Re: String lifetime (C API)

[Asko Kauppi <askok@...>]

One handy trick, for doing dynamic string manipulation within C  
functions, involves pushing and/or combining strings using the Lua  
stack functions (concat etc.), getting a pointer to the result string  
(no C buffer needed), popping it out (no changes to stack) but still  
having the result string useful. Very neat, should be compulsory in  
C! ;P

-asko


Asko Kauppi kirjoitti 28.2.2006 kello 8.09:

> But it _is_ safe, and it's actually a very, very, very :) nice  
> trick to use in C/Lua interfaces.
>
> It is safe to use the returned string, simply because the garbage  
> collector never runs until you return from your C function back to  
> Lua. Thus, you can: get a string, pop it, use the pointer.
>
> If you still need the string later, after returning from the C  
> function, then there's no guarantees (since the garbage collection  
> eventually runs).
>
> -asko
>
> > > I know this is not always safe because the garbage collector might
> > > take the string after it is popped off this stack but in this  
> > > case the
> > > string still exists in the table and will not be collected until
> > > "table" is collected (table is still on the stack).  So is this
> > > example safe or is the string from lua_getfield actually a different
> > > string than the one in the table?