414e7679cb
msleep/mtx_sleep or the various cv_*wait*() routines. Currently, the "unlock" behavior of PDROP and cv_wait_unlock() with Giant is not permitted as it is will be confusing since Giant is fully unrecursed and unlocked during a thread sleep. This is handy for subsystems which wish to allow unlocked drivers to continue to use Giant such as CAM, the new TTY layer, and the new USB stack. CAM currently uses a hack that I told Scott to use because I really didn't want to permit this behavior, and the TTY and USB patches both have various patches to permit this. MFC after: 2 weeks
234 lines
5.4 KiB
Groff
234 lines
5.4 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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.\" $FreeBSD$
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.\"
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.Dd June 5, 2007
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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_sig ,
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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_sig "struct cv *cvp" "lock" "int timo"
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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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.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 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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