Re: Heap usage and variable argument C functions

[Asko Kauppi <asko.kauppi@...>]

21.12.2004 kello 05:55, Cory Bloyd kirjoitti:

> I am assuming this does not apply to C functions called from Lua.  C
> functions do not inform the interpreter as to how many arguments they
> expect so they technically always work in vararg mode.  It is OK though
> because they can interpret the parameter stack directly without needing
> a temporary table.  Am I correct?

Well, you can also think Lua functions (non-vararg) work the same as 
the C ones. What I mean is, if you feed a function more params than the 
prototype says, it won't complain (just ignore the extraneous params). 
So, I would see C function binding just going this same path (and not 
even being vararg-like). In practise, you are right. :)

> I ask because we have a C function that will be called very frequently
> from Lua and will need to accept a varying number of arguments.  We are
> yet another game development house adopting Lua.  Therefore the time
> spent in the garbage collector is a very sensitive issue for us.

I'm doing this all the time (having C func with varying params) and 
it's not only efficient, but also very effective for the API 
simplicity. Does the operator overloading thing (same function name, 
"it'll know what you mean" ;).

> Summarizing my understanding... The following is a complete list of
> actions that involve garbage collectable allocation in Lua:
> *Loading chunks of bytecode
> *Creating tables, functions, strings, threads or userdata
> *Resizing tables
> *Calling Lua functions with variable parameters

Hmm.. changing variable values?  
(string,threads,userdata,functions,tables)  If the old reference is not 
referred to from elsewhere, it'll eventually get gc'd.

You can avoid this by keeping an extra ref to everything you create, in 
some dummy table. I would not. :)

> always constructed incrementally.  Therefore the collector could
> possibly be triggered during a table construction because of the
> fragmentation they induce.

Hmm.. (again).  I'm not a Lua internals expert, but what would be gc'ed 
if things are only added to the table?  Fragmantation - what do you 
mean by that?

> Many thanks!
>  Cory

No prob.  This is the way things work; welcome to the community, too. ;)

-ak