126 lines
4.0 KiB
Groff
126 lines
4.0 KiB
Groff
.\"
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.\" Copyright (C) 2001 Jason Evans <jasone@FreeBSD.org>. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice(s), this list of conditions and the following disclaimer as
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.\" the first lines of this file unmodified other than the possible
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.\" addition of one or more copyright notices.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice(s), this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDER(S) ``AS IS'' AND ANY
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.\" EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
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.\" WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
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.\" DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER(S) BE LIABLE FOR ANY
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.\" DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
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.\" (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
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.\" SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
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.\" CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH
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.\" DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd June 14, 2004
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.Dt SEMA 9
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.Os
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.Sh NAME
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.Nm sema ,
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.Nm sema_init ,
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.Nm sema_destroy ,
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.Nm sema_post ,
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.Nm sema_wait ,
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.Nm sema_timedwait ,
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.Nm sema_trywait ,
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.Nm sema_value
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.Nd kernel counting semaphore
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.Sh SYNOPSIS
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.In sys/types.h
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.In sys/lock.h
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.In sys/sema.h
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.Ft void
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.Fn sema_init "struct sema *sema" "int value" "const char *description"
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.Ft void
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.Fn sema_destroy "struct sema *sema"
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.Ft void
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.Fn sema_post "struct sema *sema"
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.Ft void
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.Fn sema_wait "struct sema *sema"
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.Ft int
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.Fn sema_timedwait "struct sema *sema" "int timo"
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.Ft int
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.Fn sema_trywait "struct sema *sema"
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.Ft int
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.Fn sema_value "struct sema *sema"
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.Sh DESCRIPTION
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Counting semaphores provide a mechanism for synchronizing access to a pool of
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resources.
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Unlike mutexes, semaphores do not have the concept of an owner, so they can also
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be useful in situations where one thread needs to acquire a resource, and
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another thread needs to release it.
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Each semaphore has an integer value associated with it.
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Posting (incrementing) always succeeds, but waiting (decrementing) can only
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successfully complete if the resulting value of the semaphore is greater than or
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equal to zero.
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.Pp
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Semaphores should not be used where mutexes and condition variables
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will suffice.
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Semaphores are a more complex synchronization mechanism than mutexes and
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condition variables, and are not as efficient.
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.Pp
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Semaphores are created with
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.Fn sema_init ,
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where
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.Fa sema
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is a pointer to space for a
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.Vt "struct sema" ,
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.Fa value
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is the initial value of the semaphore, and
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.Fa description
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is a pointer to a null-terminated character string that describes the semaphore.
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Semaphores are destroyed with
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.Fn sema_destroy .
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A semaphore is posted (incremented) with
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.Fn sema_post .
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A semaphore is waited on (decremented) with
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.Fn sema_wait ,
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.Fn sema_timedwait ,
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or
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.Fn sema_trywait .
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The
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.Fa timo
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argument to
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.Fn sema_timedwait
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specifies the minimum time in ticks to wait before returning with failure.
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.Fn sema_value
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is used to read the current value of the semaphore.
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.Sh RETURN VALUES
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.Fn sema_value
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returns the current value of the semaphore.
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.Pp
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If decrementing the semaphore would result in its value being negative,
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.Fn sema_trywait
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returns 0 to indicate failure.
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Otherwise, a non-zero value is returned to indicate success.
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.Pp
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.Fn sema_timedwait
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returns 0 if waiting on the semaphore succeeded; otherwise a
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non-zero error code is returned.
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.Sh ERRORS
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.Fn sema_timedwait
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will fail if:
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.Bl -tag -width Er
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.It Bq Er EWOULDBLOCK
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Timeout expired.
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.El
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.Sh SEE ALSO
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.Xr condvar 9 ,
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.Xr mtx_pool 9 ,
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.Xr mutex 9 ,
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.Xr sx 9
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