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 = 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_,‘\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