ACE_Message_Block消息数据类
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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_,\'\\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; }
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
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