C++多线程同步的几种方式

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文章目录

Overview

C++的多线程同步方式有这么几种:

  • mutex
    • lock_guard
    • unique_lock
  • condition_variable
  • future
    • promise
    • packaged_task
    • async

C++11并没有提供semaphore的API,信号量太容易出错了(too error prone),通过组合互斥锁(mutex)和条件变量(condition variable)可以达到相同的效果,且更加安全。

mutex

官网介绍:mutex
它包含以下三个部分:

  • Mutex type
  • Locks:lock_guardunique_lock
  • Functions:try_locklock

example:
mutex的实现也很简单,在进入临界区之前调用该变量(mtx)的lock函数,出临界区之前调用该变量的(mtx)的unlock函数。所以程序会连续输出50个'*'或者连续输出50个'$',而不会'*' '$'交替输出。

但是考虑这样一个问题,std::cout << '\\n',这里发生异常会发生什么?抛出异常后,意味着mtx.unlock()不会被执行,即锁没有被释放,整个程序进入不了临界区,该程序往往会挂死。

// mutex example
#include <iostream>       // std::cout
#include <thread>         // std::thread
#include <mutex>          // std::mutex

std::mutex mtx;           // mutex for critical section

void print_block (int n, char c) 
  // critical section (exclusive access to std::cout signaled by locking mtx):
  mtx.lock();
  for (int i=0; i<n; ++i)  std::cout << c; 
  std::cout << '\\n';
  mtx.unlock();


int main ()

  std::thread th1 (print_block,50,'*');
  std::thread th2 (print_block,50,'$');

  th1.join();
  th2.join();

  return 0;

Possible output (order of lines may vary, but characters are never mixed):

**************************************************
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$

lock_guard

在构造时,互斥对象被调用线程锁定,而在销毁时,互斥对象被解锁。它是最简单的锁,作为具有自动持续时间的对象特别有用,该持续时间一直持续到其上下文结束。这样,可以保证在抛出异常的情况下互斥对象已正确解锁。

example

// lock_guard example
#include <iostream>       // std::cout
#include <thread>         // std::thread
#include <mutex>          // std::mutex, std::lock_guard
#include <stdexcept>      // std::logic_error

std::mutex mtx;

void print_even (int x) 
  if (x%2==0) std::cout << x << " is even\\n";
  else throw (std::logic_error("not even"));


void print_thread_id (int id) 
  try 
    // using a local lock_guard to lock mtx guarantees unlocking on destruction / exception:
    std::lock_guard<std::mutex> lck (mtx);
    print_even(id);
  
  catch (std::logic_error&) 
    std::cout << "[exception caught]\\n";
  


int main ()

  std::thread threads[10];
  // spawn 10 threads:
  for (int i=0; i<10; ++i)
    threads[i] = std::thread(print_thread_id,i+1);

  for (auto& th : threads) th.join();

  return 0;

possible output

[exception caught]
2 is even
[exception caught]
4 is even
[exception caught]
6 is even
[exception caught]
8 is even
[exception caught]
10 is even

unique_lock

unique_lock基本用法和lock_guard一致,在构造函数和析构函数中进行锁操作,不同的地方在于它提供了非常多构造函数。

example

// unique_lock example
#include <iostream>       // std::cout
#include <thread>         // std::thread
#include <mutex>          // std::mutex, std::unique_lock

std::mutex mtx;           // mutex for critical section

void print_block (int n, char c) 
  // critical section (exclusive access to std::cout signaled by lifetime of lck):
  std::unique_lock<std::mutex> lck (mtx);
  for (int i=0; i<n; ++i)  std::cout << c; 
  std::cout << '\\n';


int main ()

  std::thread th1 (print_block,50,'*');
  std::thread th2 (print_block,50,'$');

  th1.join();
  th2.join();

  return 0;

condition_variable

条件变量是一个对象,可以阻塞线程,直到被通知恢复。当调用其等待功能之一时,它使用unique_lock(通过互斥锁)来锁定线程。该线程将保持阻塞状态,直到被另一个在同一个condition_variable对象上调用通知功能的线程唤醒为止。
Wait functions

  • wait
    • Wait until notified (public member function )
  • wait_for
    • Wait for timeout or until notified (public member function )
  • wait_until
    • Wait until notified or time point (public member function )

Notify functions

  • notify_one
    • Notify one (public member function )
  • notify_all
    • Notify all (public member function )

example

// condition_variable::notify_one
#include <iostream>           // std::cout
#include <thread>             // std::thread
#include <mutex>              // std::mutex, std::unique_lock
#include <condition_variable> // std::condition_variable

std::mutex mtx;
std::condition_variable produce,consume;

int cargo = 0;     // shared value by producers and consumers

void consumer () 
  std::unique_lock<std::mutex> lck(mtx);
  while (cargo==0) consume.wait(lck);
  std::cout << cargo << '\\n';
  cargo=0;
  produce.notify_one();


void producer (int id) 
  std::unique_lock<std::mutex> lck(mtx);
  while (cargo!=0) produce.wait(lck);
  cargo = id;
  consume.notify_one();


int main ()

  std::thread consumers[10],producers[10];
  // spawn 10 consumers and 10 producers:
  for (int i=0; i<10; ++i) 
    consumers[i] = std::thread(consumer);
    producers[i] = std::thread(producer,i+1);
  

