An application sends the EM_GETMARGINS message to retrieve the widths of the left and right margins for an edit control.
EM_GETMARGINS 
wParam = 0;  // not used, must be zero 
lParam = 0;  // not used, must be zero 
 This message has no parameters.
Returns the width of the left margin in the low-order word, and the width of the right margin in the high-order word.
  Windows NT: Requires version 4.0 or later.
  Windows: Requires Windows 95 or later.
  Windows CE: Requires version 1.0 or later.
  Header: Declared in winuser.h.
Edit Controls Overview, Edit Control Messages, EM_SETMARGINS