.Fa priority
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includes the
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.Dv PCATCH
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flag, signals are checked before and after sleeping, otherwise signals are
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not checked.
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If
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.Dv PCATCH
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is set and a signal needs to be delivered,
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.Er ERESTART
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is returned if the current system call should be restarted if
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possible, and
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.Er EINTR
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is returned if the system call should be interrupted by the signal
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(return
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.Er EINTR ) .
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If
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.Dv PBDRY
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flag is specified in addition to
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.Dv PCATCH ,
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then the sleeping thread is not stopped while sleeping upon delivery of
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.Dv SIGSTOP
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or other stop action.
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Instead, it is waken up, assuming that stop occurs on reaching a stop
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point when returning to usermode.
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The flag should be used when sleeping thread owns resources, for instance
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vnode locks, that should be freed timely.
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.Pp
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The parameter
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.Fa wmesg
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is a string describing the sleep condition for tools like
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.Xr ps 1 .
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Due to the limited space of those programs to display arbitrary strings,
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this message should not be longer than 6 characters.
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.Pp
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The parameter
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.Fa timo
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specifies a timeout for the sleep.
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If
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.Fa timo
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is not 0,
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then the thread will sleep for at most
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.Fa timo No / Va hz
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seconds.
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If the timeout expires,
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then the sleep function will return
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.Er EWOULDBLOCK .
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.Pp
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Several of the sleep functions including
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.Fn msleep ,
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.Fn msleep_spin ,
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and the locking primitive sleep routines specify an additional lock
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parameter.
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The lock will be released before sleeping and reacquired
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before the sleep routine returns.
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If
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.Fa priority
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includes the
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.Dv PDROP
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flag, then
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the lock will not be reacquired before returning.
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The lock is used to ensure that a condition can be checked atomically,
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and that the current thread can be suspended without missing a
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change to the condition, or an associated wakeup.
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In addition, all of the sleep routines will fully drop the
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.Va Giant
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mutex
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(even if recursed)
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while the thread is suspended and will reacquire the
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.Va Giant
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mutex before the function returns.
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Note that the
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.Va Giant
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mutex may be specified as the lock to drop.
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In that case, however, the
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.Dv PDROP
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flag is not allowed.
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.Pp
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To avoid lost wakeups,
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either a lock should be used to protect against races,
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or a timeout should be specified to place an upper bound on the delay due
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to a lost wakeup.
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As a result,
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the
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.Fn tsleep
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function should only be invoked with a timeout of 0 when the
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.Va Giant
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mutex is held.
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.Pp
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The
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.Fn msleep
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function requires that
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.Fa mtx
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reference a default, i.e. non-spin, mutex.
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Its use is deprecated in favor of
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.Xr mtx_sleep 9
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which provides identical behavior.
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.Pp
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The
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.Fn msleep_spin
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function requires that
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.Fa mtx
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reference a spin mutex.
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The
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.Fn msleep_spin
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function does not accept a
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.Fa priority
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parameter and thus does not support changing the current thread's priority,
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the
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.Dv PDROP
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flag,
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or catching signals via the
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.Dv PCATCH
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flag.
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.Pp
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The
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.Fn pause
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function is a wrapper around
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.Fn tsleep
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that suspends execution of the current thread for the indicated timeout.
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The thread can not be awakened early by signals or calls to
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.Fn wakeup
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or
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.Fn wakeup_one .
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.Pp
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The
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.Fn wakeup_one
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function makes the first thread in the queue that is sleeping on the
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parameter
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.Fa chan
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runnable.
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This reduces the load when a large number of threads are sleeping on
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the same address, but only one of them can actually do any useful work
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when made runnable.
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.Pp
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Due to the way it works, the
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.Fn wakeup_one
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function requires that only related threads sleep on a specific
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.Fa chan
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address.
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It is the programmer's responsibility to choose a unique
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.Fa chan
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value.
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The older
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.Fn wakeup
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function did not require this, though it was never good practice
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for threads to share a
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.Fa chan
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value.
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When converting from
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.Fn wakeup
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to
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.Fn wakeup_one ,
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pay particular attention to ensure that no other threads wait on the
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same
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.Fa chan .
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.Sh RETURN VALUES
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If the thread is awakened by a call to
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.Fn wakeup
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or
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.Fn wakeup_one ,
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the
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.Fn msleep ,
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.Fn msleep_spin ,
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.Fn tsleep ,
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and locking primitive sleep functions return 0.
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Otherwise, a non-zero error code is returned.
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.Sh ERRORS
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.Fn msleep ,
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.Fn msleep_spin ,
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.Fn tsleep ,
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and the locking primitive sleep functions will fail if:
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.Bl -tag -width Er
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.It Bq Er EINTR
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The
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.Dv PCATCH
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flag was specified, a signal was caught, and the system call should be
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interrupted.
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.It Bq Er ERESTART
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The
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.Dv PCATCH
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flag was specified, a signal was caught, and the system call should be
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restarted.
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.It Bq Er EWOULDBLOCK
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A non-zero timeout was specified and the timeout expired.
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.El
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.Sh SEE ALSO
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.Xr ps 1 ,
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.Xr locking 9 ,
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.Xr malloc 9 ,
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.Xr mi_switch 9 ,
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.Xr mtx_sleep 9 ,
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.Xr rw_sleep 9 ,
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.Xr sx_sleep 9
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.Sh HISTORY
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The functions
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.Fn sleep
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and
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.Fn wakeup
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were present in
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.At v1 .
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They were probably also present in the preceding
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PDP-7 version of
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.Ux .
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They were the basic process synchronization model.
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.Pp
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The
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.Fn tsleep
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function appeared in
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.Bx 4.4
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and added the parameters
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.Fa wmesg
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and
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.Fa timo .
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The
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.Fn sleep
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function was removed in
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.Fx 2.2 .
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The
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.Fn wakeup_one
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function appeared in
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.Fx 2.2 .
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The
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.Fn msleep
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function appeared in
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.Fx 5.0 ,
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and the
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.Fn msleep_spin
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function appeared in
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.Fx 6.2 .
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The
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.Fn pause
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function appeared in
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.Fx 7.0 .
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.Sh AUTHORS
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.An -nosplit
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This manual page was written by
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.An J\(:org Wunsch Aq joerg@FreeBSD.org .
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