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C++11 并发指南六( <atomic> 类型详解二 std::atomic )

2013-09-05 22:32 by Haippy, ... 阅读, ... 评论, 收藏, 编辑

C++11 并发指南六(atomic 类型详解一 atomic_flag 介绍)  一文介绍了 C++11 中最简单的原子类型 std::atomic_flag,但是 std::atomic_flag 过于简单,只提供了 test_and_set 和 clear 两个 API,不能满足其他需求(如 store, load, exchange, compare_exchange 等),因此本文将介绍功能更加完善的 std::atomic 类。

std::atomic 基本介绍

std::atomic 是模板类,一个模板类型为 T 的原子对象中封装了一个类型为 T 的值。

template <class T> struct atomic;

原子类型对象的主要特点就是从不同线程访问不会导致数据竞争(data race)。因此从不同线程访问某个原子对象是良性 (well-defined) 行为,而通常对于非原子类型而言,并发访问某个对象(如果不做任何同步操作)会导致未定义 (undifined) 行为发生。

C++11 标准中的基本 std::atomic 模板定义如下:

template < class T > struct atomic {
    bool is_lock_free() const volatile;
    bool is_lock_free() const;
    void store(T, memory_order = memory_order_seq_cst) volatile;
    void store(T, memory_order = memory_order_seq_cst);
    T load(memory_order = memory_order_seq_cst) const volatile;
    T load(memory_order = memory_order_seq_cst) const;
    operator  T() const volatile;
    operator  T() const;
    T exchange(T, memory_order = memory_order_seq_cst) volatile;
    T exchange(T, memory_order = memory_order_seq_cst);
    bool compare_exchange_weak(T &, T, memory_order, memory_order) volatile;
    bool compare_exchange_weak(T &, T, memory_order, memory_order);
    bool compare_exchange_strong(T &, T, memory_order, memory_order) volatile;
    bool compare_exchange_strong(T &, T, memory_order, memory_order);
    bool compare_exchange_weak(T &, T, memory_order = memory_order_seq_cst) volatile;
    bool compare_exchange_weak(T &, T, memory_order = memory_order_seq_cst);
    bool compare_exchange_strong(T &, T, memory_order = memory_order_seq_cst) volatile;
    bool compare_exchange_strong(T &, T, memory_order = memory_order_seq_cst);
    atomic() = default;
    constexpr atomic(T);
    atomic(const atomic &) = delete;
    atomic & operator=(const atomic &) = delete;
    atomic & operator=(const atomic &) volatile = delete;
    T operator=(T) volatile;
    T operator=(T);
};

另外,C++11 标准库 std::atomic 提供了针对整形(integral)和指针类型的特化实现,分别定义如下:

针对整形(integal)的特化,其中 integal 代表了如下类型char, signed char, unsigned char, short, unsigned short, int, unsigned int, long, unsigned long, long long, unsigned long long, char16_t, char32_t, wchar_t:

template <> struct atomic<integral> {
    bool is_lock_free() const volatile;
    bool is_lock_free() const;

    void store(integral, memory_order = memory_order_seq_cst) volatile;
    void store(integral, memory_order = memory_order_seq_cst);

    integral load(memory_order = memory_order_seq_cst) const volatile;
    integral load(memory_order = memory_order_seq_cst) const;

    operator integral() const volatile;
    operator integral() const;

    integral exchange(integral, memory_order = memory_order_seq_cst) volatile;
    integral exchange(integral, memory_order = memory_order_seq_cst);

    bool compare_exchange_weak(integral&, integral, memory_order, memory_order) volatile;
    bool compare_exchange_weak(integral&, integral, memory_order, memory_order);

    bool compare_exchange_strong(integral&, integral, memory_order, memory_order) volatile;
    bool compare_exchange_strong(integral&, integral, memory_order, memory_order);

    bool compare_exchange_weak(integral&, integral, memory_order = memory_order_seq_cst) volatile;
    bool compare_exchange_weak(integral&, integral, memory_order = memory_order_seq_cst);

    bool compare_exchange_strong(integral&, integral, memory_order = memory_order_seq_cst) volatile;
    bool compare_exchange_strong(integral&, integral, memory_order = memory_order_seq_cst);

