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 1, 2006 30.Dt SEMA 9 31.Os 32.Sh NAME 33.Nm sema , 34.Nm sema_init , 35.Nm sema_destroy , 36.Nm sema_post , 37.Nm sema_wait , 38.Nm sema_timedwait , 39.Nm sema_trywait , 40.Nm sema_value 41.Nd kernel counting semaphore 42.Sh SYNOPSIS 43.In sys/types.h 44.In sys/lock.h 45.In sys/sema.h 46.Ft void 47.Fn sema_init "struct sema *sema" "int value" "const char *description" 48.Ft void 49.Fn sema_destroy "struct sema *sema" 50.Ft void 51.Fn sema_post "struct sema *sema" 52.Ft void 53.Fn sema_wait "struct sema *sema" 54.Ft int 55.Fn sema_timedwait "struct sema *sema" "int timo" 56.Ft int 57.Fn sema_trywait "struct sema *sema" 58.Ft int 59.Fn sema_value "struct sema *sema" 60.Sh DESCRIPTION 61Counting semaphores provide a mechanism for synchronizing access to a pool of 62resources. 63Unlike mutexes, semaphores do not have the concept of an owner, so they can also 64be useful in situations where one thread needs to acquire a resource, and 65another thread needs to release it. 66Each semaphore has an integer value associated with it. 67Posting (incrementing) always succeeds, but waiting (decrementing) can only 68successfully complete if the resulting value of the semaphore is greater than or 69equal to zero. 70.Pp 71Semaphores should not be used where mutexes and condition variables 72will suffice. 73Semaphores are a more complex synchronization mechanism than mutexes and 74condition variables, and are not as efficient. 75.Pp 76Semaphores are created with 77.Fn sema_init , 78where 79.Fa sema 80is a pointer to space for a 81.Vt "struct sema" , 82.Fa value 83is the initial value of the semaphore, and 84.Fa description 85is a pointer to a null-terminated character string that describes the semaphore. 86Semaphores are destroyed with 87.Fn sema_destroy . 88A semaphore is posted (incremented) with 89.Fn sema_post . 90A semaphore is waited on (decremented) with 91.Fn sema_wait , 92.Fn sema_timedwait , 93or 94.Fn sema_trywait . 95The 96.Fa timo 97argument to 98.Fn sema_timedwait 99specifies the minimum time in ticks to wait before returning with failure. 100.Fn sema_value 101is used to read the current value of the semaphore. 102.Sh RETURN VALUES 103The 104.Fn sema_value 105function 106returns the current value of the semaphore. 107.Pp 108If decrementing the semaphore would result in its value being negative, 109.Fn sema_trywait 110returns 0 to indicate failure. 111Otherwise, a non-zero value is returned to indicate success. 112.Pp 113The 114.Fn sema_timedwait 115function 116returns 0 if waiting on the semaphore succeeded; otherwise a 117non-zero error code is returned. 118.Sh ERRORS 119The 120.Fn sema_timedwait 121function will fail if: 122.Bl -tag -width Er 123.It Bq Er EWOULDBLOCK 124Timeout expired. 125.El 126.Sh SEE ALSO 127.Xr condvar 9 , 128.Xr locking 9 , 129.Xr mtx_pool 9 , 130.Xr mutex 9 , 131.Xr rwlock 9 , 132.Xr sx 9 133