forked from boostorg/smart_ptr
109 lines
3.0 KiB
C++
109 lines
3.0 KiB
C++
#ifndef BOOST_DETAIL_ATOMIC_COUNT_HPP_INCLUDED
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#define BOOST_DETAIL_ATOMIC_COUNT_HPP_INCLUDED
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// MS compatible compilers support #pragma once
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#if defined(_MSC_VER) && (_MSC_VER >= 1020)
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# pragma once
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#endif
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//
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// boost/detail/atomic_count.hpp - thread/SMP safe reference counter
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//
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// Copyright (c) 2001, 2002 Peter Dimov and Multi Media Ltd.
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//
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// Distributed under the Boost Software License, Version 1.0. (See
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// accompanying file LICENSE_1_0.txt or copy at
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// http://www.boost.org/LICENSE_1_0.txt)
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//
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// typedef <implementation-defined> boost::detail::atomic_count;
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//
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// atomic_count a(n);
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//
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// (n is convertible to long)
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//
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// Effects: Constructs an atomic_count with an initial value of n
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//
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// a;
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//
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// Returns: (long) the current value of a
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//
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// ++a;
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//
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// Effects: Atomically increments the value of a
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// Returns: nothing
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//
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// --a;
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//
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// Effects: Atomically decrements the value of a
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// Returns: (long) zero if the new value of a is zero,
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// unspecified non-zero value otherwise (usually the new value)
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//
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// Important note: when --a returns zero, it must act as a
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// read memory barrier (RMB); i.e. the calling thread must
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// have a synchronized view of the memory
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//
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// On Intel IA-32 (x86) memory is always synchronized, so this
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// is not a problem.
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//
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// On many architectures the atomic instructions already act as
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// a memory barrier.
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//
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// This property is necessary for proper reference counting, since
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// a thread can update the contents of a shared object, then
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// release its reference, and another thread may immediately
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// release the last reference causing object destruction.
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//
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// The destructor needs to have a synchronized view of the
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// object to perform proper cleanup.
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//
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// Original example by Alexander Terekhov:
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//
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// Given:
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//
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// - a mutable shared object OBJ;
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// - two threads THREAD1 and THREAD2 each holding
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// a private smart_ptr object pointing to that OBJ.
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//
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// t1: THREAD1 updates OBJ (thread-safe via some synchronization)
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// and a few cycles later (after "unlock") destroys smart_ptr;
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//
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// t2: THREAD2 destroys smart_ptr WITHOUT doing any synchronization
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// with respect to shared mutable object OBJ; OBJ destructors
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// are called driven by smart_ptr interface...
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//
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#include <boost/config.hpp>
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#ifndef BOOST_HAS_THREADS
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namespace boost
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{
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namespace detail
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{
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typedef long atomic_count;
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}
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}
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#elif defined(BOOST_AC_USE_PTHREADS)
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# include <boost/detail/atomic_count_pthreads.hpp>
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#elif defined(WIN32) || defined(_WIN32) || defined(__WIN32__)
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# include <boost/detail/atomic_count_win32.hpp>
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#elif defined(__GLIBCPP__) || defined(__GLIBCXX__)
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# include <boost/detail/atomic_count_gcc.hpp>
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#elif defined(BOOST_HAS_PTHREADS)
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# define BOOST_AC_USE_PTHREADS
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# include <boost/detail/atomic_count_pthreads.hpp>
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#else
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// Use #define BOOST_DISABLE_THREADS to avoid the error
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#error Unrecognized threading platform
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#endif
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#endif // #ifndef BOOST_DETAIL_ATOMIC_COUNT_HPP_INCLUDED
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