    integral fetch_add(integral, memory_order = memory_order_seq_cst) volatile;
    integral fetch_add(integral, memory_order = memory_order_seq_cst);

    integral fetch_sub(integral, memory_order = memory_order_seq_cst) volatile;
    integral fetch_sub(integral, memory_order = memory_order_seq_cst);

    integral fetch_and(integral, memory_order = memory_order_seq_cst) volatile;
    integral fetch_and(integral, memory_order = memory_order_seq_cst);

    integral fetch_or(integral, memory_order = memory_order_seq_cst) volatile;
    integral fetch_or(integral, memory_order = memory_order_seq_cst);

    integral fetch_xor(integral, memory_order = memory_order_seq_cst) volatile;
    integral fetch_xor(integral, memory_order = memory_order_seq_cst);
    
    atomic() = default;
    constexpr atomic(integral);
    atomic(const atomic&) = delete;

    atomic& operator=(const atomic&) = delete;
    atomic& operator=(const atomic&) volatile = delete;
    
    integral operator=(integral) volatile;
    integral operator=(integral);
    
    integral operator++(int) volatile;
    integral operator++(int);
    integral operator--(int) volatile;
    integral operator--(int);
    integral operator++() volatile;
    integral operator++();
    integral operator--() volatile;
    integral operator--();
    integral operator+=(integral) volatile;
    integral operator+=(integral);
    integral operator-=(integral) volatile;
    integral operator-=(integral);
    integral operator&=(integral) volatile;
    integral operator&=(integral);
    integral operator|=(integral) volatile;
    integral operator|=(integral);
    integral operator^=(integral) volatile;
    integral operator^=(integral);
};

针对指针的特化:

template <class T> struct atomic<T*> {
    bool is_lock_free() const volatile;
    bool is_lock_free() const;

    void store(T*, memory_order = memory_order_seq_cst) volatile;
    void store(T*, memory_order = memory_order_seq_cst);

    T* load(memory_order = memory_order_seq_cst) const volatile;
    T* load(memory_order = memory_order_seq_cst) const;

    operator T*() const volatile;
    operator T*() const;

    T* exchange(T*, memory_order = memory_order_seq_cst) volatile;
    T* exchange(T*, memory_order = memory_order_seq_cst);

    bool compare_exchange_weak(T*&, T*, memory_order, memory_order) volatile;
    bool compare_exchange_weak(T*&, T*, memory_order, memory_order);

    bool compare_exchange_strong(T*&, T*, memory_order, memory_order) volatile;
    bool compare_exchange_strong(T*&, T*, memory_order, memory_order);

    bool compare_exchange_weak(T*&, T*, memory_order = memory_order_seq_cst) volatile;
    bool compare_exchange_weak(T*&, T*, memory_order = memory_order_seq_cst);

    bool compare_exchange_strong(T*&, T*, memory_order = memory_order_seq_cst) volatile;
    bool compare_exchange_strong(T*&, T*, memory_order = memory_order_seq_cst);

    T* fetch_add(ptrdiff_t, memory_order = memory_order_seq_cst) volatile;
    T* fetch_add(ptrdiff_t, memory_order = memory_order_seq_cst);

    T* fetch_sub(ptrdiff_t, memory_order = memory_order_seq_cst) volatile;
    T* fetch_sub(ptrdiff_t, memory_order = memory_order_seq_cst);

    atomic() = default;
    constexpr atomic(T*);
    atomic(const atomic&) = delete;

    atomic& operator=(const atomic&) = delete;
    atomic& operator=(const atomic&) volatile = delete;

    T* operator=(T*) volatile;
    T* operator=(T*);
    T* operator++(int) volatile;
    T* operator++(int);
    T* operator--(int) volatile;
    T* operator--(int);
    T* operator++() volatile;
    T* operator++();
    T* operator--() volatile;
    T* operator--();
    T* operator+=(ptrdiff_t) volatile;
    T* operator+=(ptrdiff_t);
    T* operator-=(ptrdiff_t) volatile;
    T* operator-=(ptrdiff_t);
};

std::atomic 成员函数

 好了,对 std::atomic 有了一个最基本认识之后我们来看 std::atomic 的成员函数吧。

std::atomic 构造函数

std::atomic 的构造函数如下:

default (1)
          atomic() noexcept = default;
initialization (2)
constexpr atomic (T val) noexcept;
copy [deleted] (3)
          atomic (const atomic&) = delete;
  1. 默认构造函数,由默认构造函数创建的 std::atomic 对象处于未初始化(uninitialized)状态,对处于未初始化(uninitialized)状态 std::atomic对象可以由 atomic_init 函数进行初始化。
  2. 初始化构造函数,由类型 T初始化一个 std::atomic对象。
  3. 拷贝构造函数被禁用。

