Xkb provides a number of functions to allocate and free subcomponents of a keyboard geometry. Use these functions to create or modify keyboard geometries. Note that these functions merely allocate space for the new element(s), and it is up to you to fill in the values explicitly in your code. These allocation functions increase sz_* but never touch num_* (unless there is an allocation failure, in which case they reset both sz_* and num_* to zero). These functions return Success if they succeed, BadAlloc if they are not able to allocate space, or BadValue if a parameter is not as expected.
XkbAllocGeomOverlayRows allocates num_needed rows and adds them to the overlay. No initialization of the rows is done.
To free rows in an overlay, use XkbFreeGeomOverlayRows.