Ruby Internal Types
While working on PlainAPM’s object allocation tracing callback, I needed a way to determine classes of arbitrary objects returned by rb_tracearg_object.
The naive way, calling rb_obj_classname was segfaulting, though, so I decided to handle this type-by-type, and either avoid the problem, or narrow down the scope of the issue, to understand it better.
Here are some notes that might save fifteen minutes to a future me, or to someone who’s starting to look into which types of objects are there in Ruby:
The internal types are defined by the ruby_value_type enum in
value_type.h.
Follow the comments from there.
A seemingly exhaustive switch based on the BUILTIN_TYPE(obj), lives
in the objspace extension’s
objspace_dump.c.
The same file also includes Ruby API functions for inspecting properties
of those types, which is especially useful for debugging.
Some of these types - T_ARRAY, T_CLASS, etc, correspond to what you usually
think of as a Ruby object. Then a good bet is to go and check the header files in the
include/ruby/internal/core directory for a definition of the corresponding struct.
Some types like T_IMEMO (this one is for debugging) or T_MOVED (related to
GC compaction) correspond to a struct without a RBasic member, so they won’t
have a class.
Last but not least, Ruby also has a T_ZOMBIE type, backed by a struct called
RZombie - a decent punk rock band name if you ask me.
And, yes, if you were wondering, there is a make_zombie function… You are welcome :)