手动释放boost locks？

Question

为了学习boost::thread的组合学，我为锁定公共互斥锁(M)的线程实现了一个简单的屏障(BR)。但是，就我在转到BR.wait()时得到的结果而言，互斥锁不会被释放，因此为了让所有线程都到达BR，需要手动释放M上的锁。所以我有以下代码：

boost::barrier BR(3);
boost::mutex M;

void THfoo(int m){
    cout<<"TH"<<m<<" started and attempts locking M\n";
    boost::lock_guard<boost::mutex> ownlock(M);

    cout<<"TH"<<m<<" locked mutex\n";
    Wait_(15); //simple wait for few milliseconds

    M.unlock(); //probably bad idea
    //boost::lock_guard<boost::mutex> ~ownlock(M);
    // this TH needs to unlock the mutex before going to barrier BR

    cout<<"TH"<<m<<" unlocked mutex\n";
    cout<<"TH"<<m<<" going to BR\n";
    BR.wait();
    cout<<"TH"<<m<<" let loose from BR\n";
}

int main()  
{  
    boost::thread TH1(THfoo,1);
    boost::thread TH2(THfoo,2);
    boost::thread TH3(THfoo,3);

    TH2.join(); //but TH2 might end before TH1, and so destroy BR and M
    cout<<"exiting main TH \n";

    return 0;  
}

而M.unlock()显然是一个糟糕的解决方案(不使用锁)；那么如何(简单地)释放锁呢？另外:如何(正确地)在main()中等待所有线程完成？(TH2.join()不好，`因为TH2可能会先完成...)；

请不要建议变通，例如使用条件变量，我也可以使用它们，但它必须可以在没有条件变量的情况下直接实现。

Answer 1

类似于：

void THfoo(int m){
  // use a scope here, this means that the lock_guard will be destroyed (and therefore mutex unlocked on exiting this scope
  {
    cout<<"TH"<<m<<" started and attempts locking M\n";
    boost::lock_guard<boost::mutex> ownlock(M);

    cout<<"TH"<<m<<" locked mutex\n";
    Wait_(15); //simple wait for few milliseconds

  }
  // This is outside of the lock
  cout<<"TH"<<m<<" unlocked mutex\n";
  cout<<"TH"<<m<<" going to BR\n";
  BR.wait();
  cout<<"TH"<<m<<" let loose from BR\n";
}

至于等待，只需在所有线程句柄上调用join (如果它们已经完成，函数将立即返回)

TH1.join();
TH2.join();
TH3.join();

Answer 2

除了在块中限定boost::lock_guard的作用域之外，您还可以使用可以显式unlock()的boost::unique_lock：

boost::unique_lock<boost::mutex> ownlock(M);

cout<<"TH"<<m<<" locked mutex\n";
Wait_(15); //simple wait for few milliseconds

ownlock.unlock();

如果您需要在稍后重新获取互斥之前释放它，这是很有用的。

至于连接，只需对所有线程句柄依次调用join()即可。

Answer 3

让它超出范围：

void THfoo(int m){
    cout<<"TH"<<m<<" started and attempts locking M\n";
    {
       boost::lock_guard<boost::mutex> ownlock(M);

       cout<<"TH"<<m<<" locked mutex\n";
       Wait_(15); //simple wait for few milliseconds
    }

    // this TH needs to unlock the mutex before going to barrier BR

    cout<<"TH"<<m<<" unlocked mutex\n";
    cout<<"TH"<<m<<" going to BR\n";
    BR.wait();
    cout<<"TH"<<m<<" let loose from BR\n";
}