Re: What is 'light' user data?

On 23 September 2013 14:44, Matthias Beyer <mail@beyermatthias.de> wrote:

Hi Lua ML!

I'm confused about the terminology: What is 'light' user data? Is is
something special? If I write a C function which pops an (in C defined
element) from the stack and creates a copy, should I push the result
of the copy operation as light user data?

Code example:

static int config_member_copy(lua_State *L) {
struct config_member_t *mem;
struct config_member_t *copy;

mem = lua_touserdata(L, 1);
lua_pop(L, 1);

copy = some_magic_copy_function(mem);

lua_pushlightuserdata(L, copy);

return 1
}

--
Mit freundlichen Grüßen,
Kind regards,
Matthias Beyer

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Happily signed with gnupg.

A lightuserdata is a black box void pointer. It can not have an individual metatable set[1] and is not garbage collected; this is stated in the manual [2] [3]

>>Is this piece of code the correct approach?

Yes.

It seems you are using C in your code but I am also unsure if you should be freeing "mem" here or is it stored somewhere in C? Also IIRC MSC does not allow implicit casts from a void pointer, so that is something to be aware of if you wanted the code to be cross platform.

[1] Setting a metatable on a light userdata sets it for all light userdata

[2] http://www.lua.org/manual/5.2/manual.html#2.1

[3] http://www.lua.org/manual/5.2/manual.html#lua_pushlightuserdata

--Liam