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95 thread

% vim: ft=mercury ts=4 sw=4 et
% Copyright (C) 2000-2001, 2003-2004, 2006-2008, 2010-2011 The University
% of Melbourne.
% Copyright (C) 2014-2018 The Mercury Team.
% This file is distributed under the terms specified in COPYING.LIB.
% File: thread.m.
% Authors: conway, wangp.
% Stability: medium.
% This module defines the Mercury concurrency interface.
% The term `concurrency' refers to threads, not necessarily to parallel
% execution of those threads. (The latter is also possible if you are using
% one of the .par grades or the Java or C# backends.)

:- module thread.
:- interface.

:- import_module io.
:- import_module maybe.

:- include_module barrier.
:- include_module channel.
:- include_module closeable_channel.
:- include_module future.
:- include_module mvar.
:- include_module semaphore.


    % Abstract type representing a thread.
:- type thread.

    % can_spawn succeeds if spawn/4 is supported in the current grade.
:- pred can_spawn is semidet.

    % can_spawn_native succeeds if spawn_native/4 is supported in the current
    % grade.
:- pred can_spawn_native is semidet.

    % spawn(Closure, IO0, IO) is true iff `IO0' denotes a list of I/O
    % transactions that is an interleaving of those performed by `Closure'
    % and those contained in `IO' - the list of transactions performed by
    % the continuation of spawn/3.
    % Operationally, spawn/3 is like spawn/4 except that Closure does not
    % accept a thread handle argument, and an exception is thrown if the
    % thread cannot be created.
:- pred spawn(pred(io, io), io, io).
:- mode spawn(pred(di, uo) is cc_multi, di, uo) is cc_multi.

    % spawn(Closure, Res, IO0, IO) creates a new thread and performs Closure in
    % that thread. On success it returns ok(Thread) where Thread is a handle to
    % the new thread. Otherwise it returns an error.
:- pred spawn(pred(thread, io, io), maybe_error(thread), io, io).
:- mode spawn(pred(in, di, uo) is cc_multi, out, di, uo) is cc_multi.

    % spawn_native(Closure, Res, IO0, IO):
    % Like spawn/4, but Closure will be performed in a separate "native thread"
    % of the environment the program is running in (POSIX thread, Windows
    % thread, Java thread, etc.).
    % spawn_native exposes a low-level implementation detail, so it is more
    % likely to change with the implementation.
    % Rationale: on the low-level C backend Mercury threads are multiplexed
    % onto a limited number of OS threads. A call to a blocking procedure
    % prevents that OS thread from making progress on another Mercury thread.
    % Also, some foreign code depends on OS thread-local state so needs to be
    % consistently executed on a dedicated OS thread to be usable.
:- pred spawn_native(pred(thread, io, io), maybe_error(thread), io, io).
:- mode spawn_native(pred(in, di, uo) is cc_multi, out, di, uo) is cc_multi.

    % yield(IO0, IO) is logically equivalent to (IO = IO0) but
    % operationally, yields the Mercury engine to some other thread
    % if one exists.
    % NOTE: this is not yet implemented in the hl*.par.gc grades; currently
    % it is a no-op in those grades.
:- pred yield(io::di, io::uo) is det.

    % num_processors(Num, !IO)
    % Retrieve the number of processors available to this process for
    % parallel execution, if known.
    % Note that the number of available processors can be different from the
    % actual number of processors/cores:
    %  + It includes hardware threads.
    %  + The Mercury grade may restrict the process to one processor.
    %  + The OS may be configured to restrict the number of processors
    %    available (e.g. cpuset(7) on Linux).
:- pred num_processors(maybe(int)::out, io::di, io::uo) is det.


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