请看下例:

#include <iostream>       // std::cout
#include <atomic>         // std::atomic, std::atomic_flag, ATOMIC_FLAG_INIT
#include <thread>         // std::thread, std::this_thread::yield
#include <vector>         // std::vector

// 由 false 初始化一个 std::atomic<bool> 类型的原子变量
std::atomic<bool> ready(false);
std::atomic_flag winner = ATOMIC_FLAG_INIT;

void do_count1m(int id)
{
    while (!ready) { std::this_thread::yield(); } // 等待 ready 变为 true.

    for (volatile int i=0; i<1000000; ++i) {} // 计数

    if (!winner.test_and_set()) {
      std::cout << "thread #" << id << " won!\n";
    }
}

int main ()
{
    std::vector<std::thread> threads;
    std::cout << "spawning 10 threads that count to 1 million...\n";
    for (int i=1; i<=10; ++i) threads.push_back(std::thread(count1m,i));
    ready = true;

    for (auto& th : threads) th.join();
    return 0;
}

std::atomic::operator=() 函数

std::atomic 的赋值操作函数定义如下:

set value (1)
T operator= (T val) noexcept;
T operator= (T val) volatile noexcept;
copy [deleted] (2)
atomic& operator= (const atomic&) = delete;
atomic& operator= (const atomic&) volatile = delete;

可以看出,普通的赋值拷贝操作已经被禁用。但是一个类型为 T 的变量可以赋值给相应的原子类型变量(相当与隐式转换),该操作是原子的,内存序(Memory Order) 默认为顺序一致性(std::memory_order_seq_cst),如果需要指定其他的内存序,需使用 std::atomic::store()。

#include <iostream>             // std::cout
#include <atomic>               // std::atomic
#include <thread>               // std::thread, std::this_thread::yield

std::atomic <int> foo = 0;

void set_foo(int x)
{
    foo = x; // 调用 std::atomic::operator=().
}

void print_foo()
{
    while (foo == 0) { // wait while foo == 0
        std::this_thread::yield();
    }
    std::cout << "foo: " << foo << '\n';
}

int main()
{
    std::thread first(print_foo);
    std::thread second(set_foo, 10);
    first.join();
    second.join();
    return 0;
}

基本 std::atomic 类型操作

本节主要介绍基本 std::atomic 类型所具备的操作(即成员函数)。我们知道 std::atomic 是模板类,一个模板类型为 T 的原子对象中封装了一个类型为 T 的值。本文<std::atomic 基本介绍>一节中也提到了 std::atomic 类模板除了基本类型以外,还针对整形和指针类型做了特化。 特化的 std::atomic 类型支持更多的操作,如 fetch_add, fetch_sub, fetch_and 等。本小节介绍基本 std::atomic 类型所具备的操作:

bool is_lock_free() const volatile noexcept;
bool is_lock_free() const noexcept;
void store (T val, memory_order sync = memory_order_seq_cst) volatile noexcept;
void store (T val, memory_order sync = memory_order_seq_cst) noexcept;
Memory Order 值Memory Order 类型
memory_order_relaxed Relaxed
memory_order_release Release
memory_order_seq_cst Sequentially consistent
#include <iostream>       // std::cout
#include <atomic>         // std::atomic, std::memory_order_relaxed
#include <thread>         // std::thread

std::atomic<int> foo(0); // 全局的原子对象 foo

void set_foo(int x)
{
    foo.store(x, std::memory_order_relaxed); // 设置(store) 原子对象 foo 的值
}

