Re: How to create another lua_State in pthread?
- From: Mike Pall <mike-1301@xxxxxxxxxx>
- To: luajit@xxxxxxxxxxxxx
- Date: Wed, 23 Jan 2013 23:30:10 +0100
Wolfgang Pupp wrote:
> Henk Boom wrote:
> > I couldn't find a way to get the returned callback through the lua API
>
> I used lua_tointeger for this (not completely sure if it'd work with a
> LuaJIT x64 build).
It does, because lua_Integer is a ptrdiff_t per luaconf.h, which
is an int64_t on x64.
> Ran into some (surprising to me) quirks though:
> print(ffi.cast('ptrdiff_t', 5)) --> cdata<int>: 0x...
> print(ffi.new('ptrdiff_t[1]', 5)[0]) --> 5
> print(ffi.new('ptrdiff_t', 5)+0) --> 5LL
>
> I can understand why the cast would give me a boxed int instead of a
> number-- I asked for it, after all, and the documentation says that
> C->Lua type conversions only happen with return values from FFI-function
> calls (or when dereferencing, thus case 2).
The canonical way to convert a cdata pointer to a Lua number is:
tonumber(ffi.cast('intptr_t', ffi.cast('void *', ptr)))
This returns a signed result, which is cheaper to handle and turns
into a no-op, when JIT-compiled. If you really need an unsigned
address (and not just an abstract identifier), then use uintptr_t.
> But I don't understand why I'm getting a different result for the third
> case (a boxed int64). Can someone explain?
ptrdiff_t is an int32_t on your platform and cdata number
arithmetic coerces to int64_t/uint64_t. Only the 64 bit types have
a tostring() metamethod that shows them as LL/ULL.
[It's rarely helpful to operate on isolated, boxed 32 bit ints.
They occur only as results of casts or as boxed enums. Plain Lua
numbers work just as well.]
--Mike
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