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debug.getuservalue
In Lua 5.1 all userdata have tables as environments. If you don't set
an environment explicitly, one is inherited by the function that
created the userdata (this is the global table _G for newproxy and
the package table for userdata created in C modules loaded via
require). Also, you can only replace the environment of a userdata
with another table.
In Lua 5.2 and 5.3 on the other hand, userdata start with nil as the
initial uservalue. You can set the uservalue to a table and back to
nil (Lua 5.3 also allows other values).
To emulate the behavior of Lua 5.3 the compatibility implementation of
debug.getuservalue for Lua 5.1 returns nil whenever the userdata
has an environment equal to _G or the package table, and the
actual environment table otherwise.
You cannot use values other than tables (or nil) in Lua 5.1 or Lua
5.2.
See also debug.setuservalue.