1.\" 2.\" Copyright (C) 2001 Jason Evans <jasone@FreeBSD.org>. All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice(s), this list of conditions and the following disclaimer as 9.\" the first lines of this file unmodified other than the possible 10.\" addition of one or more copyright notices. 11.\" 2. Redistributions in binary form must reproduce the above copyright 12.\" notice(s), 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 COPYRIGHT HOLDER(S) ``AS IS'' AND ANY 16.\" EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED 17.\" WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE 18.\" DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER(S) BE LIABLE FOR ANY 19.\" DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES 20.\" (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR 21.\" SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER 22.\" CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 23.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 24.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH 25.\" DAMAGE. 26.\" 27.\" $FreeBSD$ 28.\" 29.Dd August 14, 2001 30.Dt SX 9 31.Os 32.Sh NAME 33.Nm sx , 34.Nm sx_init , 35.Nm sx_destroy , 36.Nm sx_slock , 37.Nm sx_xlock , 38.Nm sx_try_slock , 39.Nm sx_try_xlock , 40.Nm sx_sunlock , 41.Nm sx_xunlock , 42.Nm sx_try_upgrade , 43.Nm sx_downgrade 44.Nd kernel shared/exclusive lock 45.Sh SYNOPSIS 46.Fd #include <sys/types.h> 47.Fd #include <sys/lock.h> 48.Fd #include <sys/mutex.h> 49.Fd #include <sys/sx.h> 50.Ft void 51.Fn sx_init "struct sx *sx" "const char *description" 52.Ft void 53.Fn sx_destroy "struct sx *sx" 54.Ft void 55.Fn sx_slock "struct sx *sx" 56.Ft void 57.Fn sx_xlock "struct sx *sx" 58.Ft int 59.Fn sx_try_slock "struct sx *sx" 60.Ft int 61.Fn sx_try_xlock "struct sx *sx" 62.Ft void 63.Fn sx_sunlock "struct sx *sx" 64.Ft void 65.Fn sx_xunlock "struct sx *sx" 66.Ft int 67.Fn sx_try_upgrade "struct sx *sx" 68.Ft void 69.Fn sx_downgrade "struct sx *sx" 70.Sh DESCRIPTION 71Shared/exclusive locks are used to protect data that are read far more often 72than they are written. 73Mutexes are inherently more efficient than shared/exclusive locks, so 74shared/exclusive locks should be used prudently. 75.Pp 76Shared/exclusive locks are created with 77.Fn sx_init , 78where 79.Fa sx 80is a pointer to space for a 81.Vt struct sx , 82and 83.Fa description 84is a pointer to a null-terminated character string that describes the 85shared/exclusive lock. 86Shared/exclusive locks are destroyed with 87.Fn sx_destroy . 88Threads acquire and release a shared lock by calling 89.Fn sx_slock 90or 91.Fn sx_try_slock 92and 93.Fn sx_sunlock . 94Threads acquire and release an exclusive lock by calling 95.Fn sx_xlock 96or 97.Fn sx_try_xlock 98and 99.Fn sx_xunlock . 100A thread can attempt to upgrade a currently owned shared lock to an exclusive 101lock by calling 102.Fn sx_try_upgrade . 103A thread that owns an exclusive lock can downgrade it to a shared lock by 104calling 105.Fn sx_downgrade . 106.Pp 107.Fn sx_try_slock 108and 109.Fn sx_try_xlock 110will return 0 if the shared/exclusive lock cannot be acquired immediately; 111otherwise the shared/exclusive lock will be acquired and a non-zero value will 112be returned. 113.Pp 114.Fn sx_try_upgrade 115will return 0 if the shared lock cannot be upgraded to an exclusive lock 116immediately; otherwise the exclusive lock will be acquired and a non-zero value 117will be returned. 118.Pp 119A thread may not own a shared lock and an exclusive lock simultaneously; 120attempting to do so will result in deadlock. 121.Sh SEE ALSO 122.Xr condvar 9 , 123.Xr mutex 9 , 124.Xr sema 9 125