关于c++:C并发与多线程-12recursivemutextimedmutexrecursivetimedmutex

window 临界区

• window 临界区资源对象与 C ++ 的 std::mutex 对象相似，能够爱护多个线程对临界区资源的拜访。

#include <iostream>
#include <thread>
#include <Windows.h>

static CRITICAL_SECTION g_winsec;

void print_block (int n, char c)
{EnterCriticalSection(&g_winsec);      // 2. 进入临界区

  for (int i=0; i<n; ++i) {std::cout << c;}

  std::cout << '\n';

  LeaveCriticalSection(&g_winsec);      // 3. 来到临界区
}

int main ()
{InitializeCriticalSection(&g_winsec); // 1. 初始化临界资源对象

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

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

  return 0;
}

输入：

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

屡次进入临界区试验

• window 临界资源对象能够在同一线程中多次重复进入，对应次数的来到，程序仍失常执行。
• std::mutex 对象只能在同一线程进行一次加锁并对应一次解锁，否则程序抛出异样。

测试 1：window 临界区

#include <iostream>
#include <thread>
#include <Windows.h>

static CRITICAL_SECTION g_winsec;

void print_block (int n, char c)
{EnterCriticalSection(&g_winsec);      // 2. 进入临界区
  EnterCriticalSection(&g_winsec);      // 屡次进入。。。EnterCriticalSection(&g_winsec);      // 屡次进入。。。for (int i=0; i<n; ++i) {std::cout << c;}

  std::cout << '\n';

  LeaveCriticalSection(&g_winsec);      // 3. 来到临界区
  LeaveCriticalSection(&g_winsec);      // 屡次来到。。。LeaveCriticalSection(&g_winsec);      // 屡次来到。。。}

int main ()
{InitializeCriticalSection(&g_winsec); // 1. 初始化临界资源对象

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

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

  return 0;
}

输入：[后果正确]

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

测试 2：std::mutex

// 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();
  mtx.lock();
  mtx.lock();
  for (int i=0; i<n; ++i) {std::cout << c;}
  std::cout << '\n';
  mtx.unlock();
  mtx.unlock();
  mtx.unlock();}

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

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

  return 0;
}

输入：

 程序异样退出 

主动析构技术

• RAII（Resource Acquisition Is Initialization）, 也称为“资源获取就是初始化”，是 C ++ 语言的一种治理资源、防止透露的习用法。
• C++ 规范保障任何状况下，已结构的对象最终会销毁，即它的析构函数最终会被调用。简略的说，RAII 的做法是应用一个对象，在其结构时获取资源，在对象生命期管制对资源的拜访使之始终保持无效，最初在对象析构的时候开释资源。

#include <iostream>
#include <thread>
#include <Windows.h>

static CRITICAL_SECTION g_winsec;

class CWinLock {
public:
    CWinLock(CRITICAL_SECTION *winsec) : m_winsec(winsec)
    {EnterCriticalSection(m_winsec); // 进入临界区
    }

    ~CWinLock()
    {LeaveCriticalSection(m_winsec);  // 来到临界区
    }

private:
    CRITICAL_SECTION *m_winsec = nullptr;
};

void print_block (int n, char c)
{CWinLock win_lock(&g_winsec);

  for (int i=0; i<n; ++i) {std::cout << c;}

  std::cout << '\n';
}

int main ()
{InitializeCriticalSection(&g_winsec); // 1. 初始化临界资源对象

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

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

  return 0;
}

输入：

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

std::recursive_mutex

• 就像互斥锁（mutex）一样，递归互斥锁（recursive_mutex）是可锁定的对象，但它容许同一线程取得对互斥锁对象的多级所有权（屡次 lock）。
• 这容许从曾经锁定它的线程锁定（或尝试锁定）互斥对象，从而取得对互斥对象的新所有权级别：互斥对象实际上将放弃对该线程的锁定，直到调用其成员 unlock 的次数与此所有权级别的次数雷同。

测试：仅演示阐明，应用 recursive_mutex 时需思考是否存在优化空间！

#include <iostream>
#include <thread>
#include <mutex> 

std::recursive_mutex mtx;           

void print_block (int n, char c) {mtx.lock();
  mtx.lock();
  mtx.lock();
  
  for (int i=0; i<n; ++i) {std::cout << c;}
  std::cout << '\n';
  
  mtx.unlock();
  mtx.unlock();
  mtx.unlock();}

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

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

  return 0;
}

输入：

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

std::timed_mutex、std::recursive_timed_mutex

std::timed_mutex

• 定时互斥锁是一个可工夫锁定的对象，旨在告诉何时要害代码须要独占拜访，就像惯例互斥锁一样，但还反对定时尝试锁定申请。

测试 1：try_lock_for

// timed_mutex::try_lock_for example
#include <iostream>       // std::cout
#include <chrono>         // std::chrono::milliseconds
#include <thread>         // std::thread
#include <mutex>          // std::timed_mutex

std::timed_mutex mtx;

void fireworks () {
  // waiting to get a lock: each thread prints "-" every 200ms:
  while (!mtx.try_lock_for(std::chrono::milliseconds(200))) {std::cout << "-";}
  // got a lock! - wait for 1s, then this thread prints "*"
  std::this_thread::sleep_for(std::chrono::milliseconds(1000));
  std::cout << "*\n";
  mtx.unlock();}

int main ()
{std::thread threads[10];
  // spawn 10 threads:
  for (int i=0; i<10; ++i)
    threads[i] = std::thread(fireworks);

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

  return 0;
}

输入：

------------------------------------*
----------------------------------------*
-----------------------------------*
------------------------*
-------------------------*
--------------------*
---------------*
--------*
-----*
*

测试 2：try_lock_until

// timed_mutex::try_lock_until example
#include <iostream>       // std::cout
#include <chrono>         // std::chrono::system_clock
#include <thread>         // std::thread
#include <mutex>          // std::timed_mutex
#include <ctime>          // std::time_t, std::tm, std::localtime, std::mktime

std::timed_mutex cinderella;

// gets time_point for next midnight:
std::chrono::time_point<std::chrono::system_clock> midnight() {
  using std::chrono::system_clock;
  std::time_t tt = system_clock::to_time_t (system_clock::now());
  struct std::tm * ptm = std::localtime(&tt);
  ++ptm->tm_mday; ptm->tm_hour=0; ptm->tm_min=0; ptm->tm_sec=0;
  return system_clock::from_time_t (mktime(ptm));
}

void carriage() {if (cinderella.try_lock_until(midnight())) {
    std::cout << "ride back home on carriage\n";
    cinderella.unlock();}
  else
    std::cout << "carriage reverts to pumpkin\n";
}

void ball() {cinderella.lock();
  std::cout << "at the ball...\n";
  cinderella.unlock();}

int main ()
{std::thread th1 (ball);
  std::thread th2 (carriage);

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

  return 0;
}

输入：

at the ball...
ride back home on carriage

std::recursive_timed_mutex

• 递归定时互斥锁将 recursive_timed 和 timed_mutex 的性能联合到一个类中：它既反对通过单个线程获取多个锁定级别又反对定时的 try_lock 申请。
• 成员函数与 timed_mutex 雷同。