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 February 26, 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.Nd kernel shared/exclusive lock 43.Sh SYNOPSIS 44.Fd #include <sys/types.h> 45.Fd #include <sys/lock.h> 46.Fd #include <sys/mutex.h> 47.Fd #include <sys/sx.h> 48.Ft void 49.Fn sx_init "struct sx *sx" "const char *description" 50.Ft void 51.Fn sx_destroy "struct sx *sx" 52.Ft void 53.Fn sx_slock "struct sx *sx" 54.Ft void 55.Fn sx_xlock "struct sx *sx" 56.Ft int 57.Fn sx_try_slock "struct sx *sx" 58.Ft int 59.Fn sx_try_xlock "struct sx *sx" 60.Ft void 61.Fn sx_sunlock "struct sx *sx" 62.Ft void 63.Fn sx_xunlock "struct sx *sx" 64.Sh DESCRIPTION 65Shared/exclusive locks are used to protect data that are read far more often 66than they are written. 67Mutexes are inherently more efficient than shared/exclusive locks, so 68shared/exclusive locks should be used prudently. 69.Pp 70Shared/exclusive locks are created with 71.Fn sx_init , 72where 73.Fa sx 74is a pointer to space for a 75.Vt struct sx , 76and 77.Fa description 78is a pointer to a null-terminated character string that describes the 79shared/exclusive lock. 80Shared/exclusive locks are destroyed with 81.Fn sx_destroy . 82Threads acquire and release a shared lock by calling 83.Fn sx_slock 84or 85.Fn sx_try_slock 86and 87.Fn sx_sunlock . 88Threads acquire and release an exclusive lock by calling 89.Fn sx_xlock 90or 91.Fn sx_try_xlock 92and 93.Fn sx_xunlock . 94.Pp 95.Fn sx_try_slock 96and 97.Fn sx_try_xlock 98will return 0 if the shared/exclusive lock cannot be acquired immediately; 99otherwise the shared/exclusive lock will be acquired and a non-zero value will 100be returned. 101.Pp 102A thread may not own a shared lock and an exclusive lock simultaneously; 103attempting to do so will result in deadlock. 104.Sh SEE ALSO 105.Xr mutex 9 106