203 lines
6.2 KiB
Groff
203 lines
6.2 KiB
Groff
.\" $NetBSD: ltsleep.9,v 1.2 2002/10/14 13:43:26 wiz Exp $
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.\"
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.\" Copyright (c) 1996, 2002 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 Paul Kranenburg.
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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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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the NetBSD
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.\" Foundation, Inc. and its contributors.
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.\" 4. Neither the name of The NetBSD Foundation nor the names of its
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.\" contributors may be used to endorse or promote products derived
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.\" from this software without specific prior written permission.
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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 September 29, 2002
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.Dt LTSLEEP 9
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.Os
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.Sh NAME
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.Nm ltsleep ,
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.Nm tsleep ,
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.Nm wakeup
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.Nd process context sleep and and wakeup
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.Sh SYNOPSIS
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.Fd #include \*[Lt]sys/proc.h\*[Gt]
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.Ft int
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.Fn "ltsleep" "void *ident" "int priority" "const char *wmesg" "int timo" "__volatile struct simplelock *slock"
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.Ft int
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.Fn "tsleep" "void *ident" "int priority" "const char *wmesg" "int timo"
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.Ft void
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.Fn "wakeup" "void *ident"
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.Sh DESCRIPTION
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These functions implement voluntary context switching.
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.Fn ltsleep
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and
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.Fn tsleep
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are used throughout the kernel whenever processing in the current context
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can not continue for any of the following reasons:
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.Bl -bullet -offset indent
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.It
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The current process needs to await the results of a pending I/O operation.
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.It
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The current process needs resources
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.Pq e.g. memory
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which are temporarily unavailable.
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.It
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The current process wants access to data-structures which are locked by
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other processes.
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.El
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.Pp
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The function
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.Fn wakeup
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is used to notify sleeping processes of possible changes to the condition
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that caused them to go to sleep.
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Typically, an awakened process will -- after it has acquired a context
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again -- retry the action that blocked its operation to see if the
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.Dq blocking
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condition has cleared.
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.Pp
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The
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.Fn ltsleep
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function takes the following arguments:
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.Bl -tag -width priority
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.It Fa ident
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An identifier of the
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.Dq wait channel
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representing the resource for which the current process needs to wait.
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This typically is the virtual address of some kernel data-structure related
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to the resource for which the process is contending.
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The same identifier must be used in a call to
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.Fn wakeup
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to get the process going again.
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.Fa ident
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should not be
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.Dv NULL .
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.It Fa priority
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The process priority to be used when the process is awakened and put on
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the queue of runnable processes.
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This mechanism is used to optimize
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.Dq throughput
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of processes executing in kernel mode.
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If the flag
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.Dv PCATCH
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is OR'ed into
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.Fa priority
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the process checks for posted signals before and after sleeping.
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If the flag
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.Dv PNORELOCK
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is OR'ed into
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.Fa priority ,
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.Fa slock
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is NOT re-locked after process resume.
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.It Fa wmesg
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A pointer to a character string indicating the reason a process is sleeping.
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The kernel does not use the string, but makes it available
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.Pq through the process structure field Li p_wmesg
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for user level utilities such as
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.Xr ps 1 .
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.It Fa timo
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If non-zero, the process will sleep for at most
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.Li timo/hz
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seconds.
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If this amount of time elapses and no
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.Fn wakeup "ident"
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has occurred, and no signal
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.Pq if Dv PCATCH No was set
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was posted,
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.Fn tsleep
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will return
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.Er EWOULDBLOCK .
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.It Fa slock
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If not NULL, the
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.Fa slock
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interlock is unlocked once the scheduler lock is acquired.
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Unless
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.Dv PNORELOCK
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was set,
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.Fa slock
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is locked again once
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the process is resumed from sleep.
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This provides wakeup-before-sleep condition protection facility.
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.El
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.Pp
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The
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.Fn tsleep
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macro is functionally equivalent to:
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.Bd -literal -offset indent
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ltsleep(ident, priority, wmesg, timo, NULL)
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.Ed
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.Pp
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The
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.Fn wakeup
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function will mark all processes which are currently sleeping on the identifier
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.Fa ident
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as runnable.
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Eventually, each of the processes will resume execution in the kernel
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context, causing a return from
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.Fn tsleep .
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Note that processes returning from sleep should always re-evaluate the
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conditions that blocked them, since a call to
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.Fn wakeup
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merely signals a
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.Em possible
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change to the blocking conditions.
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For example, when two or more processes are waiting for an exclusive-access
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lock
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.Pq see Xr lock 9 ,
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only one of them will succeed in acquiring the lock when it is released.
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All others will have to go back to sleep and wait for the next opportunity.
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.Sh RETURN VALUES
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.Fn ltsleep
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returns 0 if it returns as a result of a
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.Fn wakeup .
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If a
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.Fn ltsleep
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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 has the
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.Dv SA_RESTART
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property
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.Pq see Xr sigaction 2 ,
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and
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.Er EINTR
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otherwise.
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If
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.Fn ltsleep
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returns because of a timeout it returns
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.Er EWOULDBLOCK .
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.Sh SEE ALSO
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.Xr sigaction 2 ,
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.Xr hz 9 ,
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.Xr lock 9
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.Sh HISTORY
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The sleep/wakeup process synchronization mechanism is very old.
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It appeared in a very early version of Unix.
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.Fn tsleep
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appeared in
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.Bx 4.4 .
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.Fn ltsleep
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appeared in
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.Nx 1.5 .
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