295 lines
7.9 KiB
Groff
295 lines
7.9 KiB
Groff
.\" $NetBSD: condvar.9,v 1.13 2008/08/06 07:08:31 skrll Exp $
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.\"
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.\" Copyright (c) 2006, 2007, 2008 The NetBSD Foundation, Inc.
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.\" All rights reserved.
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.\"
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.\" This code is derived from software contributed to The NetBSD Foundation
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.\" by Andrew Doran.
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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 NETBSD FOUNDATION, INC. AND CONTRIBUTORS
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.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
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.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
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.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
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.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
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.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
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.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
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.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
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.\" POSSIBILITY OF SUCH DAMAGE.
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.\"
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.Dd June 4, 2008
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.Dt CONDVAR 9
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.Os
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.Sh NAME
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.Nm cv ,
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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_has_waiters
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.Nd condition variables
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.Sh SYNOPSIS
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.In sys/condvar.h
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.Ft void
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.Fn cv_init "kcondvar_t *cv" "const char *wmesg"
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.Ft void
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.Fn cv_destroy "kcondvar_t *cv"
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.Ft void
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.Fn cv_wait "kcondvar_t *cv" "kmutex_t *mtx"
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.Ft int
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.Fn cv_wait_sig "kcondvar_t *cv" "kmutex_t *mtx"
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.Ft int
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.Fn cv_timedwait "kcondvar_t *cv" "kmutex_t *mtx" "int ticks"
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.Ft int
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.Fn cv_timedwait_sig "kcondvar_t *cv" "kmutex_t *mtx" "int ticks"
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.Ft void
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.Fn cv_signal "kcondvar_t *cv"
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.Ft void
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.Fn cv_broadcast "kcondvar_t *cv"
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.Ft bool
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.Fn cv_has_waiters "kcondvar_t *cv"
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.Pp
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.Cd "options DIAGNOSTIC"
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.Cd "options LOCKDEBUG"
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.Sh DESCRIPTION
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Condition variables (CVs) are used in the kernel to synchronize access
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to resources that are limited (for example, memory) and to wait for
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pending I/O operations to complete.
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.Pp
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The
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.Vt kcondvar_t
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type provides storage for the CV object.
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This should be treated as an opaque object and not examined directly by
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consumers.
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.Sh OPTIONS
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.Bl -tag -width abcd
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.It Cd "options DIAGNOSTIC"
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.Pp
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Kernels compiled with the
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.Dv DIAGNOSTIC
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option perform basic sanity checks on CV operations.
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.It Cd "options LOCKDEBUG"
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.Pp
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Kernels compiled with the
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.Dv LOCKDEBUG
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option perform potentially CPU intensive sanity checks
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on CV operations.
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.El
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.Sh FUNCTIONS
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.Bl -tag -width abcd
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.It Fn cv_init "cv" "wmesg"
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.Pp
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Initialize a CV for use.
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No other operations can be performed on the CV until it has been initialized.
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.Pp
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The
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.Fa wmesg
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argument specifies a string of no more than 8 characters that describes
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the resource or condition associated with the CV.
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The kernel does not use this argument directly but makes it available for
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utilities such as
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.Xr ps 1
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to display.
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.It Fn cv_destroy "cv"
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.Pp
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Release resources used by a CV.
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The CV must not be in use when it is destroyed, and must not be used afterwards.
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.It Fn cv_wait "cv" "mtx"
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.Pp
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Cause the current LWP to wait non-interruptably for access to a resource,
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or for an I/O operation to complete.
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The LWP will resume execution when awoken by another thread using
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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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.Fa mtx
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specifies a kernel mutex to be used as an interlock, and must be held by the
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calling LWP on entry to
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.Fn cv_wait .
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It will be released once the LWP has prepared to sleep, and will be reacquired
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before
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.Fn cv_wait
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returns.
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.Pp
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A small window exists between testing for availability of a resource and
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waiting for the resource with
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.Fn cv_wait ,
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in which the resource may become available again.
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The interlock is used to guarantee that the resource will not be signalled
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as available until the calling LWP has begun to wait for it.
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.Pp
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Non-interruptable waits have the potential to deadlock the system, and so must
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be kept short (typically, under one second).
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.It Fn cv_wait_sig "cv" "mtx"
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.Pp
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As per
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.Fn cv_wait ,
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but causes the current LWP to wait interruptably.
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If the LWP receives a signal, or is interrupted by another condition such
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as its containing process exiting, the wait is ended early and an error
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code returned.
