How two C-closures can share an upvalue??

[slonik.az@...]

Hi Lua Experts,
Lua closures can share their private state with each other by sharing
their "outer local" variables. In the following code snippet "local
cnt" is accessible by both cnt_step() and cnt_get() and no other
function in the world could touch it.

function cnt_init()
  local cnt= 0
  function cnt_step() cnt= cnt + 1 end
  function cnt_get()  return cnt; end
end
cnt_init();

I would like to achieve the same effect using C-closures. But I cannot
figure out how?
Roberto's book "Programming in Lua, 2-nd edition" that I recently
bought shows how a C-closure stores its individual state in an upvalue
(lua_pushcclosure). But here I need two functions to have an access to
the same upvalue.
Any help is greatly appreciated.

Thanks and Best Regards,
--Leo--