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<title>12.3 串行通信与重叠I/O</title>
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</center></div><font FACE="Times New Roman" SIZE="3"><b><p ALIGN="CENTER"></b></font><font color="#3973DE" FACE="Times New Roman" size="4">12.3 </font><font color="#3973DE" size="4">串行通信与重叠</font><font color="#3973DE" FACE="Times New Roman" size="4">I/O</font><font FACE="Times New Roman" size="4"></p>
</font><font FACE="Times New Roman" SIZE="3"><p ALIGN="JUSTIFY">Win 32</font><font SIZE="3">系统为串行通信提供了全新的服务。传统的</font><font FACE="Times New Roman" SIZE="3">OpenComm</font><font SIZE="3">、</font><font FACE="Times New Roman" SIZE="3">ReadComm</font><font SIZE="3">、</font><font FACE="Times New Roman" SIZE="3">WriteComm</font><font SIZE="3">、</font><font FACE="Times New Roman" SIZE="3">CloseComm</font><font SIZE="3">等函数已经过时，</font><font FACE="Times New Roman" SIZE="3">WM_COMMNOTIFY</font><font SIZE="3">消息也消失了。取而代之的是文件</font><font FACE="Times New Roman" SIZE="3">I/O</font><font SIZE="3">函数提供的打开和关闭通信资源句柄及读写操作的基本接口。</font><font FACE="Times New Roman" SIZE="3"></p>
<p ALIGN="JUSTIFY"></font><font SIZE="3">新的文件</font><font FACE="Times New Roman" SIZE="3">I/O</font><font SIZE="3">函数</font><font FACE="Times New Roman" SIZE="3">(CreateFile</font><font SIZE="3">、</font><font FACE="Times New Roman" SIZE="3">ReadFile</font><font SIZE="3">、</font><font FACE="Times New Roman" SIZE="3">WriteFile</font><font SIZE="3">等</font><font FACE="Times New Roman" SIZE="3">)</font><font SIZE="3">支持重叠式输入输出，这使得线程可以从费时的</font><font FACE="Times New Roman" SIZE="3">I/O</font><font SIZE="3">操作中解放出来，从而极大地提高了程序的运行效率。</font><font FACE="Times New Roman" SIZE="3"></p>
<b><p ALIGN="JUSTIFY"></b></font><font color="#3973DE" FACE="Times New Roman" SIZE="3">12.3.1
</font><font SIZE="3" color="#3973DE">串行口的打开和关闭</font><font FACE="Times New Roman" SIZE="3"></p>
<p ALIGN="JUSTIFY">Win 32</font><font SIZE="3">系统把文件的概念进行了扩展。无论是文件、通信设备、命名管道、邮件槽、磁盘、还是控制台，都是用</font><font FACE="Times New Roman" SIZE="3">API</font><font SIZE="3">函数</font><font FACE="Times New Roman" SIZE="3">CreateFile</font><font SIZE="3">来打开或创建的。该函数的声明为：</font><font FACE="Times New Roman" SIZE="3"></p>
<p ALIGN="JUSTIFY">HANDLE CreateFile(</p>
<p ALIGN="JUSTIFY">LPCTSTR lpFileName, // </font><font SIZE="3">文件名 </font><font FACE="Times New Roman" SIZE="3"></p>
<p ALIGN="JUSTIFY">DWORD dwDesiredAccess, // </font><font SIZE="3">访问模式 </font><font FACE="Times New Roman" SIZE="3"></p>
<p ALIGN="JUSTIFY">DWORD dwShareMode, // </font><font SIZE="3">共享模式 </font><font FACE="Times New Roman" SIZE="3"></p>
<p ALIGN="JUSTIFY">LPSECURITY_ATTRIBUTES lpSecurityAttributes, // </font><font SIZE="3">通常为</font><font FACE="Times New Roman" SIZE="3">NULL</p>
<p ALIGN="JUSTIFY">DWORD dwCreationDistribution, // </font><font SIZE="3">创建方式 </font><font FACE="Times New Roman" SIZE="3"></p>
<p ALIGN="JUSTIFY">DWORD dwFlagsAndAttributes, // </font><font SIZE="3">文件属性和标志</font><font FACE="Times New Roman" SIZE="3"></p>
<p ALIGN="JUSTIFY">HANDLE hTemplateFile // </font><font SIZE="3">临时文件的句柄，通常为</font><font FACE="Times New Roman" SIZE="3">NULL </p>
<blockquote>
<p ALIGN="JUSTIFY">);</p>
</blockquote>
