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