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.\" $Id: sleep.9,v 1.12 1998/12/21 10:34:53 dillon 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.Fd #include <sys/param.h> 40.Fd #include <sys/systm.h> 41.Fd #include <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.Dv EWOULDBLOCK 101if the timeout expires. If 102.Dv PCATCH 103is set and a signal needs to be delivered, 104.Dv ERESTART 105is returned if the current system call should be restarted if 106possible, and 107.Dv EINTR 108is returned if the system call should be interrupted by the signal 109.Pq return Dv EINTR . 110.Pp 111.Nm Sleep 112is the traditional form. It doesn't let you specify a timeout nor a 113.Ar wmesg , 114hence its use is deprecated. 115.Pp 116.Nm Asleep 117implements the new asynchronous sleep function. It takes the same arguments 118as 119.Fn tsleep 120and places the process on the appropriate wait queue, but 121.Fn asleep 122leaves the process runnable and returns immediately. The caller is then 123expected to, at some point in the future, call 124.Fn await 125to actually wait for the previously queued wait condition. 126If 127.Fn asleep 128is called several times, only the most recent call is effective. 129.Fn asleep 130may be called with an 131.Ar ident 132value of NULL 133to remove any previously queued condition. 134.Pp 135.Nm Await 136implements the new asynchronous wait function. When 137.Fn asleep 138is called on an identifier it associates the process with that 139identifier but does not block. 140.Fn await 141will actually block the process until 142.Fn wakeup 143is called on that identifier any time after the 144.Fn asleep . 145If 146.Fn wakeup 147is called after you 148.Fn asleep 149but before you 150.Fn await 151then the 152.Fn await 153call is effectively a NOP. 154If 155.Fn await 156is called multiple times without an intervening 157.Fn asleep , 158the 159.Fn await 160is effectively a NOP but will also call 161.Fn mswitch 162for safety. The 163.Fn await 164function allows you to override the priority and timeout values to be used. 165If the value -1 is specified for an argument, the value is taken from the 166previous 167.Fn asleep 168call. If -1 is passed for the priority you must be prepared to catch signal 169conditions if the prior call to 170.Fn asleep 171specified it in its priority. If -1 is passed for the timeout you must be 172prepared to catch a timeout condition if the prior call to 173.Fn asleep 174specified a timeout. When you use -1, it is usually a good idea to not make 175assumptions as to the arguments used by the prior 176.Fn asleep 177call. 178.Pp 179The 180.Fn asleep 181and 182.Fn await 183functions are mainly used by the kernel to shift the burden of blocking 184away from extremely low level routines and to push it onto their callers. 185This in turn allows more complex interlocking code to 186.Em backout 187of a temporary resource failure 188(such as lack of memory) in order to release major locks prior to actually 189blocking, and to then retry the operation on wakeup. This key feature is 190expected to be heavily used in SMP situations in order to allow code to make 191better use of spinlocks. A spinlock, by its very nature, cannot be used 192around code that might block. It is hoped that these capabilities will 193make it easier to migrate the SMP master locks deeper into the kernel. 194.Pp 195These routines may also be used to avoid nasty spl*() calls to get around 196race conditions with simple conditional test/wait interlocks. You simple 197call 198.Fn asleep 199prior to your test, then conditonally 200.Fn await 201only if the test fails. It is usually a good idea to cancel an 202.Fn asleep 203if you wind up never calling the related 204.Fn await , 205but it is not required. If you do not want to waste cpu calling 206.Fn asleep 207unnecessarily, you can surround the whole thing with a second test. The 208race condition is still handled by the inside 209.Fn asleep 210call. 211.Sh RETURN VALUES 212See above. 213.Sh SEE ALSO 214.Xr ps 1 , 215.Xr malloc 9 216.Sh HISTORY 217The sleep/wakeup process synchronization mechanism is very old. It 218appeared in a very early version of Unix. 219.Pp 220.Nm Tsleep 221appeared in 222.Bx 4.4 . 223.Pp 224.Nm Asleep/await 225first appeared in FreeBSD-3.0.1 and is designed to shift the burden of blocking 226away from extremely low level routines and push it up to their callers. 227.Sh AUTHORS 228This man page has been written by 229.ie t J\(:org Wunsch. 230.el Joerg Wunsch. 231asleep/await designed and written by Matthew Dillon. 232