Compares the user data for the key quark on object with oldval, and if they are the same, replaces oldval with newval.
This is like a typical atomic compare-and-exchange operation, for user data on an object.
If the previous value was replaced then ownership of the old value (oldval) is passed to the caller, including the registered destroy notify for it (passed out in old_destroy). It’s up to the caller to free this as needed, which may or may not include using old_destroy as sometimes replacement should not destroy the object in the normal way.
a GQuark, naming the user data pointer
the old value to compare against
the new value
a destroy notify for the new value
destroy notify for the existing value
TRUE if the existing value for quark was replaced by newval, FALSE otherwise.