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A Window Procedure

WindowProc: proc( hwndClient, msg, mp1, mp2) options( byvalue linkage( system )) returns( MRESULT );

Dcl hwndClient HWND; Dcl msg ULONG; Dcl ( mp1, mp2 ) MPARAM; Dcl ps HPS; Dcl mr MRESULT;
mr = null();

select ( msg );
when( WM_PAINT )
do;
ps = WinBeginPaint( hwndClient, null(), null());
Call WinEndPaint( ps );
end;

otherwise
mr = WinDefWindowProc( hwndClient, msg, mp1, mp2 );
end;
return( mr );
end;