学步园

CE_Mutex包装为同步多线程控制提供了一种相对优雅的方法，它们仍是潜在地易错的，因为开发者有可能会忘记调用release方法（或是由于程序员的疏忽，或是由于C++异常的发生）。为改善应用的健壮性，ACE同步机制有效地利用了C++类构造器和析构器语义。为确保ACE_Mutex锁被自动获取和释放，ACE提供了名为ACE_Guard的助手类，定义如下：

ACE_Guard类的对象定义一个代码块，在块开始时获取ACE_Mutex，而在块结束时自动释放。
注意ACE_Guard类被定义为模板，由互斥机制参数化。有若干不同类型的互斥语义[33]。每种互斥共有一个通用接口（也就是，acquire/release），但具有不同的序列化和性能属性。ACE支持的两种互斥是非递归和递归锁。

// -*- C++ -*-

//==========================================================================
/**
 *  @file    Null_Mutex.h
 *
 *  $Id: Null_Mutex.h 91626 2010-09-07 10:59:20Z johnnyw $
 *
 *   Moved from Synch.h.
 *
 *  @author Douglas C. Schmidt <schmidt@cs.wustl.edu>
 */
//==========================================================================

#ifndef ACE_NULL_MUTEX_H
#define ACE_NULL_MUTEX_H
#include /**/ "ace/pre.h"

#include "ace/os_include/os_errno.h"

#if !defined (ACE_LACKS_PRAGMA_ONCE)
# pragma once
#endif /* ACE_LACKS_PRAGMA_ONCE */

#include "ace/Global_Macros.h"
#include "ace/OS_Memory.h"


ACE_BEGIN_VERSIONED_NAMESPACE_DECL

class ACE_Time_Value;

/**
 * @class ACE_Null_Mutex
 *
 * @brief Implement a do nothing ACE_Mutex, i.e., all the methods are
 * no ops.
 */
class ACE_Export ACE_Null_Mutex
{
public:
  ACE_Null_Mutex (const ACE_TCHAR * = 0)
    : lock_ (0) {}
  ~ACE_Null_Mutex (void) {}
  /// Return 0.
  int remove (void) {return 0;}

  /// Return 0.
  int acquire (void) {return 0;}

  /// Return -1 with @c errno == @c ETIME.
  int acquire (ACE_Time_Value &) {errno = ETIME; return -1;}

  /// Return -1 with @c errno == @c ETIME.
  int acquire (ACE_Time_Value *) {errno = ETIME; return -1;}

  /// Return 0.
  int tryacquire (void) {return 0;}

  /// Return 0.
  int release (void) {return 0;}

  /// Return 0.
  int acquire_write (void) {return 0;}

  /// Return 0.
  int tryacquire_write (void) {return 0;}

  /// Return 0.
  int tryacquire_write_upgrade (void) {return 0;}

  /// Return 0.
  int acquire_read (void) {return 0;}

  /// Return 0.
  int tryacquire_read (void) {return 0;}

  /// Dump the state of an object.
  void dump (void) const {}

  /// Declare the dynamic allocation hooks.
  //ACE_ALLOC_HOOK_DECLARE;

  int lock_; // A dummy lock.
};

// FUZZ: disable check_for_ACE_Guard

template <class ACE_LOCK>
class ACE_Guard;

/**
 * @brief Template specialization of ACE_Guard for the
 * ACE_Null_Mutex.
 *
 * This specialization is useful since it helps to speedup
 * performance of the "Null_Mutex" considerably.
 */
template<>
class ACE_Export ACE_Guard<ACE_Null_Mutex>
{
public:
  // = Initialization and termination methods.
  ACE_Guard (ACE_Null_Mutex &) {}
  ACE_Guard (ACE_Null_Mutex &, int) {}
  ACE_Guard (ACE_Null_Mutex &, int, int) {}
#if defined (ACE_WIN32)
  ~ACE_Guard (void) {}
#endif /* ACE_WIN32 */

  int acquire (void) { return 0; }
  int tryacquire (void) { return 0; }
  int release (void) { return 0; }
  void disown (void) {}
  int locked (void) { return 1; }
  int remove (void) { return 0; }
  void dump (void) const {}

private:

  // Disallow copying and assignment.
  ACE_Guard (const ACE_Guard<ACE_Null_Mutex> &);
  void operator= (const ACE_Guard<ACE_Null_Mutex> &);

};

template <class ACE_LOCK>
class ACE_Write_Guard;

/**
 * @brief Template specialization of ACE_Write_Guard for the
 * ACE_Null_Mutex.
 */
template<>
class ACE_Export ACE_Write_Guard<ACE_Null_Mutex>
  : public ACE_Guard<ACE_Null_Mutex>
{
public:
  ACE_Write_Guard (ACE_Null_Mutex &m)
    : ACE_Guard<ACE_Null_Mutex> (m) {}
  ACE_Write_Guard (ACE_Null_Mutex &m, int blocked)
    : ACE_Guard<ACE_Null_Mutex> (m, blocked) {}

  int acquire_write (void) { return 0; }
  int acquire (void) { return 0; }
  int tryacquire_write (void) { return 0; }
  int tryacquire (void) { return 0; }
  void dump (void) const {}
};

template <class ACE_LOCK>
class ACE_Read_Guard;

/**
 * @brief Template specialization of ACE_Read)Guard for the
 * ACE_Null_Mutex.
 */
template<>
class ACE_Export ACE_Read_Guard<ACE_Null_Mutex>
  : public ACE_Guard<ACE_Null_Mutex>
{
public:
  ACE_Read_Guard (ACE_Null_Mutex &m)
    : ACE_Guard<ACE_Null_Mutex> (m) {}
  ACE_Read_Guard (ACE_Null_Mutex &m, int blocked)
    : ACE_Guard<ACE_Null_Mutex> (m, blocked) {}

  int acquire_read (void) { return 0; }
  int acquire (void) { return 0; }
  int tryacquire_read (void) { return 0; }
  int tryacquire (void) { return 0; }
  void dump (void) const {}
};

// FUZZ: enable check_for_ACE_Guard

template <class T> class ACE_Malloc_Lock_Adapter_T;

/**
 * @brief Template specialization of ACE_Malloc_Lock_Adapter_T for the
 * ACE_Null_Mutex.
 */
template<>
class ACE_Export ACE_Malloc_Lock_Adapter_T<ACE_Null_Mutex>
{
public:
  ACE_Null_Mutex * operator () (const ACE_TCHAR *name)
  {
    ACE_Null_Mutex *p;
    ACE_NEW_RETURN (p, ACE_Null_Mutex (name), 0);
    return p;
  }
};

ACE_END_VERSIONED_NAMESPACE_DECL

#include /**/ "ace/post.h"
#endif /* ACE_NULL_MUTEX_H */

与ACE Lock类相比

ACE_Guard的优点：获取了一个锁，他总会释放它，不会因为没有release，而出现线程死锁。

ACE_Guard的缺点：ACE_Guard变量无效时，他才自动调用release。

ACE_Guard类似局部变量，变量无效时，他才会自动调用release。这就出现了一个问题。当在ACE_Guard变量有效时程序出现阻塞，ACE_Guard变量就没有机会自动调用release了，这时死锁就出现了。