c++11新特性之atomic

1. std::atomic_flag

　　std::atomic_flag是一個原子的布爾類型，可支持兩種原子操作：

• test_and_set, 如果atomic_flag對象被設置，則返回true; 如果atomic_flag對象未被設置，則設置之，返回false
• clear. 清楚atomic_flag對象

　　std::atomic_flag可用於多線程之間的同步操作，類似於linux中的信號量。使用atomic_flag可實現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對int, char, bool等數據結構進行原子性封裝，在多線程環境中，對std::atomic對象的訪問不會造成競爭-冒險。利用std::atomic可實現數據結構的無鎖設計。

#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，執行read=true之后，所有線程結束空等。winner被初始化為ATOMIC_FLAG_INIT，最先執行winner.test_and_set並返回false的線程為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;
}

　　在上例總，set_foo用於設置atomic<int>對象的值，print_foo用於打印atomic<int>對象的值。std::atomic對象的值的讀取和寫入可使用load和store實現。

　　

#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。