Re: metatype __gc inconsistency
- From: Tomash Brechko <tomash.brechko@xxxxxxxxx>
- To: luajit@xxxxxxxxxxxxx
- Date: Fri, 19 Sep 2014 23:57:52 +0400
2014-09-19 23:23 GMT+04:00 Cosmin Apreutesei <cosmin.apreutesei@xxxxxxxxx>:
> Is it possible that you're hitting undefined
> behavior and just escaped a crash in this particular example?
>
No, on Linux x86-64 the ABI for a scalar and a struct wrapping scalar is
the same (likely so on any ABI as there are little reasons to do
otherwise). I faked malloc/sin just for the sake of example, I don't know
any universally available function that returns a struct by value. Also
note that accessing struct value (in example with sin()) and other
metatable methods (__len) work as expected, so ABI part is safe (and just
for the case I actually checked it right now with the real function
returning a struct :D - works the same way).
As has been cited earlier, the docs say "The __gc metamethod only applies
to struct/union types and performs an implicit ffi.gc()
<http://luajit.org/ext_ffi_api.html#ffi_gc> call during creation of an
instance." So my guess that at some moment this particular case has been
overlooked and no internal ffi.gc() call has been added for structs
returned by value, though they are likely boxed with the code sequence
similar to ffi.new() call. Other metamethods work because they also have
to work for pointer to struct and hence take a different code path.
--
Tomash Brechko
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