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201 lines
4.9 KiB
Groff
201 lines
4.9 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 December 11, 2000
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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_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_waitq_remove ,
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.Nm cv_waitq_empty ,
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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" "struct mtx *mp"
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.Ft int
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.Fn cv_wait_sig "struct cv *cvp" "struct mtx *mp"
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.Ft int
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.Fn cv_timedwait "struct cv *cvp" "struct mtx *mp" "int timo"
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.Ft int
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.Fn cv_timedwait_sig "struct cv *cvp" "struct mtx *mp" "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_waitq_remove "struct thread *td"
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.Ft int
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.Fn cv_waitq_empty "struct cv *cvp"
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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_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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to unblock all waiters.
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.Fn cv_waitq_remove
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removes a waiting thread from a condition variable wait queue, if it is on one.
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.Fn cv_waitq_empty
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reports whether there are any waiters on
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.Fa cvp .
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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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A thread must hold
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.Fa mp
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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_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 mp
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is atomically released before the thread is blocked, then atomically reacquired
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before the function call returns.
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All waiters must pass the same
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.Fa mp
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in conjunction with
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.Fa cvp .
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A thread must hold
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.Fa mp
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while calling
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.Fn cv_signal
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or
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.Fn cv_broadcast ,
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even though it isn't passed as an argument.
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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_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_waitq_empty
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returns non-zero if there are no threads on the wait queue; 0 otherwise.
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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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An unmasked signal was caught.
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.It Bq Er ERESTART
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A masked signal was caught.
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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 msleep 9 ,
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.Xr mutex 9 ,
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.Xr sema 9 ,
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.Xr sx 9
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