The goal is to wrap the ffi symbols and new ones in one table

wraplib

local ffi = require( "ffi" )

local function wraplib( lib, options )
   local function LOG(...)
      if options.verbose then
	 print(...)
      end
   end
   return setmetatable ( 
      {},
      {
	 __index =
	    function( t, k )
	       local v = rawget( t, k )
	       if v ~= nil then
		  LOG( "__index: returning from table <" .. tostring(t) .. ">\n" .. 
                       "                   the symbol " .. tostring(k) .. " = " .. tostring(v) )
		  return v
	       end
	       local ok, v = xpcall(function() return lib[k] end, function() end)
	       if ok then
		  LOG( "__index: importing from library <" .. tostring(lib) .. ">\n" ..
		       "                   into table <" .. tostring(t)   .. ">\n" ..
		       "                   the symbol " .. tostring(k) .. " = " .. tostring(v) .. "\n" )
		  rawset( t, k, v );
		  return v
	       end
	       LOG( "__index: can't find in table <" .. tostring(t) .. ">\n" ..
                    "                    the symbol " .. tostring(k) .. "\n" )
	       error( "symbol " .. tostring( k ) .. " does not exist" )
	    end,

	 __newindex = 
	    function( t, k, nv )
	       local ok, v = xpcall(function() return lib[k] end, function() end)
	       if ok then
		  error("symbol " .. tostring(k) .. " already exist, imported with value " .. tostring(v))
	       end
	       local v = rawget( t, k )
	       if v ~= nil then
		  error("symbol " .. tostring(k) .. " already exist, created with value " .. tostring(v))
	       end
	       LOG( "__newindex: adding to table <" .. tostring(t) .. ">\n" ..
                    "                   the symbol " .. tostring(k) .. " = " .. tostring(nv) .. "\n" )
	       rawset( t, k, nv )
	    end
      }
   )
end

print("\n")

local lib = wraplib( ffi.load( "curl" ), { verbose = true } )

ffi.cdef[[
      typedef void* CURL;
      CURL* curl_easy_init();
]]
ffi.cdef[[
      void curl_easy_cleanup( CURL* );
]]

print("1")
for k,v in pairs(lib) do print(k,v) end
print("2")

local curl = lib.curl_easy_init()

print("3")
for k,v in pairs(lib) do print(k,v) end
print("4")

print( lib["curl_easy_init"] )
print( lib["curl_easy_init"] )
print( lib["curl_easy_init"] )

function lib.curl_new_function( string )
   print( string )
end

--lib.curl_new_function( "test" )

--print( lib.curl_new_function )

function lib.curl_easy_cleanup1( curl )
   print( "This might work" )
end

lib.curl_easy_cleanup(curl)

ffi.cdef[[
      void curl_easy_cleanup( CURL* );
]]

function lib.curl_easy_cleanup( curl )
   print( "This should not work" )
end

lib.curl_easy_cleanup( curl )

lib.curl_easy_cleanup = function( curl )
   print( "This should not work 1" )
end

lib.curl_easy_cleanup( curl )

lib.curl_easy_cleanup = function( curl )
   print( "This should not work 2" )
end

lib.curl_easy_cleanup( curl )

local mt = getmetatable(lib)
for k,v in pairs(mt) do print(k,v) end

for k,v in pairs(lib) do print(k,v) end