Equality vs. Identity of userdata?

[Stephan Herrmann <stephan@...>]

Hi,

What exactly is the behaviour of lua_pushusertag(u,t) ?
I experience it to create a new lua object of type userdata only
once for each value of u. Is this behaviour part of the specification,
or implementation detail? Are two _equal_ userdata objects in Lua
always _identical_??

The reason for asking is reference-counting and garbage collection.
I have a C datastructure containing a reference-count and I wanted to
use the same count for references both from C and from Lua. So my
idea was, every time I "export" a C-object to Lua (using lua-pushusertag),
I increment the reference-count. Then I have a "gc" tagmethod, which
calls a free routine that decrements the ref-count and if 0 is reached
frees my resources. So it is transparent, who keeps references
to an object. Whenever the last ref is removed, the object is really freed.

>From my experience I would have to keep two ref-"counts": one for C and 
a boolean "count" for Lua, because a userdata seems to be referenced at 
most once from Lua. The reverse conclusion seems to be, that two calls
to lua_pushusertag(u,t) with the same u entail at most one call to the
"gc" tagmethod. Is this correct?

A convenient solution would be, if I knew whether lua_pushusertag
created a new object or recycled an existing one. Only in the first
case I would increment my ref-count.


Stephan