C++11中的使用

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原子库为细粒度的原子操作提供组件，允许无锁并发编程。涉及同一对象的每个原子操作，相对于任何其他原子操作是不可分的。原子对象不具有数据竞争(data race)。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。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。

atomic是C++标准程序库中的一个头文件，定义了C++11标准中的一些表示线程、并发控制时原子操作的类与方法等。此头文件主要声明了两大类原子对象：std::atomic和std::atomic_flag，另外还声明了一套C风格的原子类型和与C兼容的原子操作的函数。在多线程并发执行时，原子操作是线程不会被打断的执行片段。一些程序设计更为注重性能和效率，需要开发lock-free的算法和数据结构，这就需要更为底层的原子操作与原子类型。原子类型对象的主要特点就是从不同线程并发访问是良性(well-defined)行为，不会导致竞争危害。与之相反，不做适当控制就并发访问非原子对象则会导致未定义(undifined)行为。

atomic_flag类：是一种简单的原子布尔类型，只支持两种操作：test_and_set(flag=true)和clear(flag=false)。跟std::atomic的其它所有特化类不同，它是锁无关的。结合std::atomic_flag::test_and_set()和std::atomic_flag::clear()，std::atomic_flag对象可以当作一个简单的自旋锁(spin lock)使用。atomic_flag只有默认构造函数，禁用拷贝构造函数，移动构造函数实际上也禁用。如果在初始化时没有明确使用宏ATOMIC_FLAG_INIT初始化，那么新创建的std::atomic_flag对象的状态是未指定的(unspecified)，既没有被set也没有被clear;如果使用该宏初始化，该std::atomic_flag对象在创建时处于clear状态。

(1)、test_and_set：返回该std::atomic_flag对象当前状态，检查flag是否被设置，若被设置直接返回true，若没有设置则设置flag为true后再返回false。该函数是原子的。

(2)、clear：清除std::atomic_flag对象的标志位，即设置atomic_flag的值为false。

std::atomic类模板：std::atomic比std::atomic_flag功能更加完善。c++11标准库std::atomic提供了针对bool类型、整形(integral)和指针类型的特化实现。每个std::atomic模板的实例化和完全特化定义一个原子类型。若一个线程写入原子对象，同时另一个线程从它读取，则行为良好定义。而且，对原子对象的访问可以按std::memory_order所指定建立线程间同步，并排序非原子的内存访问。std::atomic可以以任何可平凡复制(Trivially Copyable)的类型T实例化。std::atomic既不可复制亦不可移动。

除了std::atomic和std::atomic_flag外，还包括了基于std::atomic_flag类的C风格API和基于std::atomic类模板的C风格API。

与原子对象初始化相关的两个宏：

(1)、ATOMIC_VAR_INIT(val)：初始化std::atomic对象。This macro expands to a token sequence suitable to initialize an atomic object (of static storage duration) with a value of val. This macro exists for compatibility with C implementations, in which it is used as a constructor-like function for(default-constructed) atomic objects; In C++, this initialization may be performed directly by the initialization constructor.

(2)、ATOMIC_FLAG_INIT：初始化std::atomic_flag对象。This macro is defined in such a way that it can be used to initialize an object of type atomic_flag to the clear state.

std::atomic：Objects of atomic types contain a value of a particular type (T). The main characteristic of atomic objects is that access to this contained value from different threads cannot cause data races (i.e., doing that is well-defined behavior, with accesses properly sequenced). Generally, for all other objects, the possibility of causing a data race for accessing the same object concurrently qualifies the operation as undefined behavior. Additionally, atomic objects have the ability to synchronize access to other non-atomic objects in their threads by specifying different memory orders.

std::atomic_flag:Atomic flags are boolean atomic objects that support two operations:test-and-set and clear.

