关于异步I/O请求被同步处理的问题。。

    有些异步I/O请求会被同步处理。。比方说，你要读的内容正好在系统缓存中，那么就可以直接copy到你的缓存中，异步I/O请求就可以同步的处理。。于是，在我们coding的时候，就要把情况考虑周全。。

if (!ReadFile(hFile,

                 pDataBuf,

                 dwSizeOfBuffer,

                 &NumberOfBytesRead,

                 &osReadOperation )

   {

      if (GetLastError() != ERROR_IO_PENDING)

      {

         // Some other error occurred while reading the file.

         ErrorReadingFile();

         ExitProcess(0);

      }

      else

         // Operation has been queued and

         // will complete in the future.

         fOverlapped = TRUE;

   }

   else

      // Operation has completed immediately.

      fOverlapped = FALSE;

   if (fOverlapped)

   {

      // Wait for the operation to complete before continuing.

      // You could do some background work if you wanted to.

      if (GetOverlappedResult( hFile,

                               &osReadOperation,

                               &NumberOfBytesTransferred,

                               TRUE))

         ReadHasCompleted(NumberOfBytesTransferred);

      else

         // Operation has completed, but it failed.

         ErrorReadingFile();

   }

   else

      ReadHasCompleted(NumberOfBytesRead);

 

   以上函数，当ReadFile 和 WriteFile返回非零值表示这个异步I/O请求被同步的处理了。。

    如果被异步处理了，或者出现error.返回false。。此时，我们必须调用GetLastError来确定是那种情况。若GetLastError返回ERROR_IO_PENDING，说明I/O请求被排入等待队列，将会被处理。。其他的返回值，说明出现了error..