lua_tolstring
const char *s = lua_tolstring(L, narg, len);
const char *s = lua_tolstring(L, -1, &l);
return lua_tolstring(L, -1, len);
const char *s = lua_tolstring(L, lua_upvalueindex(1), &ls);
const char *p = lua_tolstring(L, lua_upvalueindex(2), &lp);
const char *news = lua_tolstring(ms->L, 3, &l);
LUA_API const char *(lua_tolstring) (lua_State *L, int idx, size_t *len);
#define lua_tostring(L,i) lua_tolstring(L, (i), NULL)
key = lua_tolstring(state, 2, &keylen);