October
14th,
2019
thread_local
thread_local数据专属于线程，类似于静态数据，在第一次使用时创建，其生命周期将绑定到线程的生命周期。因为是专属于线程，可以在不加锁时修改。
#include <iostream>
#include <string>
#include <thread>
#include <mutex>
thread_local unsigned int rage = 1;
std::mutex cout_mutex;
void increase_rage(const std::string& thread_name)
{
++rage; // modifying outside a lock is okay; this is a thread-local variable
std::lock_guard<std::mutex> lock(cout_mutex);
std::cout << "Rage counter for " << thread_name << ": " << rage << '\n';
}
int main()
{
std::thread a(increase_rage, "a"), b(increase_rage, "b");
{
std::lock_guard<std::mutex> lock(cout_mutex);
std::cout << "Rage counter for main: " << rage << '\n';
}
a.join();
b.join();
}
condition_variable
std::condition_variable::notify_all()：唤醒所有线程。如果当前没有等待线程，则该函数什么也不做。std::condition_variable::wait()：当前线程获得锁lck之后，调用wait()会自动释放锁lck，使得其他被锁阻塞的线程继续执行。一旦获得通知notify，wait()会自动获得锁。
#include <iostream>
#include <thread>
#include <mutex>
#include <condition_variable>
std::mutex mtx; // 全局互斥锁.
std::condition_variable cv; // 全局条件变量.
bool ready = false; // 全局标志位.
void do_print_id(int id)
{
std::unique_lock <std::mutex> lck(mtx);
while (!ready) // 如果标志位不为 true, 则等待...
cv.wait(lck); // 当前线程被阻塞, 当全局标志位变为 true 之后,
// 线程被唤醒, 继续往下执行打印线程编号id.
std::cout << "thread " << id << '\n';
}
void go()
{
std::unique_lock <std::mutex> lck(mtx);
ready = true; // 设置全局标志位为 true.
cv.notify_all(); // 唤醒所有线程.
}
int main()
{
std::thread threads[10];
for (int i = 0; i < 10; ++i)
threads[i] = std::thread(do_print_id, i);
std::cout << "10 threads ready to race...\n";
go();
for (auto & th:threads)
th.join();
return 0;
}