ACE_Message_Block消息数据类

　　ACE_Message_Block

　　ACE_Message_Block用于构建“固定”和“可变”长度的消息。ACE_Message_Block可以将多条消息连接在一起，形成一个链表，从而支持复合消息。ACE_Message_Block内部结构图如下：

　　头文件“Message_Block.h”。

1：ACE_Message_Block初始化与释放

　　初始化一般用以下操作实现：

ACE_NEW_NORETURN (m_pRcvmb,ACE_Message_Block (1024));

ACE_Message_Block* p = new ACE_Message_Block(1024);

 　　这两种方式都可以实现ACE_Message_Block的new操作。ACE定义了一组申请内存的宏，内部都实现了new操作符。这组宏定义在如下：

#  if defined (ACE_HAS_NEW_NOTHROW)
#    define ACE_NEW_RETURN(POINTER,CONSTRUCTOR,RET_VAL) 
   do { POINTER = new (ACE_nothrow) CONSTRUCTOR; 
     if (POINTER == 0) { errno = ENOMEM; return RET_VAL; } 
   } while (0)
#    define ACE_NEW(POINTER,CONSTRUCTOR) 
   do { POINTER = new(ACE_nothrow) CONSTRUCTOR; 
     if (POINTER == 0) { errno = ENOMEM; return; } 
   } while (0)
#    define ACE_NEW_NORETURN(POINTER,CONSTRUCTOR) 
   do { POINTER = new(ACE_nothrow) CONSTRUCTOR; 
     if (POINTER == 0) { errno = ENOMEM; } 
   } while (0)

#  else

#    define ACE_NEW_RETURN(POINTER,CONSTRUCTOR,RET_VAL) 
   do { try { POINTER = new CONSTRUCTOR; } 
     catch (ACE_bad_alloc) { ACE_del_bad_alloc errno = ENOMEM; POINTER = 0; return RET_VAL; } 
   } while (0)

#    define ACE_NEW(POINTER,CONSTRUCTOR) 
   do { try { POINTER = new CONSTRUCTOR; } 
     catch (ACE_bad_alloc) { ACE_del_bad_alloc errno = ENOMEM; POINTER = 0; return; } 
   } while (0)

#    define ACE_NEW_NORETURN(POINTER,CONSTRUCTOR) 
   do { try { POINTER = new CONSTRUCTOR; } 
     catch (ACE_bad_alloc) { ACE_del_bad_alloc errno = ENOMEM; POINTER = 0; } 
   } while (0)
#  endif /* ACE_HAS_NEW_NOTHROW */

 　　值得注意的是，ACE_Message_Block有多个构造函数，最常用的一个构造函数定义为：

  ACE_Message_Block (size_t size,
                     ACE_Message_Type type = MB_DATA,
                     ACE_Message_Block *cont = 0,
                     const char *data = 0,
                     ACE_Allocator *allocator_strategy = 0,
                     ACE_Lock *locking_strategy = 0,
                     unsigned long priority = ACE_DEFAULT_MESSAGE_BLOCK_PRIORITY,
                     const ACE_Time_Value &execution_time = ACE_Time_Value::zero,
                     const ACE_Time_Value &deadline_time = ACE_Time_Value::max_time,
                     ACE_Allocator *data_block_allocator = 0,
                     ACE_Allocator *message_block_allocator = 0);

 　　在该构造函数内部，ACE_Message_Block调用了init_i函数，init_i内部调用了ACE_Data_Block的构造函数。ACE_Data_Block定义了一个char* base_ 指针，其构造函数会调用C风格的malloc方法为base_申请大小为size的空间。也就是说，ACE_Message_Block真正的数据载体是ACE_Data_Block。其实现代码为：

//ACE_Message_Block内部申请ACE_Data_Block的空间
ACE_NEW_MALLOC_RETURN (db,static_cast<ACE_Data_Block *> (data_block_allocator->malloc (sizeof (ACE_Data_Block))),
                             ACE_Data_Block (size,
                                             msg_type,
                                             msg_data,
                                             allocator_strategy,
                                             locking_strategy,
                                             flags,
                                             data_block_allocator),-1);

