Instructs the system to initialize the object, so that operations that take a significant amount of time to process return ERROR_IO_PENDING. When the operation is finished, the specified event is set to the signaled state. When you specify FILE_FLAG_OVERLAPPED, the ReadFile and WriteFile functions must specify an OVERLAPPED structure. That is, when FILE_FLAG_OVERLAPPED is specified, an application must perform overlapped reading and writing. When FILE_FLAG_OVERLAPPED is specified, the system does not maintain the file pointer. The file position must be passed as part of the lpOverlapped parameter (pointing to an OVERLAPPED structure) to the ReadFile and WriteFile functions. This flag also enables more than one operation to be performed simultaneously with the handle (a simultaneous read and write operation, for example).