1.\" 2.\" Copyright (c) 1996 Joerg Wunsch 3.\" 4.\" All rights reserved. 5.\" 6.\" Redistribution and use in source and binary forms, with or without 7.\" modification, are permitted provided that the following conditions 8.\" are met: 9.\" 1. Redistributions of source code must retain the above copyright 10.\" notice, this list of conditions and the following disclaimer. 11.\" 2. Redistributions in binary form must reproduce the above copyright 12.\" notice, this list of conditions and the following disclaimer in the 13.\" documentation and/or other materials provided with the distribution. 14.\" 15.\" THIS SOFTWARE IS PROVIDED BY THE DEVELOPERS ``AS IS'' AND ANY EXPRESS OR 16.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 17.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 18.\" IN NO EVENT SHALL THE DEVELOPERS BE LIABLE FOR ANY DIRECT, INDIRECT, 19.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 20.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 21.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 22.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 23.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 24.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 25.\" 26.\" $FreeBSD: src/share/man/man9/sleep.9,v 1.18.2.5 2001/12/17 11:30:19 ru Exp $ 27.\" " 28.Dd December 17, 1998 29.Os 30.Dt SLEEP 9 31.Sh NAME 32.Nm sleep , 33.Nm tsleep , 34.Nm asleep , 35.Nm await , 36.Nm wakeup 37.Nd wait for events 38.Sh SYNOPSIS 39.In sys/param.h 40.In sys/systm.h 41.In sys/proc.h 42.Ft int 43.Fn tsleep "void *ident" "int priority" "const char *wmesg" "int timo" 44.Ft int 45.Fn asleep "void *ident" "int priority" "const char *wmesg" "int timo" 46.Ft int 47.Fn await "int priority" "int timo" 48.Ft void 49.Fn wakeup "void *ident" 50.Ft void 51.Fn wakeup_one "void *ident" 52.Sh DESCRIPTION 53The functions 54.Fn tsleep 55and 56.Fn wakeup 57handle event-based process blocking. If a process must wait for an 58external event, it is put on sleep by 59.Nm tsleep . 60The parameter 61.Ar ident 62is an arbitrary address that uniquely identifies the event on which 63the process is being asleep. All processes sleeping on a single 64.Ar ident 65are woken up later by 66.Nm wakeup , 67often called from inside an interrupt routine, to indicate that the 68resource the process was blocking on is available now. 69.Pp 70The parameter 71.Ar wmesg 72is a string describing the sleep condition for tools like 73.Xr ps 1 . 74Due to the limited space of those programs to display arbitrary strings, 75this message should not be longer than 6 characters. 76.Pp 77The 78.Fn wakeup_one 79function is used to make the first process in the queue that is 80sleeping on the parameter 81.Fa ident 82runnable. This can prevent the system from becoming saturated 83when a large number of processes are sleeping on the same address, 84but only one of them can actually do any useful work when made 85runnable. 86.Pp 87.Nm Tsleep 88is the general sleep call. Suspends the current process until a wakeup is 89performed on the specified identifier. The process will then be made 90runnable with the specified 91.Ar priority . 92Sleeps at most 93.Ar timo 94\&/ hz seconds (0 means no timeout). If 95.Ar pri 96includes the 97.Dv PCATCH 98flag, signals are checked before and after sleeping, else signals are 99not checked. Returns 0 if awakened, 100.Er EWOULDBLOCK 101if the timeout expires. If 102.Dv PCATCH 103is set and a signal needs to be delivered, 104.Er ERESTART 105is returned if the current system call should be restarted if 106possible, and 107.Er EINTR 108is returned if the system call should be interrupted by the signal 109(return 110.Er EINTR ) . 111.Pp 112.Nm Asleep 113implements the new asynchronous sleep function. It takes the same arguments 114as 115.Fn tsleep 116and places the process on the appropriate wait queue, but 117.Fn asleep 118leaves the process runnable and returns immediately. The caller is then 119expected to, at some point in the future, call 120.Fn await 121to actually wait for the previously queued wait condition. 122If 123.Fn asleep 124is called several times, only the most recent call is effective. 125.Fn asleep 126may be called with an 127.Ar ident 128value of NULL 129to remove any previously queued condition. 130.Pp 131.Nm Await 132implements the new asynchronous wait function. When 133.Fn asleep 134is called on an identifier it associates the process with that 135identifier but does not block. 136.Fn await 137will actually block the process until 138.Fn wakeup 139is called on that identifier any time after the 140.Fn asleep . 141If 142.Fn wakeup 143is called after you 144.Fn asleep 145but before you 146.Fn await 147then the 148.Fn await 149call is effectively a NOP. 150If 151.Fn await 152is called multiple times without an intervening 153.Fn asleep , 154the 155.Fn await 156is effectively a NOP but will also call 157.Fn mswitch 158for safety. The 159.Fn await 160function allows you to override the priority and timeout values to be used. 161If the value -1 is specified for an argument, the value is taken from the 162previous 163.Fn asleep 164call. If -1 is passed for the priority you must be prepared to catch signal 165conditions if the prior call to 166.Fn asleep 167specified it in its priority. If -1 is passed for the timeout you must be 168prepared to catch a timeout condition if the prior call to 169.Fn asleep 170specified a timeout. When you use -1, it is usually a good idea to not make 171assumptions as to the arguments used by the prior 172.Fn asleep 173call. 174.Pp 175The 176.Fn asleep 177and 178.Fn await 179functions are mainly used by the kernel to shift the burden of blocking 180away from extremely low level routines and to push it onto their callers. 181This in turn allows more complex interlocking code to 182.Em backout 183of a temporary resource failure 184(such as lack of memory) in order to release major locks prior to actually 185blocking, and to then retry the operation on wakeup. This key feature is 186expected to be heavily used in SMP situations in order to allow code to make 187better use of spinlocks. A spinlock, by its very nature, cannot be used 188around code that might block. It is hoped that these capabilities will 189make it easier to migrate the SMP master locks deeper into the kernel. 190.Pp 191These routines may also be used to avoid nasty spl*() calls to get around 192race conditions with simple conditional test/wait interlocks. You simply 193call 194.Fn asleep 195prior to your test, then conditionally 196.Fn await 197only if the test fails. It is usually a good idea to cancel an 198.Fn asleep 199if you wind up never calling the related 200.Fn await , 201but it is not required. If you do not want to waste cpu calling 202.Fn asleep 203unnecessarily, you can surround the whole thing with a second test. The 204race condition is still handled by the inside 205.Fn asleep 206call. 207.Sh RETURN VALUES 208See above. 209.Sh SEE ALSO 210.Xr ps 1 , 211.Xr malloc 9 212.Sh HISTORY 213The sleep/wakeup process synchronization mechanism is very old. It 214appeared in a very early version of Unix. 215.Pp 216.Nm Tsleep 217appeared in 218.Bx 4.4 . 219.Pp 220.Nm Asleep Ns / Ns Nm await 221first appeared in 222.Fx 3.0 223and is designed to shift the burden of blocking 224away from extremely low level routines and push it up to their callers. 225.Pp 226.Nm Sleep 227used to be the traditional form. It doesn't let you specify a timeout or a 228.Ar wmesg , 229hence it has been discontinued. 230.Sh AUTHORS 231.An -nosplit 232This man page was written by 233.An J\(:org Wunsch . 234.Nm Asleep 235and 236.Nm await 237were designed and written by 238.An Matthew Dillon . 239