void print_foo()
{
    int x;
    do {
        x = foo.load(std::memory_order_relaxed); // 读取(load) 原子对象 foo 的值
    } while (x == 0);
    std::cout << "foo: " << x << '\n';
}

int main ()
{
    std::thread first(print_foo); // 线程 first 打印 foo 的值
    std::thread second(set_foo, 10); // 线程 second 设置 foo 的值
    first.join();
    second.join();
    return 0;
}
T load (memory_order sync = memory_order_seq_cst) const volatile noexcept;
T load (memory_order sync = memory_order_seq_cst) const noexcept;
Memory Order 值Memory Order 类型
memory_order_relaxed Relaxed
memory_order_consume Consume
memory_order_acquire Acquire
memory_order_seq_cst Sequentially consistent
#include <iostream>       // std::cout
#include <atomic>         // std::atomic, std::memory_order_relaxed
#include <thread>         // std::thread

std::atomic<int> foo(0); // 全局的原子对象 foo

void set_foo(int x)
{
    foo.store(x, std::memory_order_relaxed); // 设置(store) 原子对象 foo 的值
}

void print_foo()
{
    int x;
    do {
        x = foo.load(std::memory_order_relaxed); // 读取(load) 原子对象 foo 的值
    } while (x == 0);
    std::cout << "foo: " << x << '\n';
}

int main ()
{
    std::thread first(print_foo); // 线程 first 打印 foo 的值
    std::thread second(set_foo, 10); // 线程 second 设置 foo 的值
    first.join();
    second.join();
    return 0;
}
operator T() const volatile noexcept;
operator T() const noexcept;
#include <iostream>       // std::cout
#include <atomic>         // std::atomic
#include <thread>         // std::thread, std::this_thread::yield

std::atomic<int> foo = 0;
std::atomic<int> bar = 0;

void set_foo(int x)
{
    foo = x;
}

void copy_foo_to_bar()
{

    // 如果 foo == 0,则该线程 yield,
    // 在 foo == 0 时, 实际也是隐含了类型转换操作,
    // 因此也包含了 operator T() const 的调用.
    while (foo == 0) std::this_thread::yield();

    // 实际调用了 operator T() const, 将foo 强制转换成 int 类型,
    // 然后调用 operator=().
    bar = static_cast<int>(foo);
}

void print_bar()
{
    // 如果 bar == 0,则该线程 yield,
    // 在 bar == 0 时, 实际也是隐含了类型转换操作,
    // 因此也包含了 operator T() const 的调用.
    while (bar == 0) std::this_thread::yield();
    std::cout << "bar: " << bar << '\n';
}

int main ()
{
    std::thread first(print_bar);
    std::thread second(set_foo, 10);
    std::thread third(copy_foo_to_bar);

    first.join();
    second.join();
    third.join();
    return 0;
}

 

T exchange (T val, memory_order sync = memory_order_seq_cst) volatile noexcept;
T exchange (T val, memory_order sync = memory_order_seq_cst) noexcept;
Memory Order 值Memory Order 类型
memory_order_relaxed Relaxed
memory_order_consume Consume
memory_order_acquire Acquire
memory_order_release Release
memory_order_acq_rel Acquire/Release
memory_order_seq_cst Sequentially consistent

请看下面例子,各个线程计数至 1M,首先完成计数任务的线程打印自己的 ID,

#include <iostream>       // std::cout
#include <atomic>         // std::atomic
#include <thread>         // std::thread
#include <vector>         // std::vector

std::atomic<bool> ready(false);
std::atomic<bool> winner(false);

void count1m (int id)
{
    while (!ready) {}                  // wait for the ready signal
    for (int i = 0; i < 1000000; ++i) {}   // go!, count to 1 million
    if (!winner.exchange(true)) { std::cout << "thread #" << id << " won!\n"; }
};

int main ()
{
    std::vector<std::thread> threads;
    std::cout << "spawning 10 threads that count to 1 million...\n";
    for (int i = 1; i <= 10; ++i) threads.push_back(std::thread(count1m,i));
    ready = true;
    for (auto& th : threads) th.join();

