.TH "Semaphore" 3 2024-02-29 OCamldoc "OCaml library"
.SH NAME
Semaphore \- Semaphores
.SH Module
Module   Semaphore
.SH Documentation
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Module
.BI "Semaphore"
 : 
.B sig end

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Semaphores
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A semaphore is a thread synchronization device that can be used to
control access to a shared resource\&.
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Two flavors of semaphores are provided: counting semaphores and
binary semaphores\&.

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.B "Since"
4.12

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.PP
.SS Counting semaphores

.PP

.PP
A counting semaphore is a counter that can be accessed concurrently
by several threads\&.  The typical use is to synchronize producers and
consumers of a resource by counting how many units of the resource
are available\&.
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The two basic operations on semaphores are:
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\-"release" (also called "V", "post", "up", and "signal"), which
increments the value of the counter\&.  This corresponds to producing
one more unit of the shared resource and making it available to others\&.
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\-"acquire" (also called "P", "wait", "down", and "pend"), which
waits until the counter is greater than zero and decrements it\&.
This corresponds to consuming one unit of the shared resource\&.

.PP
.I module Counting : 
.B sig end

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.PP
.SS Binary semaphores

.PP

.PP
Binary semaphores are a variant of counting semaphores
where semaphores can only take two values, 0 and 1\&.
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A binary semaphore can be used to control access to a single
shared resource, with value 1 meaning "resource is available" and
value 0 meaning "resource is unavailable"\&.
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The "release" operation of a binary semaphore sets its value to 1,
and "acquire" waits until the value is 1 and sets it to 0\&.
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A binary semaphore can be used instead of a mutex (see module
.ft B
Mutex
.ft R
) when the mutex discipline (of unlocking the mutex from the
thread that locked it) is too restrictive\&.  The "acquire" operation
corresponds to locking the mutex, and the "release" operation to
unlocking it, but "release" can be performed in a thread different
than the one that performed the "acquire"\&.  Likewise, it is safe
to release a binary semaphore that is already available\&.
.PP
.I module Binary : 
.B sig end

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