source: NonGTP/Boost/boost/detail/atomic_count.hpp @ 857

Revision 857, 3.1 KB checked in by igarcia, 19 years ago (diff)
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[857]1#ifndef BOOST_DETAIL_ATOMIC_COUNT_HPP_INCLUDED
2#define BOOST_DETAIL_ATOMIC_COUNT_HPP_INCLUDED
3
4// MS compatible compilers support #pragma once
5
6#if defined(_MSC_VER) && (_MSC_VER >= 1020)
7# pragma once
8#endif
9
10//
11//  boost/detail/atomic_count.hpp - thread/SMP safe reference counter
12//
13//  Copyright (c) 2001, 2002 Peter Dimov and Multi Media Ltd.
14//
15// Distributed under the Boost Software License, Version 1.0. (See
16// accompanying file LICENSE_1_0.txt or copy at
17// http://www.boost.org/LICENSE_1_0.txt)
18//
19//  typedef <implementation-defined> boost::detail::atomic_count;
20//
21//  atomic_count a(n);
22//
23//    (n is convertible to long)
24//
25//    Effects: Constructs an atomic_count with an initial value of n
26//
27//  a;
28//
29//    Returns: (long) the current value of a
30//
31//  ++a;
32//
33//    Effects: Atomically increments the value of a
34//    Returns: nothing
35//
36//  --a;
37//
38//    Effects: Atomically decrements the value of a
39//    Returns: (long) zero if the new value of a is zero,
40//      unspecified non-zero value otherwise (usually the new value)
41//
42//    Important note: when --a returns zero, it must act as a
43//      read memory barrier (RMB); i.e. the calling thread must
44//      have a synchronized view of the memory
45//
46//    On Intel IA-32 (x86) memory is always synchronized, so this
47//      is not a problem.
48//
49//    On many architectures the atomic instructions already act as
50//      a memory barrier.
51//
52//    This property is necessary for proper reference counting, since
53//      a thread can update the contents of a shared object, then
54//      release its reference, and another thread may immediately
55//      release the last reference causing object destruction.
56//
57//    The destructor needs to have a synchronized view of the
58//      object to perform proper cleanup.
59//
60//    Original example by Alexander Terekhov:
61//
62//    Given:
63//
64//    - a mutable shared object OBJ;
65//    - two threads THREAD1 and THREAD2 each holding
66//      a private smart_ptr object pointing to that OBJ.
67//
68//    t1: THREAD1 updates OBJ (thread-safe via some synchronization)
69//      and a few cycles later (after "unlock") destroys smart_ptr;
70//
71//    t2: THREAD2 destroys smart_ptr WITHOUT doing any synchronization
72//      with respect to shared mutable object OBJ; OBJ destructors
73//      are called driven by smart_ptr interface...
74//
75
76#include <boost/config.hpp>
77
78#ifndef BOOST_HAS_THREADS
79
80namespace boost
81{
82
83namespace detail
84{
85
86typedef long atomic_count;
87
88}
89
90}
91
92#elif defined(BOOST_AC_USE_PTHREADS)
93#  include <boost/detail/atomic_count_pthreads.hpp>
94#elif defined(WIN32) || defined(_WIN32) || defined(__WIN32__)
95#  include <boost/detail/atomic_count_win32.hpp>
96#elif defined(__GLIBCPP__) || defined(__GLIBCXX__)
97#  include <boost/detail/atomic_count_gcc.hpp>
98#elif defined(BOOST_HAS_PTHREADS)
99#  define BOOST_AC_USE_PTHREADS
100#  include <boost/detail/atomic_count_pthreads.hpp>
101#else
102
103// Use #define BOOST_DISABLE_THREADS to avoid the error
104#error Unrecognized threading platform
105
106#endif
107
108#endif // #ifndef BOOST_DETAIL_ATOMIC_COUNT_HPP_INCLUDED
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