luaL_optlstring

luaL_optlstring[-0, +0, v]

const char *luaL_optlstring (lua_State *L,
                             int arg,
                             const char *d,
                             size_t *l);

If the function argument arg is a string, returns this string. If this argument is absent or is nil, returns d. Otherwise, raises an error.

If l is not NULL, fills the position *l with the result's length. If the result is NULL (only possible when returning d and d == NULL), its length is considered zero.

This function uses lua_tolstring to get its result, so all conversions and caveats of that function apply here.