//ACE_Data_Block内部为base_申请大小为size的空间
if (msg_data == 0)
    {
      ACE_ALLOCATOR (this->base_,(char *) this->allocator_strategy_->malloc (size));
#if defined (ACE_INITIALIZE_MEMORY_BEFORE_USE)
      (void) ACE_OS::memset (this->base_,'',size);
#endif /* ACE_INITIALIZE_MEMORY_BEFORE_USE */
    }

 　　释放ACE_Message_Block，调用release方法即可。release方法会将消息的引用计数减1，如果消息的引用计数为0，则释放该消息。

2：ACE_Message_Block写入数据　　

 　　ACE_Message_Block内部有读地址和写地址，ACE_Message_Block的长度是写地址减去读地址的值。其定义为：

//读写地址
  /// Pointer to beginning of next read.
  size_t rd_ptr_;
  /// Pointer to beginning of next write.
  size_t wr_ptr_;

//长度
ACE_Message_Block::length (void) const
{
  ACE_TRACE ("ACE_Message_Block::length");
  return this->wr_ptr_ - this->rd_ptr_;
}

 　　rd_ptr()和wr_ptr()分别用于设置和获取读写地址的值。

　　将buffer中的数据复制到ACE_Message_Block中，需要调用copy函数。copy函数内部用memcpy实现，将buf的size个BYTE拷贝到以wr_ptr_地址为首的地址上，并将wr_ptr_的值加上size，其实现为：

int ACE_Message_Block::copy (const char *buf, size_t n)
{
  ACE_TRACE ("ACE_Message_Block::copy");
  /*size_t len = static_cast<size_t> (this->end () - this->wr_ptr ());*/
  // Note that for this to work correct, end () *must* be >= mark ().
  size_t len = this->space ();
  if (len < n)
    {
      errno = ENOSPC;
      return -1;
    }
  else
    {
      (void) ACE_OS::memcpy (this->wr_ptr (),buf,n);
      this->wr_ptr (n);
      return 0;
    }
}

3：ACE_Message_Block复制操作

　　ACE_Message_Block提供了clone和duplicate两个操作，clone是深复制，duplicate是浅复制，仅为消息的引用计数加1。

4：ACE_Message_Block消息链

　　ACE_Message_Block内部定义3个指针：

/// Pointer to next message block in the chain.
  ACE_Message_Block *cont_;
  /// Pointer to next message in the list.
  ACE_Message_Block *next_;
  /// Pointer to previous message in the list.
  ACE_Message_Block *prev_;

　　分别用重载函数cont()、next()、prev()来设置和获取邻居消息。其中，cont用于将复合消息连接在一起，next和prev用于连接消息链上的简单消息。

　　一个消息链的示例如下：

#include "ace/OS.h"
#include "ace/Message_Block.h"

int main (int argc, char *argv[])
{
  ACE_Message_Block *head = new ACE_Message_Block (BUFSIZ);
  ACE_Message_Block *mblk = head;

  for (;;) {
    ssize_t nbytes = ACE::read_n (ACE_STDIN,mblk->wr_ptr (),mblk->size () ) ;
    if (nbytes <=  0)
      break; // Break out at EOF or error.
    // Advance the write pointer to the end of the buffer.
    mblk->wr_ptr (nbytes);
    // Allocate message block and chain it at the end of list.
    mblk->cont (new ACE_Message_Block (BUFSIZ));
    mblk = mblk->cont ();
  }
  // Print the contents of the list to the standard output.
  for (mblk = head; mblk != 0; mblk = mblk->cont ())
    ACE::write_n (ACE_STDOUT, mblk->rd_ptr (), mblk->length ());
  head->release (); // This releases all the memory in the chain.
  return 0;
}

5：size()、length()、space()、capacity()

　　一张图说明ACE_Message_Block这几个函数的含义：

　　length= wr_ptr - rd_ptr;

　　space = mark - wr_ptr;

　　size = mark - base;

　　capacity = end - base;

　　其中，capacity和size的关系参考stl的capacity和size。int size (size_t length)可以动态申请空间。

 C++ Network Programming. Volume 1: Mastering Complexity with ACE and Patterns