/* push functions and arguments */
lua_getglobal(L, "f");  /* function to be called */
lua_pushnumber(L, x);   /* push 1st argument */
lua_pushnumber(L, y);   /* push 2nd argument */

/* do the call (2 arguments, 1 result) */
if (lua_pcall(L, 2, 1, 0) != 0)
    error(L, "error running function `f': %s", lua_tostring(L, -1));

/* do something with the result */