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;}

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。

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