Re: String parameters when calling C from Lua

[Mike Pall <mikelu-0605@...>]

Hi,

Daniel Collins wrote:
> When calling a C function from lua with a string argument, is that
> string copied onto the stack that the C function gets? Or does the stack
> contain a pointer to the original string?

No. Yes.

The Lua stack is just an array of TValue's. In your case this
boils down to an int (for the type) and a pointer (to the string
plus a header) for each stack slot.

Lua strings are immutable, shared and pre-hashed. Constant
strings (i.e. t.key or t["key"]) are hashed once when the script
is loaded.

A string lookup is then just a couple instructions in the fast
path. See luaH_getstr() and realize that the hashstr() macro is
just a bitmask operation and an array lookup. Hashing is done
once when a Lua string is created (see luaS_newlstr()), not when
it is used as a key.

A corollary of this is that you should avoid converting C strings
to Lua strings over and over again. lua_push[l]string() has to
rehash it and lookup it up in the shared string table. Try to
keep repeatedly used Lua strings in upvalues or arrays. Or,
better yet, in Lua code.

> I am wondering about the most
> efficient way to implement localisable string tables. I can see two ways
> of doing this but I want to use the most efficient implementation. The
> two methods can be boiled down to:
> 
> Method One:
> -- StringTable.lua
> stringtable = 
> {
>     mainmenu = "main",
>     quit = "quit",
>     yes = "yes"
> }
> 
> -- SomeMenu.lua
> require "StringTable"
> callSomeCFunction(stringtable.yes)

One hash table lookup.

> Method Two:
> -- StringTable.lua
> LoadStringData("stringdata.bin")    -- loads the actual strings into a C
> array
> 
> stringtable = 
> {
>     mainmenu = 1,
>     quit = 2,
>     yes = 3
> }
> 
> -- SomeMenu.lua
> require "StringTable"
> callSomeCFunction(stringtable.yes)    -- integer argument indexes the C
> array loaded earlier

One hash table lookup and on the C side a double->int conversion
and one array lookup. Slower.

General rule: Lua's hash tables are very fast. Particularly when
used with constant strings. Use them if you can.

Trying to avoid a hash lookup does not pay off. And a workaround
may be slower due to other issues (as seen above).

Bye,
     Mike