ThunkProc - Remarks

Pointer conversion is normally performed automatically by the operating system. An application needs to provide its own pointer-conversion procedures only for application-defined messages which may be passed from 16-bit code to 32-bit code.

A pointer-conversion procedure is associated with a window by the WinSetWindowThunkProc and WinSetClassThunkProc functions.

The logic of the pointer-conversion procedure is as follows:

• Convert each message parameter, if necessary. This may include converting any data structures to which the parameter points.

• Call the window procedure referenced by the pWndProc parameter, supplying as arguments hwnd, usmsg, mpParam1 and mpParam2.

• Collect the return value and, if necessary, convert it.

  Note that structures to which the return value might point cannot be converted.

• Convert any structures referenced by message parameters which might have been modified by the window procedure. Note that the pointer-conversion procedure should ensure that the original memory is still available before converting the structures.

A pointer-conversion procedure should process only those messages that it recognizes. On receiving unrecognized messages, it should set usmsg to 0.