Xkb provides functions to add a single new element to the top-level keyboard geometry. In each case the num_ * fields of the corresponding structure is incremented by 1. These functions do not change sz_* unless there is no more room in the array. Some of these functions fill in the values of the element's structure from the arguments. For other functions, you must explicitly write code to fill the structure's elements.
The top-level geometry description includes a list of geometry properties. A geometry property associates an arbitrary string with an equally arbitrary name. Programs that display images of keyboards can use geometry properties as hints, but they are not interpreted by Xkb. No other geometry structures refer to geometry properties.
XkbAddGeomOverlay adds an overlay with the specified name to the specified section. The new overlay is created with space allocated for sz_rows rows. If an overlay with name name already exists in the section, a pointer to the existing overlay is returned. XkbAddGeomOverlay returns NULL if any of the parameters is empty or if it was not able to allocate space for the overlay. To allocate space for an arbitrary number of overlays to a section, use the XkbAllocGeomOverlay function.
typedef struct _XkbOverlayRec {
Atom name; /* overlay name */
XkbSectionPtr section_under; /* the section under this
overlay */
unsigned short num_rows; /* number of rows in the
rows array */
unsigned short sz_rows; /* size of the rows array
*/
XkbOverlayRowPtr rows; /* array of rows in the
overlay */
XkbBoundsPtr bounds; /* bounding box for the
overlay */
} XkbOverlayRec,*XkbOverlayPtr;