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I am revising some of my code which uses a light userdata stored in the registry to allow Lua functions coded in C to access a C++ object. This is a common issue which will come up in a number of different contexts such as multi-threading.


While the registry solution works fine, it occurs to me that it would be more efficient to store this frequently accessed pointer in the userdata supplied to lua_newstate. However, while this is passed back to the memory allocator function, there is no API to recover it from the lua_State. Examination of the lua sources enabled me to write such a function fairly trivially, but it occurs that this would be a useful addition to the API – say:


void *lua_getallocud(lua_State *L)


I know about the LUAI_EXTRASPACE feature to stash extra data in the state, but using this requires recompilation of the Lua code library whereas the above approach could be used as-is.