Casting mrb_atexit_func to void * is undefined behavior in C and C++. However I doubt this will be a problem. On POSIX and Windows systems it's okay to do so.

Maybe it's preferable to:

• replace mrb_atexit_func with mrb_func_t
• change the data type of the stack to struct RProc *

@cremno You're right.
Seems like Kernel.at_exit proc stack should be stored to other place.

Do we really need it? I prefer explicit use of ensure clause.

@matz I think script only mrbgem require these feature since it can't use mrb_*_gem_final.

It should be quite rare case. As far as I know there's no gem using valid *_final yet.
If you really want finalization, you can always write C part. We don't have to spoil gem writers.

@matz I agree.
Making gem writer to write C finalizer is better.