Hello,
I use a similar solution for sharing data, but instead of :
lua_pushlightuserdata( L, &myUDMapKey );
lua_gettable( L, LUA_REGISTRYINDEX );
I use a metatable that I have create to insert c_pointer as key and
fullUserData as value that gives something like:
lua_getmetatable( L, "myKeyMap" );
lua_pushlightuserdata(L, c_pointer);
lua_gettable(L,-2)
lua_remove(L,-2)
Is there a lot of overhead using a metatable instead of the
lua_registryindex table?
thanx
David
Mark Hamburg wrote:
Technically, this isn't safe. Amongst other things, if the userdata has been
collected, this call corrupts the Lua state.
That being said, I've written this code (though I later took it out).
Someone else also posted essentially the same thing a month or two ago. So,
it's clearly an issue that a lot of people encounter.
The safe way to do this is to maintain a weak table mapping the userdata
pointers as light userdata to the full userdata objects. Pushing then
becomes something like the following:
lua_pushlightuserdata( L, &myUDMapKey );
lua_gettable( L, LUA_REGISTRYINDEX );
lua_pushlightuserdata( L, theUserData );
lua_gettable( L, -2 );
lua_remove( L, -2 );
(I used a light userdata rather than an integer to key the registry because
it's easier to guarantee unique.)
Mark
on 7/25/05 5:45 PM, Kriss at Kriss@XIXs.com wrote:
I was just thinking about tidying up some code and want to do the
following.
cache a pointer returned from lua_touserdata() on the C side (I am
already doing this anyway)
at some point later on, push that userdata onto the stack to be used as
a table look up key, I can guarantee that the userdata will not have
been GCd at this point. At the moment I am using a unique integer as a
table key, but it would be simpler to use the userdata instead.
however there is no lua_pushuserdata()
Is it safe just to use a modified version of lua_newuserdata() that
doesnt allocate any new userdata but takes a previously allocated
pointer and pushes it on the stack?
EG some untested 5.0 code :)
LUA_API void lua_pushuserdata (lua_State *L, void *data) {
lua_lock(L);
luaC_checkGC(L);
setuvalue(L->top, ((Udata *)(data))-1);
api_incr_top(L);
lua_unlock(L);
}
or is there some more magic surrounding userdata I should be aware of.
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