The only place where zbx_calculate_macro_function() is called it is called like so:

				ret = zbx_calculate_macro_function(*data + token.data.func_macro.func.l,
						token.data.func_macro.func.r - token.data.func_macro.func.l + 1,
						&replace_to);

The only use of the second parameter inside zbx_calculate_macro_function() is passing it's address to zbx_function_parse() which just overwrites the addressed memory.