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% vim: ft=mercury ts=4 sw=4 et
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% Copyright (C) 2000-2001,2003-2004, 2006-2007, 2009-2011 The University of Melbourne.
% Copyright (C) 2014-2018 The Mercury Team.
% This file is distributed under the terms specified in COPYING.LIB.
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%
% File: thread.semaphore.m.
% Main author: conway.
% Stability: medium.
%
% This module implements a simple semaphore data type for allowing
% threads to synchronise with one another.
%
% The operations in this module are no-ops in the hlc grades that do not
% contain a .par component.
%
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:- module thread.semaphore.
:- interface.

:- import_module bool.
:- import_module io.

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:- type semaphore.

    % init(Count, Sem, !IO) creates a new semaphore `Sem' with its counter
    % initialized to `Count'.
    %
:- pred init(int::in, semaphore::uo, io::di, io::uo) is det.

    % init(Sem, !IO) creates a new semaphore `Sem' with its counter
    % initialized to 0.
    %
:- pred init(semaphore::uo, io::di, io::uo) is det.

    % Sem = init(Count) returns a new semaphore `Sem' with its counter
    % initialized to `Count'.
    %
:- impure func impure_init(int::in) = (semaphore::uo) is det.

    % signal(Sem, !IO) increments the counter associated with `Sem'
    % and if the resulting counter has a value greater than 0, it wakes
    % one or more threads that are waiting on this semaphore (if
    % any).
    %
:- pred signal(semaphore::in, io::di, io::uo) is det.

    % wait(Sem, !IO) blocks until the counter associated with `Sem'
    % becomes greater than 0, whereupon it wakes, decrements the
    % counter and returns.
    %
:- pred wait(semaphore::in, io::di, io::uo) is det.

    % try_wait(Sem, Succ, !IO) is the same as wait/3, except that
    % instead of blocking, it binds `Succ' to a boolean indicating
    % whether the call succeeded in obtaining the semaphore or not.
    %
:- pred try_wait(semaphore::in, bool::out, io::di, io::uo) is det.

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