See: http://lists.freedesktop.org/archives/xcb/2013-October/008689.html

To quote from that mail (more details are in there):

First, the summary if you don't want to read the whole story: It looks
like _every_ structure having a fixed size field after a variadic
field/list is broken! Most other extensions are not affected as they've
the variadic stuff at the end of a structure - not intermixed with fixed
size fields.

[...]

    This struct has a
    - fixed size field (nameLength)
    - variadic list (name)
    - fixed size field (valueLength)
    - variadic list (value).

    Now, look at the generated structure:
    typedef struct xcb_xkb_property_t {
        uint16_t nameLength;
        uint16_t valueLength; <-- doesn't belong here
    } xcb_xkb_property_t;