<p ALIGN="JUSTIFY"></font><font SIZE="3">如果调用成功，那么该函数返回文件的句柄，如果调用失败，则函数返回</font><font FACE="Times New Roman" SIZE="3">INVALID_HANDLE_VALUE</font><font SIZE="3">。</font><font FACE="Times New Roman" SIZE="3"></p>
<p ALIGN="JUSTIFY"></font><font SIZE="3">如果想要用重叠</font><font FACE="Times New Roman" SIZE="3">I/O</font><font SIZE="3">方式（参见</font><font FACE="Times New Roman" SIZE="3">12.3.3</font><font SIZE="3">）打开</font><font FACE="Times New Roman" SIZE="3">COM2</font><font SIZE="3">口，则一般应象清单</font><font FACE="Times New Roman" SIZE="3">12.4</font><font SIZE="3">那样调用</font><font FACE="Times New Roman" SIZE="3">CreateFile</font><font SIZE="3">函数。注意在打开一个通信端口时，应该以独占方式打开，另外要指定</font><font FACE="Times New Roman" SIZE="3">GENERIC_READ</font><font SIZE="3">、</font><font FACE="Times New Roman" SIZE="3">GENERIC_WRITE</font><font SIZE="3">、</font><font FACE="Times New Roman" SIZE="3">OPEN_EXISTING</font><font SIZE="3">和</font><font FACE="Times New Roman" SIZE="3">FILE_ATTRIBUTE_NORMAL</font><font SIZE="3">等属性。如果要打开重叠</font><font FACE="Times New Roman" SIZE="3">I/O</font><font SIZE="3">，则应该指定 </font><font FACE="Times New Roman" SIZE="3">FILE_FLAG_OVERLAPPED</font><font SIZE="3">属性。</font><font FACE="Times New Roman" SIZE="3"></p>
<p ALIGN="JUSTIFY"></font><b><font SIZE="3"> </p>
<p ALIGN="JUSTIFY">清单</font><font FACE="Times New Roman" SIZE="3">12.4</b></p>
<p ALIGN="JUSTIFY">HANDLE hCom;</p>
<p ALIGN="JUSTIFY">DWORD dwError;</p>
<p ALIGN="JUSTIFY">hCom=CreateFile(</font><font SIZE="3">“</font><font FACE="Times New Roman" SIZE="3">COM2</font><font SIZE="3">”</font><font FACE="Times New Roman" SIZE="3">, // </font><font SIZE="3">文件名</font><font FACE="Times New Roman" SIZE="3"></p>
<p ALIGN="JUSTIFY">GENERIC_READ | GENERIC_WRITE, // </font><font SIZE="3">允许读和写 </font><font FACE="Times New Roman" SIZE="3"></p>
<p ALIGN="JUSTIFY">0, // </font><font SIZE="3">独占方式</font><font FACE="Times New Roman" SIZE="3"></p>
<p ALIGN="JUSTIFY">NULL, </p>
<p ALIGN="JUSTIFY">OPEN_EXISTING, //</font><font SIZE="3">打开而不是创建</font><font FACE="Times New Roman" SIZE="3"></p>
<p ALIGN="JUSTIFY">FILE_ATTRIBUTE_NORMAL | FILE_FLAG_OVERLAPPED, // </font><font SIZE="3">重叠方式</font><font FACE="Times New Roman" SIZE="3"></p>
<p ALIGN="JUSTIFY">NULL</p>
<p ALIGN="JUSTIFY">);</p>
<p ALIGN="JUSTIFY">if(hCom = = INVALID_HANDLE_VALUE)</p>
<p ALIGN="JUSTIFY">{</p>
<p ALIGN="JUSTIFY">dwError=GetLastError( );</p>
<p ALIGN="JUSTIFY"><b>. . .</b> // </font><font SIZE="3">处理错误</font><font FACE="Times New Roman" SIZE="3"></p>
<p ALIGN="JUSTIFY">}</p>
<p ALIGN="JUSTIFY"></font><font SIZE="3">当不再使用文件句柄时，应该调用</font><font FACE="Times New Roman" SIZE="3">CloseHandle</font><font SIZE="3">函数关闭之。</font><font FACE="Times New Roman" SIZE="3"></p>
<b><p ALIGN="JUSTIFY"></b></font><font color="#3973DE" FACE="Times New Roman" SIZE="3">12.3.2
</font><font SIZE="3" color="#3973DE">串行口的初始化</font><font FACE="Times New Roman" SIZE="3"></p>
<p ALIGN="JUSTIFY"></font><font SIZE="3">在打开通信设备句柄后，常常需要对串行口进行一些初始化工作。这需要通过一个</font><font FACE="Times New Roman" SIZE="3">DCB</font><font SIZE="3">结构来进行。</font><font FACE="Times New Roman" SIZE="3">DCB</font><font SIZE="3">结构包含了诸如波特率、每个字符的数据位数、奇偶校验和停止位数等信息。在查询或配置置串行口的属性时，都要用</font><font FACE="Times New Roman" SIZE="3">DCB</font><font SIZE="3">结构来作为缓冲区。</font><font FACE="Times New Roman" SIZE="3"></p>
