Xkb provides several functions that make it easier to apply the compatibility map to configure a client-side Xkb keyboard mapping, given a core protocol representation of part or all of a keyboard mapping. Obtain a core protocol representation of a keyboard mapping from an actual server (by using XGetKeyboardMapping, for example), a data file, or some other source.
XkbUpdateMapFromCore interprets input argument information representing a keyboard map in core format to update the Xkb keyboard description passed in xkb. Only a portion of the Xkb map is updated - the portion corresponding to keys with keycodes in the range first_key through first_key + num_keys - 1. If XkbUpdateMapFromCore is being called in response to a MappingNotify event, first_key and num_keys are reported in the MappingNotify event. core_keysyms contains the keysyms corresponding to the keycode range being updated, in core keyboard description order. map_width is the number of keysyms per key in core_keysyms. Thus, the first map_width entries in core_keysyms are for the key with keycode first_key, the next map_width entries are for key first_key + 1, and so on.
In addition to modifying the Xkb keyboard mapping in xkb, XkbUpdateMapFromCore backfills the changes structure whose address is passed in changes to indicate the modifications that were made. You may then use changes in subsequent calls such as XkbSetMap, to propagate the local modifications to a server.