chap12_3.htm

MFC开发基础教程

      </font><font FACE="Times New Roman" SIZE="3"><p ALIGN="JUSTIFY">} 
      COMMTIMEOUTS,*LPCOMMTIMEOUTS;</p>
    </blockquote>
    <p ALIGN="JUSTIFY">COMMTIMEOUTS</font><font SIZE="3">结构的成员都以毫秒为单位。总超时的计算公式是：</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY"></font><font SIZE="3">总超时</font><font FACE="Times New Roman" SIZE="3">=</font><font SIZE="3">时间系数×要求读</font><font FACE="Times New Roman" SIZE="3">/</font><font SIZE="3">写的字符数 </font><font FACE="Times New Roman" SIZE="3">+ </font><font SIZE="3">时间常量</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY"></font><font SIZE="3">例如，如果要读入</font><font FACE="Times New Roman" SIZE="3">10</font><font SIZE="3">个字符，那么读操作的总超时的计算公式为：</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY"></font><font SIZE="3">读总超时＝</font><font FACE="Times New Roman" SIZE="3">ReadTotalTimeoutMultiplier</font><font SIZE="3">×</font><font FACE="Times New Roman" SIZE="3">10 + ReadTotalTimeoutConstant</p>
    <p ALIGN="JUSTIFY"></font><font SIZE="3">可以看出，间隔超时和总超时的设置是不相关的，这可以方便通信程序灵活地设置各种超时。</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY"></font><font SIZE="3">如果所有写超时参数均为</font><font FACE="Times New Roman" SIZE="3">0</font><font SIZE="3">，那么就不使用写超时。如果</font><font FACE="Times New Roman" SIZE="3">ReadIntervalTimeout</font><font SIZE="3">为</font><font FACE="Times New Roman" SIZE="3">0</font><font SIZE="3">，那么就不使用读间隔超时，如果</font><font FACE="Times New Roman" SIZE="3">ReadTotalTimeoutMultiplier</font><font SIZE="3">和</font><font FACE="Times New Roman" SIZE="3">ReadTotalTimeoutConstant</font><font SIZE="3">都为</font><font FACE="Times New Roman" SIZE="3">0</font><font SIZE="3">，则不使用读总超时。如果读间隔超时被设置成</font><font FACE="Times New Roman" SIZE="3">MAXDWORD</font><font SIZE="3">并且两个读总超时为</font><font FACE="Times New Roman" SIZE="3">0</font><font SIZE="3">，那么在读一次输入缓冲区中的内容后读操作就立即完成，而不管是否读入了要求的字符。</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY"></font><font SIZE="3">在用重叠方式读写串行口时，虽然</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">在完成操作以前就可能返回，但超时仍然是起作用的。在这种情况下，超时规定的是操作的完成时间，而不是</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">的返回时间。</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY"></font><font SIZE="3">清单</font><font FACE="Times New Roman" SIZE="3">12.5</font><font SIZE="3">列出了一段简单的串行口初始化代码。</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">12.5 </font><font SIZE="3">打开并初始化串行口</font></b><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY">HANDLE hCom;</p>
    <p ALIGN="JUSTIFY">DWORD dwError;</p>
    <p ALIGN="JUSTIFY">DCB dcb;</p>
    <p ALIGN="JUSTIFY">COMMTIMEOUTS TimeOuts;</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">　</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY">SetupComm( hCom, 1024, 1024 ) //</font><font SIZE="3">缓冲区的大小为</font><font FACE="Times New Roman" SIZE="3">1024</p>
    <p ALIGN="JUSTIFY"></font><font SIZE="3">　</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY">TimeOuts. ReadIntervalTimeout=1000; </p>
    <p ALIGN="JUSTIFY">TimeOuts.ReadTotalTimeoutMultiplier=500; </p>
    <p ALIGN="JUSTIFY">TimeOuts.ReadTotalTimeoutConstant=5000; </p>
    <p ALIGN="JUSTIFY">TimeOuts.WriteTotalTimeoutMultiplier=500; </p>
    <p ALIGN="JUSTIFY">TimeOuts.WriteTotalTimeoutConstant=5000;</p>
    <p ALIGN="JUSTIFY">SetCommTimeouts(hCom, &amp;TimeOuts); // </font><font SIZE="3">设置超时</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY"></font><font SIZE="3">　</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY">GetCommState(hCom, &amp;dcb);</p>
    <p ALIGN="JUSTIFY">dcb.BaudRate=2400; // </font><font SIZE="3">波特率为</font><font FACE="Times New Roman" SIZE="3">2400</p>
    <p ALIGN="JUSTIFY">dcb.ByteSize=8; // </font><font SIZE="3">每个字符有</font><font FACE="Times New Roman" SIZE="3">8</font><font SIZE="3">位</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY">dcb.Parity=NOPARITY; //</font><font SIZE="3">无校验</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY">dcb.StopBits=ONESTOPBIT; //</font><font SIZE="3">一个停止位</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY">SetCommState(hCom, &amp;dcb);</p>
