Issue #7220 has been updated by brixen (Brian Ford). If StringIO manifests this behavior as a copy of IO's behavior, both should be consider bugs. RubySpec has a quarantined spec for this behavior in IO with a comment stating that there are platform incompatibilities with this IO "state aliasing": https://github.com/rubyspec/rubyspec/blob/master/core/io/dup_spec.rb#L34-54 IO#dup creates an instance with a different underlying fd (#fileno): https://github.com/rubyspec/rubyspec/blob/master/core/io/dup_spec.rb#L30-32 There is no other Ruby core class that causes aliasing when calling #dup. String#dup, for example, is called in code precisely to create a new String so the original would not be mutated. That IO and StringIO do cause this aliasing is a deviation from the typical behavior of #dup, makes no sense, and is not at all required. It's perfectly possible to do the following: sasha:rubinius brian$ cat foobar.txt 123456 sasha:rubinius brian$ irb 1.9.3p286 :001 > a = File.open("foobar.txt", "r") => #<File:foobar.txt> 1.9.3p286 :002 > b = File.open("foobar.txt", "r") => #<File:foobar.txt> 1.9.3p286 :003 > a.getc => "1" 1.9.3p286 :004 > a.pos => 1 1.9.3p286 :005 > b.pos => 0 1.9.3p286 :006 > b.getc => "1" 1.9.3p286 :007 > a.fileno => 5 1.9.3p286 :008 > b.fileno => 6 1.9.3p286 :009 > c = b.dup => #<File:foobar.txt> 1.9.3p286 :010 > c.fileno => 7 Thanks, Brian ---------------------------------------- Bug #7220: StringIO#initialize_copy causes aliasing between the objects https://bugs.ruby-lang.org/issues/7220#change-31627 Author: brixen (Brian Ford) Status: Open Priority: Normal Assignee: Category: Target version: ruby -v: ruby 1.9.3p286 (2012-10-12 revision 37165) [x86_64-darwin10.8.0] Calling StringIO#initialize_copy causes the two objects to share the position, and eof status. Is this a bug? sasha:rubinius brian$ irb 1.9.3p286 :001 > require 'stringio' => true 1.9.3p286 :002 > a = StringIO.new "abcdefuq" => #<StringIO:0x00000101016a88> 1.9.3p286 :003 > b = StringIO.new => #<StringIO:0x00000101010728> 1.9.3p286 :004 > b.send :initialize_copy, a => #<StringIO:0x00000101010728> 1.9.3p286 :005 > a.pos => 0 1.9.3p286 :006 > b.pos => 0 1.9.3p286 :007 > b.getc => "a" 1.9.3p286 :008 > a.pos => 1 1.9.3p286 :009 > a.getc => "b" 1.9.3p286 :010 > b.pos => 2 1.9.3p286 :011 > b.read => "cdefuq" 1.9.3p286 :012 > a.eof? => true Thanks, Brian -- http://bugs.ruby-lang.org/