XkbResizeKeySyms - Man Page
Change the number of symbols bound to a key
KeySym * XkbResizeKeySyms (XkbDescRec *xkb, int key, int needed);
keyboard description to be changed
keycode for key to modify
new number of keysyms required for key
XkbResizeKeySyms reserves the space needed for needed keysyms and returns a pointer to the beginning of the new array that holds the keysyms. It adjusts the offset field of the key_sym_map entry for the key if necessary and can also change the syms, num_syms, and size_syms fields of xkb->map if it is necessary to reallocate the syms array. XkbResizeKeySyms does not modify either the width or number of groups associated with the key.
If needed is greater than the current number of keysyms for the key, XkbResizeKeySyms initializes all new keysyms in the array to NoSymbol.
Because the number of symbols needed by a key is normally computed as width * number of groups, and XkbResizeKeySyms does not modify either the width or number of groups for the key, a discrepancy exists upon return from XkbResizeKeySyms between the space allocated for the keysyms and the number required. The unused entries in the list of symbols returned by XkbResizeKeySyms are not preserved across future calls to any of the map editing functions, so you must update the key symbol mapping (which updates the width and number of groups for the key) before calling another allocator function. A call to XkbChangeTypesOfKey will update the mapping.
If any allocation errors occur while resizing the number of symbols bound to the key, XkbResizeKeySyms returns NULL.
A change to the number of symbols bound to a key should be accompanied by a change in the number of actions bound to a key. Refer to XkbResizeKeyActions for more information on changing the number of actions bound to a key.
XkbGetMap(3), XkbGetUpdatedMap(3), XkbResizeKeyActions(3), XkbSetMap(3).