When the IDirectDrawSurface3::Flip method is called, the primary surface and back buffer are exchanged. However, the exchange may not occur immediately. For example, if a previous flip has not finished, or if it did not succeed, this method returns DDERR_WASSTILLDRAWING. In the samples included with this Programmer's Reference, the IDirectDrawSurface3::Flip call continues to loop until it returns DD_OK. Also, a IDirectDrawSurface3::Flip call does not complete immediately. It schedules a flip for the next time a vertical blank occurs on the system.
An application that waits until the DDERR_WASSTILLDRAWING value is not returned is very inefficient. Instead, you could create a function in your application that calls the IDirectDrawSurface3::GetFlipStatus method on the back buffer to determine if the previous flip has finished.
If the previous flip has not finished and the call returns DDERR_WASSTILLDRAWING, your application can use the time to perform another task before it checks the status again. Otherwise, you can perform the next flip. The following example demonstrates this concept:
while(lpDDSBack->GetFlipStatus(DDGFS_ISFLIPDONE) ==
DDERR_WASSTILLDRAWING);
// Waiting for the previous flip to finish. The application can
// perform another task here.
ddrval = lpDDSPrimary->Flip(NULL, 0);
You can use the IDirectDrawSurface3::GetBltStatus method in much the same way to determine whether a blit has finished. Because IDirectDrawSurface3::GetFlipStatus and IDirectDrawSurface3::GetBltStatus return immediately, you can use them periodically in your application with little loss in speed.