201 lines
5.6 KiB
Groff
201 lines
5.6 KiB
Groff
.\" Copyright (c) 2006 Gleb Smirnoff <glebius@FreeBSD.org>
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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 AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER 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
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.\" SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd January 30, 2006
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.Dt RWLOCK 9
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.Os
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.Sh NAME
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.Nm rwlock
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.Nm rw_init ,
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.Nm rw_rlock ,
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.Nm rw_wlock ,
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.Nm rw_assert ,
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.Nm rw_runlock ,
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.Nm rw_wunlock ,
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.Nm rw_initialized ,
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.Nm rw_destroy ,
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.Nm RW_SYSINIT ,
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.Nd kernel synchronization primitives
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.Sh SYNOPSIS
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.In sys/param.h
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.In sys/lock.h
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.In sys/rwlock.h
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.Ft void
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.Fn rw_init "struct rwlock *rwlock" "const char *name"
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.Ft void
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.Fn rw_rlock "struct rwlock *rwlock"
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.Ft void
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.Fn rw_wlock "struct rwlock *rwlock"
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.Ft void
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.Fn rw_runlock "struct rwlock *rwlock"
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.Ft void
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.Fn rw_wunlock "struct rwlock *rwlock"
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.Ft int
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.Fn rw_initialized "struct rwlock *rwlock"
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.Ft void
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.Fn rw_destroy "struct rwlock *rwlock"
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.Pp
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.Cd "options INVARIANTS"
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.Cd "options INVARIANT_SUPPORT"
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.Ft void
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.Fn rw_assert "struct rwlock *rwlock" "int what"
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.In sys/kernel.h
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.Fn RW_SYSINIT "name" "struct rwlock *rwlock" "const char *description"
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.Sh DESCRIPTION
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Read/write locks are a method of thread synchronization, which
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allows several threads have shared access to protected data, or
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one thread have exclusive access.
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The threads that share access are known as
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.Em readers
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since they should only read the protected data, while thread
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with exclusive access is known as a
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.Em writer
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since it can modify protected data.
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.Pp
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Although the description of read/write locks looks very similar
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to description of
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.Xr sx 9
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locks, their usage pattern is different.
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The read/write locks can be treated as mutexes (see
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.Xr mutex 9 )
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with shared/exclusive semantics.
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Unlike
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.Xr sx 9 ,
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an
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.Nm
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can be locked while holding a non-spin mutex;
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.Nm
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cannot be held while sleeping.
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The
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.Nm
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locks have priority propagation like mutexes, but priority
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can be propagated only to exclusive holder.
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This limitation comes from the fact that shared owners
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are anonymous.
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Another important property is that shared holder of
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.Nm
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can recurse on it.
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.Ss Macros and Functions
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.Bl -tag -width indent
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.It Fn rw_init "struct rwlock *rwlock" "const char *name"
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Initialize structure located at
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.Fa rwlock
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as read/write lock, described by name
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.Fa name .
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The description is used solely for debugging purposes.
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This function must be used before any other manipulations
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with the lock.
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.It Fn rw_rlock "struct rwlock *rwlock"
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Lock the
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.Fa rwlock
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as reader.
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If any thread holds this lock exclusively, the current thread blocks,
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and its priority is propagated to exclusive holder.
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The
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.Fn rw_rlock
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function can be called when the thread has already acquired reader
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access on
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.Fa rwlock .
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This is called
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.Dq "recursing on a lock" .
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.It Fn rw_wlock "struct rwlock *rwlock"
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Lock the
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.Fa rwlock
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as writer.
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If there are any shared owners of the lock, the current thread blocks.
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The
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.Fn rw_wlock
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function cannot be called recursively.
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.It Fn rw_runlock "struct rwlock *rwlock"
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This function releases shared lock, previously acquired by
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.Fn rw_rlock .
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.It Fn rw_wunlock "struct rwlock *rwlock"
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This function releases exclusive lock, previously acquired by
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.Fn rw_wlock .
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.It Fn rw_initialized "struct rwlock *rwlock"
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This function returns non-zero if the
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.Fa rwlock
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has been initialized, and zero otherwise.
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.It Fn rw_destroy "struct rwlock *rwlock"
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This functions destroys a lock previously initialized with
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.Fn rw_init .
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The
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.Fa rwlock
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must be unlocked.
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.It Fn rw_assert "struct rwlock *rwlock" "int what"
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This function allows assertions specified in
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.Fa what
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to be made about
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.Fa rwlock .
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If the assertions are not true and the kernel is compiled
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with
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.Cd "options INVARIANTS"
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and
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.Cd "options INVARIANT_SUPPORT" ,
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the kernel will panic.
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Currently the following assertions are supported:
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.Bl -tag -width ".Dv RA_UNLOCKED"
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.It Dv RA_LOCKED
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Assert that current thread is either shared or exclusive owner
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of the
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.Nm
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pointed to by the first argument.
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.It Dv RA_RLOCKED
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Assert that current thread is shared owner of the
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.Nm
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pointed
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to by the first argument.
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.It Dv RA_WLOCKED
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Assert that current thread is exclusive owner of the
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.Nm
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pointed
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to by the first argument.
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.It Dv RA_UNLOCKED
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Assert that current thread is neither shared nor exclusive owner
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of the
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.Nm
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pointed to by the first argument.
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.El
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.El
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.Sh SEE ALSO
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.Xr condvar 9 ,
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.Xr mutex 9 ,
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.Xr panic 9 ,
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.Xr sema 9 ,
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.Xr sx 9
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.Sh HISTORY
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These
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functions 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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The
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.Nm
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facility was written by
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.An "John Baldwin" .
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This manual page was written by
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.An "Gleb Smirnoff" .
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