    return 0;
}
(1)
bool compare_exchange_weak (T& expected, T val,
           memory_order sync = memory_order_seq_cst) volatile noexcept;
bool compare_exchange_weak (T& expected, T val,
           memory_order sync = memory_order_seq_cst) noexcept;
(2)
bool compare_exchange_weak (T& expected, T val,
           memory_order success, memory_order failure) volatile noexcept;
bool compare_exchange_weak (T& expected, T val,
           memory_order success, memory_order failure) noexcept;
  • 相等,则用 val 替换原子对象的旧值。
  • 不相等,则用原子对象的旧值替换 expected ,因此调用该函数之后,如果被该原子对象封装的值与参数 expected 所指定的值不相等,expected 中的内容就是原子对象的旧值。
Memory Order 值Memory Order 类型
memory_order_relaxed Relaxed
memory_order_consume Consume
memory_order_acquire Acquire
memory_order_release Release
memory_order_acq_rel Acquire/Release
memory_order_seq_cst Sequentially consistent
#include <iostream>       // std::cout
#include <atomic>         // std::atomic
#include <thread>         // std::thread
#include <vector>         // std::vector

// a simple global linked list:
struct Node { int value; Node* next; };
std::atomic<Node*> list_head(nullptr);

void append(int val)
{
    // append an element to the list
    Node* newNode = new Node{val, list_head};

    // next is the same as: list_head = newNode, but in a thread-safe way:
    while (!list_head.compare_exchange_weak(newNode->next,newNode)) {}
    // (with newNode->next updated accordingly if some other thread just appended another node)
}

int main ()
{
    // spawn 10 threads to fill the linked list:
    std::vector<std::thread> threads;
    for (int i = 0; i < 10; ++i) threads.push_back(std::thread(append, i));
    for (auto& th : threads) th.join();

    // print contents:
    for (Node* it = list_head; it!=nullptr; it=it->next)
        std::cout << ' ' << it->value;

    std::cout << '\n';

    // cleanup:
    Node* it; while (it=list_head) {list_head=it->next; delete it;}

    return 0;
}
9 8 7 6 5 4 3 2 1 0

 

(1)
bool compare_exchange_strong (T& expected, T val,
           memory_order sync = memory_order_seq_cst) volatile noexcept;
bool compare_exchange_strong (T& expected, T val,
           memory_order sync = memory_order_seq_cst) noexcept;
(2)
bool compare_exchange_strong (T& expected, T val,
           memory_order success, memory_order failure) volatile noexcept;
bool compare_exchange_strong (T& expected, T val,
           memory_order success, memory_order failure) noexcept;
  • 相等,则用 val 替换原子对象的旧值。
  • 不相等,则用原子对象的旧值替换 expected ,因此调用该函数之后,如果被该原子对象封装的值与参数 expected 所指定的值不相等,expected 中的内容就是原子对象的旧值。
Memory Order 值Memory Order 类型
memory_order_relaxed Relaxed
memory_order_consume Consume
memory_order_acquire Acquire
memory_order_release Release
memory_order_acq_rel Acquire/Release
memory_order_seq_cst Sequentially consistent
#include <iostream>       // std::cout
#include <atomic>         // std::atomic
#include <thread>         // std::thread
#include <vector>         // std::vector

// a simple global linked list:
struct Node { int value; Node* next; };
std::atomic<Node*> list_head(nullptr);

void append(int val)
{
    // append an element to the list
    Node* newNode = new Node{val, list_head};

    // next is the same as: list_head = newNode, but in a thread-safe way:

    while (!(list_head.compare_exchange_strong(newNode->next, newNode)));
    // (with newNode->next updated accordingly if some other thread just appended another node)
}

int main ()
{
    // spawn 10 threads to fill the linked list:
    std::vector<std::thread> threads;
    for (int i = 0; i < 10; ++i) threads.push_back(std::thread(append, i));
    for (auto& th : threads) th.join();

    // print contents:
    for (Node* it = list_head; it!=nullptr; it=it->next)
        std::cout << ' ' << it->value;

    std::cout << '\n';

    // cleanup:
    Node* it; while (it=list_head) {list_head=it->next; delete it;}

    return 0;
}


好了,本文花了大量的篇幅介绍 std::atomic 基本类型,下一篇博客我会给大家介绍 C++11 的标准库中std::atomic 针对整形(integral)和指针类型的特化版本做了哪些改进。