    <b><p ALIGN="JUSTIFY"></font><font SIZE="3">　</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY"></font></b><font color="#3973DE" FACE="Times New Roman" SIZE="3">12.3.3 
    </font><font SIZE="3" color="#3973DE">重叠</font><font color="#3973DE" FACE="Times New Roman" SIZE="3">I/O</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，既可以同步执行，也可以重叠（异步）执行。在同步执行时，函数直到操作完成后才返回。这意味着在同步执行时线程会被阻塞，从而导致效率下降。在重叠执行时，即使操作还未完成，调用的函数也会立即返回。费时的</font><font FACE="Times New Roman" SIZE="3">I/O</font><font SIZE="3">操作在后台进行，这样线程就可以干别的事情。例如，线程可以在不同的句柄上同时执行</font><font FACE="Times New Roman" SIZE="3">I/O</font><font SIZE="3">操作，甚至可以在同一句柄上同时进行读写操作。“重叠”一词的含义就在于此。</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY">ReadFile</font><font SIZE="3">函数只要在串行口输入缓冲区中读入指定数量的字符，就算完成操作。而</font><font FACE="Times New Roman" SIZE="3">WriteFile</font><font SIZE="3">函数不但要把指定数量的字符拷入到输出缓冲中，而且要等这些字符从串行口送出去后才算完成操作。</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY">ReadFile</font><font SIZE="3">和</font><font FACE="Times New Roman" SIZE="3">WriteFile</font><font SIZE="3">函数是否为执行重叠操作是由</font><font FACE="Times New Roman" SIZE="3">CreateFile</font><font SIZE="3">函数决定的。如果在调用</font><font FACE="Times New Roman" SIZE="3">CreateFile</font><font SIZE="3">创建句柄时指定了</font><font FACE="Times New Roman" SIZE="3">FILE_FLAG_OVERLAPPED</font><font SIZE="3">标志，那么调用</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">对该句柄进行的读写操作就是重叠的，如果未指定重叠标志，则读写操作是同步的。</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY"></font><font SIZE="3">函数</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">的参数和返回值很相似。这里仅列出</font><font FACE="Times New Roman" SIZE="3">ReadFile</font><font SIZE="3">函数的声明：</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY">BOOL ReadFile(</p>
    <blockquote>
      <p ALIGN="JUSTIFY">HANDLE hFile, // </font><font SIZE="3">文件句柄</p>
      </font><font FACE="Times New Roman" SIZE="3"><p ALIGN="JUSTIFY">LPVOID lpBuffer, // </font><font SIZE="3">读缓冲区</p>
      </font><font FACE="Times New Roman" SIZE="3"><p ALIGN="JUSTIFY">DWORD 
      nNumberOfBytesToRead, // </font><font SIZE="3">要求读入的字节数 </font><font FACE="Times New Roman" SIZE="3"></p>
      <p ALIGN="JUSTIFY">LPDWORD lpNumberOfBytesRead, // </font><font SIZE="3">实际读入的字节数</p>
      </font><font FACE="Times New Roman" SIZE="3"><p ALIGN="JUSTIFY">LPOVERLAPPED lpOverlapped 
      // </font><font SIZE="3">指向一个</font><font FACE="Times New Roman" SIZE="3">OVERLAPPED</font><font SIZE="3">结构</p>
      </font><font FACE="Times New Roman" SIZE="3"><p ALIGN="JUSTIFY">); //</font><font SIZE="3">若返回</font><font FACE="Times New Roman" SIZE="3">TRUE</font><font SIZE="3">则表明操作成功</font></p>
    </blockquote>
    <p><font SIZE="3">　</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY"></font><font SIZE="3">需要注意的是如果该函数因为超时而返回，那么返回值是</font><font FACE="Times New Roman" SIZE="3">TRUE</font><font SIZE="3">。参数</font><font FACE="Times New Roman" SIZE="3">lpOverlapped</font><font SIZE="3">在重叠操作时应该指向一个</font><font FACE="Times New Roman" SIZE="3">OVERLAPPED</font><font SIZE="3">结构，如果该参数为</font><font FACE="Times New Roman" SIZE="3">NULL</font><font SIZE="3">，那么函数将进行同步操作，而不管句柄是否是由</font><font FACE="Times New Roman" SIZE="3">FILE_FLAG_OVERLAPPED</font><font SIZE="3">标志建立的。</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY"></font><font SIZE="3">当</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">返回</font><font FACE="Times New Roman" SIZE="3">FALSE</font><font SIZE="3">时，不一定就是操作失败，线程应该调用</font><font FACE="Times New Roman" SIZE="3">GetLastError</font><font SIZE="3">函数分析返回的结果。例如，在重叠操作时如果操作还未完成函数就返回，那么函数就返回</font><font FACE="Times New Roman" SIZE="3">FALSE</font><font SIZE="3">，而且</font><font FACE="Times New Roman" SIZE="3">GetLastError</font><font SIZE="3">函数返回</font><font FACE="Times New Roman" SIZE="3">ERROR_IO_PENDING</font><font SIZE="3">。