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.
XkbAllocGeometry allocates a keyboard geometry and adds it to the keyboard description specified by xkb. The keyboard description should be obtained via the XkbGetKeyboard or XkbAllocKeyboard functions. The sizes parameter specifies the number of elements to be reserved for the subcomponents of the keyboard geometry and can be zero or more. These subcomponents include the properties, colors, shapes, sections, and doodads.