<p ALIGN="JUSTIFY"></font><font SIZE="3">调用</font><font FACE="Times New Roman" SIZE="3">GetCommState</font><font SIZE="3">函数可以获得串口的配置，该函数把当前配置填充到一个</font><font FACE="Times New Roman" SIZE="3">DCB</font><font SIZE="3">结构中。一般在用</font><font FACE="Times New Roman" SIZE="3">CreateFile</font><font SIZE="3">打开串行口后，可以调用</font><font FACE="Times New Roman" SIZE="3">GetCommState</font><font SIZE="3">函数来获取串行口的初始配置。要修改串行口的配置，应该先修改</font><font FACE="Times New Roman" SIZE="3">DCB</font><font SIZE="3">结构，然后再调用</font><font FACE="Times New Roman" SIZE="3">SetCommState</font><font SIZE="3">函数用指定的</font><font FACE="Times New Roman" SIZE="3">DCB</font><font SIZE="3">结构来设置串行口。</font><font FACE="Times New Roman" SIZE="3"></p>
<p ALIGN="JUSTIFY"></font><font SIZE="3">除了在</font><font FACE="Times New Roman" SIZE="3">DCB</font><font SIZE="3">中的设置外，程序一般还需要设置</font><font FACE="Times New Roman" SIZE="3">I/O</font><font SIZE="3">缓冲区的大小和超时。</font><font FACE="Times New Roman" SIZE="3">Windows</font><font SIZE="3">用</font><font FACE="Times New Roman" SIZE="3">I/O</font><font SIZE="3">缓冲区来暂存串行口输入和输出的数据，如果通信的速率较高，则应该设置较大的缓冲区。调用</font><font FACE="Times New Roman" SIZE="3">SetupComm</font><font SIZE="3">函数可以设置串行口的输入和输出缓冲区的大小。</font><font FACE="Times New Roman" SIZE="3"></p>
<p ALIGN="JUSTIFY"></font><font SIZE="3">在用</font><font FACE="Times New Roman" SIZE="3">ReadFile</font><font SIZE="3">和</font><font FACE="Times New Roman" SIZE="3">WriteFile</font><font SIZE="3">读写串行口时，需要考虑超时问题。如果在指定的时间内没有读出或写入指定数量的字符，那么</font><font FACE="Times New Roman" SIZE="3">ReadFile</font><font SIZE="3">或</font><font FACE="Times New Roman" SIZE="3">WriteFile</font><font SIZE="3">的操作就会结束。要查询当前的超时设置应调用</font><font FACE="Times New Roman" SIZE="3">GetCommTimeouts</font><font SIZE="3">函数，该函数会填充一个</font><font FACE="Times New Roman" SIZE="3">COMMTIMEOUTS</font><font SIZE="3">结构。调用</font><font FACE="Times New Roman" SIZE="3">SetCommTimeouts</font><font SIZE="3">可以用某一个</font><font FACE="Times New Roman" SIZE="3">COMMTIMEOUTS</font><font SIZE="3">结构的内容来设置超时。</font><font FACE="Times New Roman" SIZE="3"></p>
<p ALIGN="JUSTIFY"></font><font SIZE="3">有两种超时：间隔超时和总超时。间隔超时是指在接收时两个字符之间的最大时延，总超时是指读写操作总共花费的最大时间。写操作只支持总超时，而读操作两种超时均支持。用</font><font FACE="Times New Roman" SIZE="3">COMMTIMEOUTS</font><font SIZE="3">结构可以规定读</font><font FACE="Times New Roman" SIZE="3">/</font><font SIZE="3">写操作的超时，该结构的定义为：</p>
<blockquote>
</font><font FACE="Times New Roman" SIZE="3"><p ALIGN="JUSTIFY">typedef struct
_COMMTIMEOUTS { </p>
<p ALIGN="JUSTIFY">DWORD ReadIntervalTimeout; // </font><font SIZE="3">读间隔超时</p>
</font><font FACE="Times New Roman" SIZE="3"><p ALIGN="JUSTIFY">DWORD
ReadTotalTimeoutMultiplier; // </font><font SIZE="3">读时间系数</p>
</font><font FACE="Times New Roman" SIZE="3"><p ALIGN="JUSTIFY">DWORD
ReadTotalTimeoutConstant; // </font><font SIZE="3">读时间常量</p>
</font><font FACE="Times New Roman" SIZE="3"><p ALIGN="JUSTIFY">DWORD
WriteTotalTimeoutMultiplier; // </font><font SIZE="3">写时间系数</p>
</font><font FACE="Times New Roman" SIZE="3"><p ALIGN="JUSTIFY">DWORD
WriteTotalTimeoutConstant; // </font><font SIZE="3">写时间常量</p>
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