The patch fixes a simple omission:
Tracer allows setting of the attribute 'stdout', but the method 'trace_func' does not respect it in the main output. - The optional output of process id and thread id respects the attribute.
BTW: The info on http://www.ruby-lang.org/en/community/ruby-core about how to submit patches via github seems outdated!? @shyouhei's fork of this repo is way behind... That's why I am sending this request to this repo.
Fix for using Kernel.printf for tracing output instead of stdout.printf
Closing this, please refer to Bug #6490 on redmine.