</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY"></font><font SIZE="3">在使用重叠</font><font FACE="Times New Roman" SIZE="3">I/O</font><font SIZE="3">时，线程需要创建</font><font FACE="Times New Roman" SIZE="3">OVERLAPPED</font><font SIZE="3">结构以供读写函数使用。</font><font FACE="Times New Roman" SIZE="3">OVERLAPPED</font><font SIZE="3">结构最重要的成员是</font><font FACE="Times New Roman" SIZE="3">hEvent</font><font SIZE="3">，</font><font FACE="Times New Roman" SIZE="3">hEvent</font><font SIZE="3">是一个事件对象句柄，线程应该用</font><font FACE="Times New Roman" SIZE="3">CreateEvent</font><font SIZE="3">函数为</font><font FACE="Times New Roman" SIZE="3">hEvent</font><font SIZE="3">成员创建一个手工重置事件，</font><font FACE="Times New Roman" SIZE="3">hEvent</font><font SIZE="3">成员将作为线程的同步对象使用。如果读写函数未完成操作就返回，就那么把</font><font FACE="Times New Roman" SIZE="3">hEvent</font><font SIZE="3">成员设置成无信号的。操作完成后（包括超时），</font><font FACE="Times New Roman" SIZE="3">hEvent</font><font SIZE="3">会变成有信号的。</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY"></font><font SIZE="3">如果</font><font FACE="Times New Roman" SIZE="3">GetLastError</font><font SIZE="3">函数返回</font><font FACE="Times New Roman" SIZE="3">ERROR_IO_PENDING</font><font SIZE="3">，则说明重叠操作还为完成，线程可以等待操作完成。有两种等待办法：一种办法是用象</font><font FACE="Times New Roman" SIZE="3">WaitForSingleObject</font><font SIZE="3">这样的等待函数来等待</font><font FACE="Times New Roman" SIZE="3">OVERLAPPED</font><font SIZE="3">结构的</font><font FACE="Times New Roman" SIZE="3">hEvent</font><font SIZE="3">成员，可以规定等待的时间，在等待函数返回后，调用</font><font FACE="Times New Roman" SIZE="3">GetOverlappedResult</font><font SIZE="3">。另一种办法是调用</font><font FACE="Times New Roman" SIZE="3">GetOverlappedResult</font><font SIZE="3">函数等待，如果指定该函数的</font><font FACE="Times New Roman" SIZE="3">bWait</font><font SIZE="3">参数为</font><font FACE="Times New Roman" SIZE="3">TRUE</font><font SIZE="3">，那么该函数将等待</font><font FACE="Times New Roman" SIZE="3">OVERLAPPED</font><font SIZE="3">结构的</font><font FACE="Times New Roman" SIZE="3">hEvent </font><font SIZE="3">事件。</font><font FACE="Times New Roman" SIZE="3">GetOverlappedResult</font><font SIZE="3">可以返回一个</font><font FACE="Times New Roman" SIZE="3">OVERLAPPED</font><font SIZE="3">结构来报告包括实际传输字节在内的重叠操作结果。</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY"></font><font SIZE="3">如果规定了读</font><font FACE="Times New Roman" SIZE="3">/</font><font SIZE="3">写操作的超时，那么当超过规定时间后，</font><font FACE="Times New Roman" SIZE="3">hEvent</font><font SIZE="3">成员会变成有信号的。因此，在超时发生后，</font><font FACE="Times New Roman" SIZE="3">WaitForSingleObject</font><font SIZE="3">和</font><font FACE="Times New Roman" SIZE="3">GetOverlappedResult</font><font SIZE="3">都会结束等待。</font><font FACE="Times New Roman" SIZE="3">WaitForSingleObject</font><font SIZE="3">的</font><font FACE="Times New Roman" SIZE="3">dwMilliseconds</font><font SIZE="3">参数会规定一个等待超时，该函数实际等待的时间是两个超时的最小值。注意</font><font FACE="Times New Roman" SIZE="3">GetOverlappedResult</font><font SIZE="3">不能设置等待的时限，因此如果</font><font FACE="Times New Roman" SIZE="3">hEvent</font><font SIZE="3">成员无信号，则该函数将一直等待下去。</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY"></font><font SIZE="3">在调用</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">之前，线程应该调用</font><font FACE="Times New Roman" SIZE="3">ClearCommError</font><font SIZE="3">函数清除错误标志。该函数负责报告指定的错误和设备的当前状态。</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">PurgeComm</font><font SIZE="3">函数可以终止正在进行的读写操作，该函数还会清除输入或输出缓冲区中的内容。</font><font FACE="Times New Roman" SIZE="3"></p>
    <p></font><b><font SIZE="3">　</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY"></font></b><font color="#3973DE" FACE="Times New Roman" SIZE="3">12.3.4 
    </font><font SIZE="3" color="#3973DE">通信事件</font><font FACE="Times New Roman" SIZE="3"></p>
    <p ALIGN="JUSTIFY"></font><font SIZE="3">在</font><font FACE="Times New Roman" SIZE="3">Windows