c++11新特性之atomic

std::atomic_flag

　　std::atomic_flag是一个原子的布尔类型，可支持两种原子操作：

test_and_set, 如果atomic_flag对象被设置，则返回true; 如果atomic_flag对象未被设置，则设置之，返回false
clear. 清楚atomic_flag对象

　　std::atomic_flag可用于多线程之间的同步操作，类似于linux中的信号量。使用atomic_flag可实现mutex.

#include <iostream>
#include <atomic>
#include <vector>
#include <thread>
#include <sstream>

std::atomic_flag lock = ATOMIC_FLAG_INIT;
std::stringstream stream;

void append_numer(int x)
{
  while (lock.test_and_set());
  stream << "thread#" << x << "\n";
  lock.clear();
}

int main()
{
  std::vector<std::thread> ths;
  for (int i=0; i<10; i++)
    ths.push_back(std::thread(append_numer, i));
  for (int i=0; i<10; i++)
    ths[i].join();
  std::cout << stream.str();
  return 0;
}

　　2. std::atomic

　　std::atomic对int, char, bool等数据结构进行原子性封装，在多线程环境中，对std::atomic对象的访问不会造成竞争-冒险。利用std::atomic可实现数据结构的无锁设计。

#include <iostream>
#include <atomic>
#include <vector>
#include <thread>
#include <sstream>

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

void count1m(int i)
{
  while (!ready);
  for (int i=0; i<1000000; i++);
  if (!winner.test_and_set())
    std::cout << "winner: " << i << std::endl;
}

int main()
{
  std::vector<std::thread> ths;
  for (int i=0; i<10; i++)
    ths.push_back(std::thread(count1m, i));
  ready = true;
  for (int i=0; i<10; i++)
    ths[i].join();
  return 0;
}

　　在上例中，执行read=true之后，所有线程结束空等。winner被初始化为ATOMIC_FLAG_INIT，最先执行winner.test_and_set并返回false的线程为winner。

#include <iostream>
#include <atomic>
#include <vector>
#include <thread>
#include <sstream>

std::atomic<int> foo(0);

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

void print_foo()
{
  while (foo == 0)
  {
    std::this_thread::yield();
  }
  std::cout << "x: " << foo << std::endl;
}
int main()
{
  std::thread print_th(print_foo);
  std::thread set_th(set_foo, 10);
  print_th.join();
  set_th.join();
  return 0;
}

　　在上例总，set_foo用于设置atomic<int>对象的值，print_foo用于打印atomic<int>对象的值。std::atomic对象的值的读取和写入可使用load和store实现。

#include <iostream>
#include <cassert>
#include <atomic>
#include <vector>
#include <unistd.h>
#include <thread>
#include <sstream>

std::atomic<int> foo(0);
std::atomic_flag lock = ATOMIC_FLAG_INIT;

void add_foo()
{
  while (1) 
  {
    foo++;
    // foo = foo + 1;
    while (lock.test_and_set());
    std::cout <<"add: " << foo << std::endl;
    lock.clear();
    usleep(1000);
  }
}

void sub_foo()
{
  while (1) 
  {
    foo--;
    // foo = foo - 1;
    while (lock.test_and_set());
    std::cout << "sub: " << foo << std::endl;
    lock.clear();
    usleep(1000);
  }
}
int main()
{
  std::thread th2 = std::thread(add_foo);
  std::thread th1 = std::thread(sub_foo);
  th1.join();
  th2.join();
  return 0;
}

　　atomic<int>支持++和--的原子操作。

来源：https://www.cnblogs.com/taiyang-li/p/5914331.html

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