Re: concat for ropes
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- Subject: Re: concat for ropes
- From: Luiz Henrique de Figueiredo <lhf@...>
- Date: Sat, 6 Dec 2008 00:25:50 -0200
> This version incorrectly changes the value at index 2 to an empty
> string (in unrope). This may cause memcpy to copy from a NULL pointer,
> although my operating system's implementation of memcpy cowardly
> avoids this.
Thanks for spotting a problem there, though I don't actually get your
explanation. What I see is that nil (or any other non-string value)
would be accepted as a 2nd arg and then sent to luaL_addvalue, where it
would be converted to NULL and then potentially crash the program, as you
point out.
> I think there also may be a problem with the stack not being large
> enough to accommodate the luaL_Buffer but I have not explored this yet.
Ignacio has spotted that. I hope the version below fixes all this...
(I really should not debug my code in public like that :-)
static int unrope (lua_State *L) {
luaL_Buffer b;
luaL_checktype(L,1,LUA_TTABLE);
lua_settop(L,2);
if (lua_isnil(L,2)) {lua_pushliteral(L,""); lua_replace(L,2);}
else luaL_checkstring(L,2);
lua_insert(L,1);
luaL_checkstack(L,MAXLEVEL-1+LUA_MINSTACK,"cannot grow stack");
lua_settop(L,MAXLEVEL+1);
luaL_buffinit(L,&b);
doconcat(L,&b,2,0);
luaL_pushresult(&b);
return 1;
}
Anyway, the point of the code was to promote unrope. But perhaps the bad code
has given unrope a bad name! (I hope not because ropes are nice and easy to
build and use in Lua; unrope was just meant as an efficient concat for ropes,
though it is a bit tricky to get right...)
Thanks for all the feedback.