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.Pp
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If
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.Fn cv_wait_sig
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returns as a result of a signal, the return value is
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.Er ERESTART
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if the signal
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has the
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.Dv SA_RESTART
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property.
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If awoken normally, the value is zero, and
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.Er EINTR
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under all other conditions.
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.It Fn cv_timedwait "cv" "mtx" "ticks"
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.Pp
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As per
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.Fn cv_wait ,
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but will return early if a timeout specified by the
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.Fa ticks
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argument expires.
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.Pp
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.Fa ticks
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is an architecture and system dependent value related to the number of
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clock interrupts per second.
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See
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.Xr hz 9
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for details.
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The
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.Xr mstohz 9
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macro can be used to convert a timeout expressed in milliseconds to
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one suitable for
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.Fn cv_timedwait .
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If the
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.Fa ticks
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argument is zero,
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.Fn cv_timedwait
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behaves exactly like
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.Fn cv_wait .
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.Pp
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If the timeout expires before the LWP is awoken, the return value is
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.Er EWOULDBLOCK .
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If awoken normally, the return value is zero.
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.It Fn cv_timedwait_sig "cv" "mtx" "ticks"
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.Pp
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As per
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.Fn cv_wait_sig ,
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but also accepts a timeout value and will return
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.Er EWOULDBLOCK
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if the timeout expires.
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.It Fn cv_signal "cv"
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.Pp
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Awaken one LWP (potentially among many) that is waiting on the specified
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condition variable.
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The mutex passed to the wait function
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.Po Fa mtx Pc
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must also be held when calling
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.Fn cv_signal .
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.Pp
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(Note that
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.Fn cv_signal
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is erroneously named in that it does not send a signal in the traditional
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sense to LWPs waiting on a CV.)
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.It Fn cv_broadcast "cv"
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.Pp
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Awaken all LWPs waiting on the specified condition variable.
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The mutex passed to the wait function
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.Po Fa mtx Pc
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must also be held when calling
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.Fn cv_broadcast .
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.It Fn cv_has_waiters "cv"
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.Pp
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Return
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.Dv true
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if one or more LWPs are waiting on the specified condition variable.
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.Pp
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.Fn cv_has_waiters
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cannot test reliably for interruptable waits.
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It should only be used to test for non-interruptable waits
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made using
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.Fn cv_wait .
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.Pp
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.Fn cv_has_waiters
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should only be used when making diagnostic assertions, and must
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be called while holding the interlocking mutex passed to
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.Fn cv_wait .
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.El
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.Sh EXAMPLES
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.Bd -literal
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Consuming a resource:
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/*
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* Lock the resource. Its mutex will also serve as the
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* interlock.
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*/
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mutex_enter(\*[Am]res-\*[Gt]mutex);
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/*
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* Wait for the resource to become available.
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*/
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while (res-\*[Gt]state == BUSY)
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cv_wait(\*[Am]res-\*[Gt]condvar, \*[Am]res-\*[Gt]mutex);
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/*
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* It's now available to us. Take ownership of the
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* resource, and consume it.
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*/
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res-\*[Gt]state = BUSY;
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mutex_exit(\*[Am]res-\*[Gt]mutex);
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consume(res);
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Releasing a resource for the next consumer to use:
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mutex_enter(\*[Am]res-\*[Gt]mutex);
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res-\*[Gt]state = IDLE;
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cv_signal(\*[Am]res-\*[Gt]condvar);
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mutex_exit(\*[Am]res-\*[Gt]mutex);
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.Ed
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.Sh CODE REFERENCES
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This section describes places within the
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.Nx
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source tree where code implementing condition variables can be found.
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All pathnames are relative to
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.Pa /usr/src .
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.Pp
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The core of the CV implementation is in
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.Pa sys/kern/kern_condvar.c .
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.Pp
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The header file
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.Pa sys/sys/condvar.h
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describes the public interface.
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.Sh SEE ALSO
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.Xr sigaction 2 ,
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.Xr errno 9 ,
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.Xr mb 9 ,
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.Xr mstohz 9 ,
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.Xr mutex 9 ,
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.Xr rwlock 9
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.Pp
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.Rs
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.%A Jim Mauro
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.%A Richard McDougall
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.%T Solaris Internals: Core Kernel Architecture
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.%I Prentice Hall
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.%D 2001
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.%O ISBN 0-13-022496-0
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.Re
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.Sh HISTORY
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The CV primitives first appeared in
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.Nx 5.0 .
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