下面是从其他文章中copy的测试代码，详细内容介绍可以参考对应的reference：

[cpp] view plain copy#include "atomic.hpp"

#include

#include

#include

#include

#include

namespace atomic {

/////////////////////////////////////////////////////

// reference: https://www.cplusplus.com/reference/atomic/atomic/atomic/

std::atomic ready(false);

// atomic_flag::atomic_flag: Constructs an atomic_flag object

// The atomic_flag is in an unspecified state on construction (either set or clear),

// unless it is explicitly initialized to ATOMIC_FLAG_INIT.

std::atomic_flag winner = ATOMIC_FLAG_INIT;

void count1m(int id)

{

while (!ready) { std::this_thread::yield(); } // wait for the ready signal

for (volatile int i = 0; i < 1000000; ++i) {} // go!, count to 1 million

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

};

int test_atomic_atomic()

{

// atomic::atomic: Constructs an atomic object

std::vector 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;

}

/////////////////////////////////////////////////////////////////////

// reference: https://www.cplusplus.com/reference/atomic/atomic/compare_exchange_weak/

// a simple global linked list:

struct Node { int value; Node* next; };

std::atomic list_head(nullptr);

void append(int val)

{ // append an element to the list

Node* oldHead = list_head;

Node* newNode = new Node{ val, oldHead };

// what follows is equivalent to: list_head = newNode, but in a thread-safe way:

while (!list_head.compare_exchange_weak(oldHead, newNode))

newNode->next = oldHead;

}

int test_atomic_compare_exchange_weak()

{

// atomic::compare_exchange_weak: Compares the contents of the atomic object's contained value with expected:

// -if true, it replaces the contained value with val(like store).

// - if false, it replaces expected with the contained value

// spawn 10 threads to fill the linked list:

std::vector 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;

}

///////////////////////////////////////////////////////////////

// reference: https://www.cplusplus.com/reference/atomic/atomic/exchange/

std::atomic 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 test_atomic_exchange()

{

// atomic::exchange: Replaces the contained value by val and returns the value it had immediately before

std::vector 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;

}

/////////////////////////////////////////////////////////

// reference: https://www.cplusplus.com/reference/atomic/atomic/load/

std::atomic foo(0);

void set_foo(int x)

{

foo.store(x, std::memory_order_relaxed); // set value atomically

}

void print_foo()

{

int x;

do {

x = foo.load(std::memory_order_relaxed); // get value atomically

} while (x == 0);

std::cout << "foo: " << x << '\n';

}

int test_atomic_load()

{

// atomic::load: Returns the contained value.

// The operation is atomic and follows the memory ordering specified by sync.

std::thread first(print_foo);

std::thread second(set_foo, 10);

first.join();

second.join();

return 0;

}

////////////////////////////////////////////////////////////////

// reference: https://www.cplusplus.com/reference/atomic/atomic/operator=/

std::atomic foo_ = 0;

void set_foo_(int x)

{

foo_ = x;

}

void print_foo_()

{

while (foo_ == 0) { // wait while foo_=0

std::this_thread::yield();

}

std::cout << "foo_: " << foo_ << '\n';

}

int test_atomic_operator()

{

// atomic::operator=: Replaces the stored value by val.

// This operation is atomic and uses sequential consistency (memory_order_seq_cst).

// To modify the value with a different memory ordering

std::thread first(print_foo_);

std::thread second(set_foo_, 10);

first.join();

second.join();

return 0;

}

///////////////////////////////////////////////////////////////////

// reference: https://www.cplusplus.com/reference/atomic/atomic/store/

int test_atomic_store()

{

// atomic::store: Replaces the contained value with val.

// The operation is atomic and follows the memory ordering specified by sync.

std::thread first(print_foo);

std::thread second(set_foo, 10);

first.join();

second.join();

return 0;

}

/////////////////////////////////////////////////////////////////////

// reference: https://www.cplusplus.com/reference/atomic/atomic_flag/clear/

std::atomic_flag lock_stream = ATOMIC_FLAG_INIT;

std::stringstream stream;

void append_number(int x)

{

while (lock_stream.test_and_set()) {}

stream << "thread #" << x << '\n';

lock_stream.clear();

}

int test_atomic_flag_atomic_clear()

{

// atomic_flag::clear: Clears the atomic_flag (i.e., sets it to false)

//Clearing the atomic_flag makes the next call to member atomic_flag::test_and_set on this object return false.

// The operation is atomic and follows the memory ordering specified by sync.

// atomic_flag::test_and_set: Sets the atomic_flag and returns whether it was already set immediately before the call

// The entire operation is atomic (an atomic read-modify-write operation): the value is not affected by other threads

// between the instant its value is read (to be returned) and the moment it is modified by this function.

std::vector threads;

for (int i = 1; i <= 10; ++i) threads.push_back(std::thread(append_number, i));

for (auto& th : threads) th.join();

std::cout << stream.str();

return 0;

}

} // namespace atomic

 

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