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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：
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 = http://www.mamicode.com/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 =http://www.mamicode.com/= 0) { ACE_ALLOCATOR (this->base_,(char *) this->allocator_strategy_->malloc (size));#if defined (ACE_INITIALIZE_MEMORY_BEFORE_USE) (void) ACE_OS::memset (this->base_,‘\0‘,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;}
C++ Network Programming. Volume 1: Mastering Complexity with ACE and Patterns
ACE_Message_Block消息数据类