  // join them back:
  for (int i=0; i<10; ++i) 
    producers[i].join();
    consumers[i].join();
  

  return 0;

Possible output (order of consumed cargoes may vary):

1
2
3
4
5
6
7
8
9
10

future

future的目标是充分利用CPU的并发性,它只能通过asyncpromisepackage_task三种方式构造。future只能移动,不可复制,需要复制时可以使用shared_future,但通常不建议使用。调用future的get()时可能会发生阻塞,直到返回值ready。future有三种姿势的等待:

  • wait():一直等待直到得到返回值
  • wait_for():设定一个超时时间;
  • wait_until():等待到某个时间点。

future有一特化版本future,返回值为空,即不返回任何值,因此仅能用于线程间通知,但却是最常用的future。

promise

promise对象可以通过调用成员get_future将此共享状态与future对象关联。调用之后,两个对象共享相同的共享状态:

  • promise:promise对象是异步提供者,在共享状态的时候设置一个值
  • future负责:future对象是异步返回对象,可以检索共享状态的值,并在必要时等待其准备就绪

example

// promise example
#include <iostream>       // std::cout
#include <functional>     // std::ref
#include <thread>         // std::thread
#include <future>         // std::promise, std::future

void print_int (std::future<int>& fut) 
  int x = fut.get();
  std::cout << "value: " << x << '\\n';


int main ()

  std::promise<int> prom;                      // create promise

  std::future<int> fut = prom.get_future();    // engagement with future

  std::thread th1 (print_int, std::ref(fut));  // send future to new thread

  prom.set_value (10);                         // fulfill promise
                                               // (synchronizes with getting the future)
  th1.join();
  return 0;

Output:

value: 10

packaged_task

很多情况下并不希望另起一个线程,因为线程是非常重要的资源。因此希望可以合理的管理线程资源,这就需要使用线程池。如何将future与线程池同时使用呢?这就需要采用package_task。package_task本质是将一个函数包装成一个future。这个task类似于std::function,有输入输出,大家可以将其认为是一个异步函数,但该异步函数并不负责执行,而是将其结果预置于一个future变量中,然后交给一个线程来实际执行,此时主线程便可以得到其返回值。

example

// packaged_task example
#include <iostream>     // std::cout
#include <future>       // std::packaged_task, std::future
#include <chrono>       // std::chrono::seconds
#include <thread>       // std::thread, std::this_thread::sleep_for

// count down taking a second for each value:
int countdown (int from, int to) 
  for (int i=from; i!=to; --i) 
    std::cout << i << '\\n';
    std::this_thread::sleep_for(std::chrono::seconds(1));
  
  std::cout << "Lift off!\\n";
  return from-to;


int main ()

  std::packaged_task<int(int,int)> tsk (countdown);   // set up packaged_task
  std::future<int> ret = tsk.get_future();            // get future

  std::thread th (std::move(tsk),10,0);   // spawn thread to count down from 10 to 0

  // ...

  int value = ret.get();                  // wait for the task to finish and get result

  std::cout << "The countdown lasted for " << value << " seconds.\\n";

  th.join();

  return 0;

Possible output:

10
9
8
7
6
5
4
3
2
1
Lift off!
The countdown lasted for 10 seconds.

async

有时某项工作很早就可以开始做(前置条件都已完备),而等待这件工作结果的任务在非常靠后的位置,这时候就需要async。换言之,如果可以尽早开始做一件事,就让其在后台运行即可,或快或慢都可以,只需在需要结果的时候运行完成就好。

example

// async example
#include <iostream>       // std::cout
#include <future>         // std::async, std::future

// a non-optimized way of checking for prime numbers:
bool is_prime (int x) 
  std::cout << "Calculating. Please, wait...\\n";
  for (int i=2; i<x; ++i) if (x%i==0) return false;
  return true;


int main ()

  // call is_prime(313222313) asynchronously:
  std::future<bool> fut = std::async (is_prime,313222313);

  std::cout << "Checking whether 313222313 is prime.\\n";
  // ...

  bool ret = fut.get();      // waits for is_prime to return

  if (ret) std::cout << "It is prime!\\n";
  else std::cout << "It is not prime.\\n";

  return 0;

Possible output (the first two lines may be in a different order, or scrambled):

Checking whether 313222313 is prime.
Calculating. Please, wait...
It is prime!

Reference

10分钟,带你掌握C++多线程同步!
Multi-threading

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