Re: one-time-set, many-times-use data

[C++ RTMP Server <crtmpserver@...>]

Hi,

Thank you for your inputs.

LUA_API lua_State *(lua_newstate) (lua_Alloc f, void *ud);
Creates a new, independent state. Returns NULL if cannot create the state (due to lack of memory). The argument f is the allocator function; Lua does all memory allocation for this state through this function. The second argument, ud, is an opaque pointer that Lua simply passes to the allocator in every call

and the next one:
LUA_API lua_Alloc (lua_getallocf) (lua_State *L, void **ud);
Returns the memory-allocation function of a given state. If ud is not NULL, Lua stores in *ud the opaque pointer passed to lua_newstate.

It doesn't say anything about the lifetime of ud. Since the lua_newstate doc clearly states that "ud, is an opaque pointer that Lua simply passes to the allocator in every call" I presume it MUST preserve it for the lifetime of lua_State. The end result is exactly what I need at 1-function-call-distance without tempering with the stack like it is required if I would want to keep my opaque pointer in the global index.

Moreover, another reason against keeping it in the global index is that I don't want lua script having access to that pointer, not even seeing it, let alone playing around with it. It only makes sense for the C/C++ side.

Another thing is that I only need a pointer so void * is perfect for my needs. Why should I temper with the internals of the lib? Can you confirm that my logic is ok?

Thank you again

On Apr 3, 2011, at 5:23 AM, Robert G. Jakabosky wrote:

> On Saturday 02, C++ RTMP Server wrote:
>> On Apr 3, 2011, at 2:42 AM, Peter Cawley wrote:
>>> On Sun, Apr 3, 2011 at 12:40 AM, C++ RTMP Server <crtmpserver@gmail.com> 
> wrote:
>>>> However, I didn't find any way of pulling it out from a lua_State.
>>> 
>>> lua_getallocf
>> 
>> Jackpot!!! That's exactly what I needed :)
> 
> I don't recommend messing with allocator's userdata value.  There is a better 
> option for doing what you want.
> 
> In the luaconf.h redefine these:
> 
> #define LUAI_EXTRASPACE  sizeof(void *) /* or the size of your custom state */
> 
> #define LUA_TO_CUSTOM_STATE(L) ((((unsigned char *)L) - LUAI_EXTRASPACE))
> inline void init_custom_state(lua_State *L) {
>  void **custom_state = LUA_TO_CUSTOM_STATE(L);
>  *custom_state = NULL;
>  /* or allocate your custom state. */
> }
> 
> inline void cleanup_custom_state(lua_State *L) {
>  void **custom_state = LUA_TO_CUSTOM_STATE(L);
>  if(*custom_state) free(*custom_state);
> }
> 
> inline void copy_custom_state(lua_State *L, lua_State *L1) {
>  void **custom_state = LUA_TO_CUSTOM_STATE(L);
>  void **custom_state1 = LUA_TO_CUSTOM_STATE(L1);
>  *custom_state1 = *custom_state;
>  /* or allocate a new custom state. */
> }
> 
> /* these are for the main states. */
> #define luai_userstateopen(L)		init_custom_state(L)
> #define luai_userstateclose(L)		cleanup_custom_state(L)
> /* these are for lua threads (i.e. coroutines) */
> #define luai_userstatethread(L,L1)	copy_custom_state(L, L1)
> #define luai_userstatefree(L)		((void)L)
> /* you might not need these. */
> #define luai_userstateresume(L,n)	((void)L)
> #define luai_userstateyield(L,n)	((void)L)
> 
> Now you can always get your custom state object from a lua state pointer using 
> the LUA_TO_CUSTOM_STATE(L) macro.  Note that you could pre-append your custom 
> state object to have a 1-to-1 mapping both ways.  Remember that coroutines 
> have there own lua_State object, which is different from the main lua_State.
> 
> -- 
> Robert G. Jakabosky
> 

------
Eugen-Andrei Gavriloaie
Web: http://www.rtmpd.com