Pointer to a window procedure.
C Declaration Method
In the header file, this is a two-part definition as shown below:
typedef MRESULT (EXPENTRY FNWP) (HWND, ULONG, MPARAM, MPARAM); typedef FNWP *PFNWP;
The first argument (HWND) is the handle of the window receiving the message. The second argument (ULONG) is a message identifier. The third argument (MPARAM) is the first message parameter (mp1). The fourth argument (MPARAM) is the second message parameter (mp2). The function returns an MRESULT.
This is the standard function definition for window procedures. Each message has a specific set of possible return codes. The window procedure must return a value that is appropriate for the message being processed.
Window procedures must be exported in the definitions file used by the linker.