C++ std::thread

c++ 11 之后有了标准的线程库：std::thread。

之前一些编译器使用 C++11 的编译参数是 -std=c++11

g++ -std=c++11 test.cpp 

std::thread 构造函数

• 默认构造函数，创建一个空的 std::thread 执行对象。
• 初始化构造函数，创建一个 std::thread 对象，该 std::thread 对象可被 joinable，新产生的线程会调用 fn 函数，该函数的参数由 args 给出。
• 拷贝构造函数(被禁用)，意味着 std::thread 对象不可拷贝构造。
• Move 构造函数，move 构造函数(move 语义是 C++11 新出现的概念，详见附录)，调用成功之后 x 不代表任何 std::thread 执行对象。

注意：可被 joinable 的 std::thread 对象必须在他们销毁之前被主线程 join 或者将其设置为 detached.

std::thread 各种构造函数例子如下：

#include <iostream>#include <utility>#include <thread>#include <chrono>#include <functional>#include <atomic>void f1(int n){    for (int i = 0; i < 5; ++i) {        std::cout << "Thread " << n << " executing\n";        std::this_thread::sleep_for(std::chrono::milliseconds(10));    }}void f2(int& n){    for (int i = 0; i < 5; ++i) {        std::cout << "Thread 2 executing\n";        ++n;        std::this_thread::sleep_for(std::chrono::milliseconds(10));    }}int main(){    int n = 0;    std::thread t1; // t1 is not a thread    std::thread t2(f1, n + 1); // pass by value    std::thread t3(f2, std::ref(n)); // pass by reference    std::thread t4(std::move(t3)); // t4 is now running f2(). t3 is no longer a thread    t2.join();    t4.join();    std::cout << "Final value of n is " << n << '\n';}

std::thread 赋值操作

• Move 赋值操作(1)，如果当前对象不可 joinable，需要传递一个右值引用(rhs)给 move 赋值操作；如果当前对象可被 joinable，则会调用 terminate() 报错。
• 拷贝赋值操作(2)，被禁用，因此 std::thread 对象不可拷贝赋值。

请看下面的例子：

#include <stdio.h>#include <stdlib.h>#include <chrono>    // std::chrono::seconds#include <iostream>  // std::cout#include <thread>    // std::thread, std::this_thread::sleep_forvoid thread_task(int n) {    std::this_thread::sleep_for(std::chrono::seconds(n));    std::cout << "hello thread "        << std::this_thread::get_id()        << " paused " << n << " seconds" << std::endl;}int main(int argc, const char *argv[]){    std::thread threads[5];    std::cout << "Spawning 5 threads...\n";    for (int i = 0; i < 5; i++) {        threads[i] = std::thread(thread_task, i + 1);    }    std::cout << "Done spawning threads! Now wait for them to join\n";    for (auto& t: threads) {        t.join();    }    std::cout << "All threads joined.\n";    return EXIT_SUCCESS;}

其他成员函数

get_id: 获取线程 ID，返回一个类型为 std::thread::id 的对象。请看下面例子：

#include <iostream>#include <thread>#include <chrono>void foo(){  std::this_thread::sleep_for(std::chrono::seconds(1));}int main(){  std::thread t1(foo);  std::thread::id t1_id = t1.get_id();  std::thread t2(foo);  std::thread::id t2_id = t2.get_id();  std::cout << "t1's id: " << t1_id << '\n';  std::cout << "t2's id: " << t2_id << '\n';  t1.join();  t2.join();}

joinable: 检查线程是否可被 join。检查当前的线程对象是否表示了一个活动的执行线程，由默认构造函数创建的线程是不能被 join 的。另外，如果某个线程 已经执行完任务，但是没有被 join 的话，该线程依然会被认为是一个活动的执行线程，因此也是可以被 join 的。

#include <iostream>#include <thread>#include <chrono>void foo(){  std::this_thread::sleep_for(std::chrono::seconds(1));}int main(){  std::thread t;  std::cout << "before starting, joinable: " << t.joinable() << '\n';  t = std::thread(foo);  std::cout << "after starting, joinable: " << t.joinable() << '\n';  t.join();}join: Join 线程，调用该函数会阻塞当前线程，直到由 *this 所标示的线程执行完毕 join 才返回。#include <iostream>#include <thread>#include <chrono>void foo(){  // simulate expensive operation  std::this_thread::sleep_for(std::chrono::seconds(1));}void bar(){  // simulate expensive operation  std::this_thread::sleep_for(std::chrono::seconds(1));}int main(){  std::cout << "starting first helper...\n";  std::thread helper1(foo);  std::cout << "starting second helper...\n";  std::thread helper2(bar);  std::cout << "waiting for helpers to finish..." << std::endl;  helper1.join();  helper2.join();  std::cout << "done!\n";}

detach: Detach 线程。 将当前线程对象所代表的执行实例与该线程对象分离，使得线程的执行可以单独进行。一旦线程执行完毕，它所分配的资源将会被释放。

调用 detach 函数之后：

• *this 不再代表任何的线程执行实例。
• joinable() == false
• get_id() == std::thread::id()

