will get some more sensible debugging information from the server as to why it's
returning badvalue

Not sure if this helps or not, but I am getting the same error from the
following code (trying to change a single KeySym assignment; values in
XkbDescRec seem to be OK):


unsigned short offset;
unsigned short width;
XkbDescPtr pXkbDesc;

pXkbDesc = XkbGetMap(display, XkbKeySymsMask, XkbUseCoreKbd);
		
XkbMapChangesRec ChgRec = {0};
		
ChgRec.changed = XkbKeySymsMask;
ChgRec.first_key_sym = keycode;
ChgRec.num_key_syms = 1;

ChgRec.min_key_code = pXkbDesc->min_key_code;
ChgRec.max_key_code = pXkbDesc->max_key_code;

offset = pXkbDesc->map->key_sym_map[keycode].offset;
width = pXkbDesc->map->key_sym_map[keycode].width;

printf("Changing symbol {%d, %d, %d} from %d to %d\n",
	   keycode, group, level,
	   pXkbDesc->map->syms[offset + group * width + level],
	   newsym);
		
pXkbDesc->map->syms[offset + group * width + level] = newsym;
if(False == XkbChangeMap(display, pXkbDesc, &ChgRec))
{
	printf("ERROR: failed to apply changes.\n");
	ret = 1;
	goto quit;
}

Sebastien, this looks strange. I am using ubunbu breezy and "make install" from
xkeyboard-config. Look:
$ setxkbmap -layout 'fr,us' -model pc105 -option '' -option 'grp:alts_toggle' -print
xkb_keymap {
        xkb_keycodes  { include "xfree86+aliases(azerty)"       };
        xkb_types     { include "complete"      };
        xkb_compat    { include "complete"      };
        xkb_symbols   { include
"pc(pc105)+fr+us:2+level3(ralt_switch_for_alts_toggle):1+level3(ralt_switch_for_alts_toggle):2+group(alts_toggle)"
  };
        xkb_geometry  { include "pc(pc105)"     };
};

It is different from what you show me here...

That is fixed with xkeyboard-config now, closing the bug. Sergey let me know if
you are interested anyway by the reply to your previous comment, but there is
probably no point to debug something working now