源码实现

其实现分布在三个文件中:port/port_posix.cc、port/port_posix.h、util/mutexlock.h
其中的实现也非常简单，只是对pthread进行了双层封装，这种封装很大程度上简化了操作。

class Mutex {
 public:
  Mutex();
  ~Mutex();

  void Lock();
  void Unlock();
  void AssertHeld() { }

 private:
  friend class CondVar;
  pthread_mutex_t mu_;

  // No copying
  Mutex(const Mutex&);
  void operator=(const Mutex&);
};

class CondVar {
 public:
  explicit CondVar(Mutex* mu);
  ~CondVar();
  void Wait();
  void Signal();
  void SignalAll();
 private:
  pthread_cond_t cv_;
  Mutex* mu_;
};
static void PthreadCall(const char* label, int result) {
  if (result != 0) {
    fprintf(stderr, "pthread %s: %s\n", label, strerror(result));
    abort();
  }
}

Mutex::Mutex() { PthreadCall("init mutex", pthread_mutex_init(&mu_, NULL)); }

Mutex::~Mutex() { PthreadCall("destroy mutex", pthread_mutex_destroy(&mu_)); }

void Mutex::Lock() { PthreadCall("lock", pthread_mutex_lock(&mu_)); }

void Mutex::Unlock() { PthreadCall("unlock", pthread_mutex_unlock(&mu_)); }

CondVar::CondVar(Mutex* mu)
    : mu_(mu) {
    PthreadCall("init cv", pthread_cond_init(&cv_, NULL));
}

CondVar::~CondVar() { PthreadCall("destroy cv", pthread_cond_destroy(&cv_)); }

void CondVar::Wait() {
  PthreadCall("wait", pthread_cond_wait(&cv_, &mu_->mu_));
}

void CondVar::Signal() {
  PthreadCall("signal", pthread_cond_signal(&cv_));
}

void CondVar::SignalAll() {
  PthreadCall("broadcast", pthread_cond_broadcast(&cv_));
}

// Helper class that locks a mutex on construction and unlocks the mutex when
// the destructor of the MutexLock object is invoked.
//
// Typical usage:
//
//   void MyClass::MyMethod() {
//     MutexLock l(&mu_);       // mu_ is an instance variable
//     ... some complex code, possibly with multiple return paths ...
//   }

class SCOPED_LOCKABLE MutexLock {
 public:
  explicit MutexLock(port::Mutex *mu) EXCLUSIVE_LOCK_FUNCTION(mu)
      : mu_(mu)  {
    this->mu_->Lock();
  }
  ~MutexLock() UNLOCK_FUNCTION() { this->mu_->Unlock(); }

 private:
  port::Mutex *const mu_;
  // No copying allowed
  MutexLock(const MutexLock&);
  void operator=(const MutexLock&);
};

如果对pthread有不熟悉的函数可以参考
https://linux.die.net/man/3/pthread_cond_init
http://www.jianshu.com/p/5024482ae821