另外，如果出错或者 joinable() == false，则会抛出 std::system_error。

#include <iostream>#include <chrono>#include <thread> void independentThread() {    std::cout << "Starting concurrent thread.\n";    std::this_thread::sleep_for(std::chrono::seconds(2));    std::cout << "Exiting concurrent thread.\n";} void threadCaller() {    std::cout << "Starting thread caller.\n";    std::thread t(independentThread);    t.detach();    std::this_thread::sleep_for(std::chrono::seconds(1));    std::cout << "Exiting thread caller.\n";} int main() {    threadCaller();    std::this_thread::sleep_for(std::chrono::seconds(5));}

swap: Swap 线程，交换两个线程对象所代表的底层句柄(underlying handles)。

#include <iostream>#include <thread>#include <chrono>void foo(){  std::this_thread::sleep_for(std::chrono::seconds(1));}void bar(){  std::this_thread::sleep_for(std::chrono::seconds(1));}int main(){  std::thread t1(foo);  std::thread t2(bar);  std::cout << "thread 1 id: " << t1.get_id() << std::endl;  std::cout << "thread 2 id: " << t2.get_id() << std::endl;  std::swap(t1, t2);  std::cout << "after std::swap(t1, t2):" << std::endl;  std::cout << "thread 1 id: " << t1.get_id() << std::endl;  std::cout << "thread 2 id: " << t2.get_id() << std::endl;  t1.swap(t2);  std::cout << "after t1.swap(t2):" << std::endl;  std::cout << "thread 1 id: " << t1.get_id() << std::endl;  std::cout << "thread 2 id: " << t2.get_id() << std::endl;  t1.join();  t2.join();}

执行结果如下：

thread 1 id: 1892thread 2 id: 2584after std::swap(t1, t2):thread 1 id: 2584thread 2 id: 1892after t1.swap(t2):thread 1 id: 1892thread 2 id: 2584

native_handle: 返回 native handle（由于 std::thread 的实现和操作系统相关，因此该函数返回与 std::thread 具体实现相关的线程句柄，例如在符合 Posix 标准的平台下(如 Unix/Linux)是 Pthread 库）。

#include <thread>#include <iostream>#include <chrono>#include <cstring>#include <pthread.h>std::mutex iomutex;void f(int num){  std::this_thread::sleep_for(std::chrono::seconds(1)); sched_param sch; int policy;  pthread_getschedparam(pthread_self(), &policy, &sch); std::lock_guard<std::mutex> lk(iomutex); std::cout << "Thread " << num << " is executing at priority "           << sch.sched_priority << '\n';}int main(){  std::thread t1(f, 1), t2(f, 2);  sched_param sch;  int policy;   pthread_getschedparam(t1.native_handle(), &policy, &sch);  sch.sched_priority = 20;  if(pthread_setschedparam(t1.native_handle(), SCHED_FIFO, &sch)) {      std::cout << "Failed to setschedparam: " << std::strerror(errno) << '\n';  }  t1.join();  t2.join();}

执行结果如下：

Thread 2 is executing at priority 0Thread 1 is executing at priority 20

hardware_concurrency [static]: 检测硬件并发特性，返回当前平台的线程实现所支持的线程并发数目，但返回值仅仅只作为系统提示(hint)。

#include <iostream>#include <thread>int main() {  unsigned int n = std::thread::hardware_concurrency();  std::cout << n << " concurrent threads are supported.\n";}

std::this_thread 命名空间中相关辅助函数介绍

get_id: 获取线程 ID。

#include <iostream>#include <thread>#include <chrono>#include <mutex>std::mutex g_display_mutex;void foo(){  std::thread::id this_id = std::this_thread::get_id();  g_display_mutex.lock();  std::cout << "thread " << this_id << " sleeping...\n";  g_display_mutex.unlock();  std::this_thread::sleep_for(std::chrono::seconds(1));}int main(){  std::thread t1(foo);  std::thread t2(foo);  t1.join();  t2.join();}

yield: 当前线程放弃执行，操作系统调度另一线程继续执行。

#include <iostream>#include <chrono>#include <thread>// "busy sleep" while suggesting that other threads run // for a small amount of timevoid little_sleep(std::chrono::microseconds us){  auto start = std::chrono::high_resolution_clock::now();  auto end = start + us;  do {      std::this_thread::yield();  } while (std::chrono::high_resolution_clock::now() < end);}int main(){  auto start = std::chrono::high_resolution_clock::now();  little_sleep(std::chrono::microseconds(100));  auto elapsed = std::chrono::high_resolution_clock::now() - start;  std::cout << "waited for "            << std::chrono::duration_cast<std::chrono::microseconds>(elapsed).count()            << " microseconds\n";}

sleep_until: 线程休眠至某个指定的时刻(time point)，该线程才被重新唤醒。

template< class Clock, class Duration >void sleep_until( const std::chrono::time_point<Clock,Duration>& sleep_time );

sleep_for: 线程休眠某个指定的时间片(time span)，该线程才被重新唤醒，不过由于线程调度等原因，实际休眠时间可能比 sleep_duration 所表示的时间片更长。

#include <iostream>#include <chrono>#include <thread>int main(){  std::cout << "Hello waiter" << std::endl;  std::chrono::milliseconds dura( 2000 );  std::this_thread::sleep_for( dura );  std::cout << "Waited 2000 ms\n";}

执行结果如下：

Hello waiterWaited 2000 ms

来源：https://github.com/forhappy/Cplusplus-Concurrency-In-Practice/blob/master/zh/chapter3-Thread/Introduction-to-Thread.md

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