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260 lines
6.0 KiB
Groff
260 lines
6.0 KiB
Groff
.\"
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.\" Copyright (C) 2000 Jason Evans <jasone@FreeBSD.org>. 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(s), this list of conditions and the following disclaimer as
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.\" the first lines of this file unmodified other than the possible
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.\" addition of one or more copyright notices.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice(s), 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 COPYRIGHT HOLDER(S) ``AS IS'' AND ANY
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.\" EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
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.\" WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
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.\" DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER(S) BE LIABLE FOR ANY
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.\" DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
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.\" (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
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.\" SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
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.\" CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH
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.\" DAMAGE.
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.\"
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.Dd February 19, 2013
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.Dt CONDVAR 9
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.Os
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.Sh NAME
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.Nm condvar ,
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.Nm cv_init ,
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.Nm cv_destroy ,
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.Nm cv_wait ,
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.Nm cv_wait_sig ,
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.Nm cv_wait_unlock ,
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.Nm cv_timedwait ,
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.Nm cv_timedwait_sbt ,
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.Nm cv_timedwait_sig ,
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.Nm cv_timedwait_sig_sbt ,
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.Nm cv_signal ,
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.Nm cv_broadcast ,
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.Nm cv_broadcastpri ,
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.Nm cv_wmesg
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.Nd kernel condition variable
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.Sh SYNOPSIS
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.In sys/param.h
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.In sys/proc.h
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.In sys/condvar.h
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.Ft void
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.Fn cv_init "struct cv *cvp" "const char *desc"
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.Ft void
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.Fn cv_destroy "struct cv *cvp"
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.Ft void
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.Fn cv_wait "struct cv *cvp" "lock"
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.Ft int
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.Fn cv_wait_sig "struct cv *cvp" "lock"
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.Ft void
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.Fn cv_wait_unlock "struct cv *cvp" "lock"
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.Ft int
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.Fn cv_timedwait "struct cv *cvp" "lock" "int timo"
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.Ft int
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.Fn cv_timedwait_sbt "struct cv *cvp" "lock" "sbintime_t sbt" \
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"sbintime_t pr" "int flags"
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.Ft int
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.Fn cv_timedwait_sig "struct cv *cvp" "lock" "int timo"
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.Ft int
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.Fn cv_timedwait_sig_sbt "struct cv *cvp" "lock" "sbintime_t sbt" \
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"sbintime_t pr" "int flags"
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.Ft void
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.Fn cv_signal "struct cv *cvp"
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.Ft void
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.Fn cv_broadcast "struct cv *cvp"
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.Ft void
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.Fn cv_broadcastpri "struct cv *cvp" "int pri"
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.Ft const char *
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.Fn cv_wmesg "struct cv *cvp"
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.Sh DESCRIPTION
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Condition variables are used in conjunction with mutexes to wait for conditions
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to occur.
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Condition variables are created with
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.Fn cv_init ,
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where
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.Fa cvp
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is a pointer to space for a
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.Vt struct cv ,
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and
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.Fa desc
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is a pointer to a null-terminated character string that describes the condition
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variable.
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Condition variables are destroyed with
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.Fn cv_destroy .
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Threads wait on condition variables by calling
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.Fn cv_wait ,
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.Fn cv_wait_sig ,
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.Fn cv_wait_unlock ,
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.Fn cv_timedwait ,
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or
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.Fn cv_timedwait_sig .
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Threads unblock waiters by calling
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.Fn cv_signal
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to unblock one waiter, or
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.Fn cv_broadcast
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or
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.Fn cv_broadcastpri
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to unblock all waiters.
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In addition to waking waiters,
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.Fn cv_broadcastpri
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ensures that all of the waiters have a priority of at least
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.Fa pri
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by raising the priority of any threads that do not.
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.Fn cv_wmesg
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returns the description string of
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.Fa cvp ,
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as set by the initial call to
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.Fn cv_init .
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.Pp
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The
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.Fa lock
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argument is a pointer to either a
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.Xr mutex 9 ,
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.Xr rwlock 9 ,
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or
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.Xr sx 9
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lock.
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A
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.Xr mutex 9
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argument must be initialized with
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.Dv MTX_DEF
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and not
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.Dv MTX_SPIN .
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A thread must hold
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.Fa lock
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before calling
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.Fn cv_wait ,
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.Fn cv_wait_sig ,
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.Fn cv_wait_unlock ,
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.Fn cv_timedwait ,
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or
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.Fn cv_timedwait_sig .
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When a thread waits on a condition,
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.Fa lock
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is atomically released before the thread is blocked, then reacquired
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before the function call returns.
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In addition, the thread 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 it 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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The
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.Fn cv_wait_unlock
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function does not reacquire the lock before returning.
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Note that the
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.Va Giant
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mutex may be specified as
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.Fa lock .
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However,
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.Va Giant
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may not be used as
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.Fa lock
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for the
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.Fn cv_wait_unlock
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function.
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All waiters must pass the same
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.Fa lock
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in conjunction with
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.Fa cvp .
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.Pp
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When
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.Fn cv_wait ,
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.Fn cv_wait_sig ,
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.Fn cv_wait_unlock ,
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.Fn cv_timedwait ,
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and
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.Fn cv_timedwait_sig
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unblock, their calling threads are made runnable.
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.Fn cv_timedwait
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and
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.Fn cv_timedwait_sig
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wait for at most
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.Fa timo
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/
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.Dv HZ
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seconds before being unblocked and returning
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.Er EWOULDBLOCK ;
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otherwise, they return 0.
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.Fn cv_wait_sig
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and
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.Fn cv_timedwait_sig
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return prematurely with a value of
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.Er EINTR
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or
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.Er ERESTART
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if a signal is caught, or 0 if signaled via
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.Fn cv_signal
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or
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.Fn cv_broadcast .
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.Pp
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.Fn cv_timedwait_sbt
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and
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.Fn cv_timedwait_sig_sbt
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functions take
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.Fa sbt
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argument instead of
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.Fa timo .
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It allows to specify relative or absolute unblock time with higher resolution
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in form of
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.Vt sbintime_t .
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The parameter
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.Fa pr
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allows to specify wanted absolute event precision.
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The parameter
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.Fa flags
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allows to pass additional
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.Fn callout_reset_sbt
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flags.
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.Sh RETURN VALUES
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If successful,
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.Fn cv_wait_sig ,
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.Fn cv_timedwait ,
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and
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.Fn cv_timedwait_sig
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return 0.
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Otherwise, a non-zero error code is returned.
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.Pp
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.Fn cv_wmesg
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returns the description string that was passed to
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.Fn cv_init .
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.Sh ERRORS
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.Fn cv_wait_sig
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and
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.Fn cv_timedwait_sig
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will fail if:
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.Bl -tag -width Er
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.It Bq Er EINTR
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A signal was caught and the system call should be interrupted.
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.It Bq Er ERESTART
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A signal was caught and the system call should be restarted.
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.El
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.Pp
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.Fn cv_timedwait
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and
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.Fn cv_timedwait_sig
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will fail if:
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.Bl -tag -width Er
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.It Bq Er EWOULDBLOCK
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Timeout expired.
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.El
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.Sh SEE ALSO
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.Xr callout 9 ,
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.Xr locking 9 ,
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.Xr mtx_pool 9 ,
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.Xr mutex 9 ,
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.Xr rwlock 9 ,
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.Xr sema 9 ,
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.Xr sleep 9 ,
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.Xr sx 9
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