Bug #21260: duping stringio objects shares cursors

• Status: Feedback
• Assignee: -
• Target version: -
• ruby -v: ruby 3.4.2 (2025-02-15 revision d2930f8e7a) +PRISM [x86_64-darwin23]
• Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN

[chucke (Tiago Cardoso)]

I've found that, if I .dup a stringio, when I read one of the objects, the cursor also moves internally on the other:

require "stringio"
s1 = StringIO.new("test")
s2 = s1.dup
s1.read #=> "test"
s2.read #=> ""
s1.rewind
s2.rewind
s2.read #=> "test"
s1.read #=> ""
s1.pos #=> 4
s2.pos #=> 4

[jeremyevans0 (Jeremy Evans)]

This is how IO#dup works. What would be the benefit of StringIO#dup behaving differently?

f = File.open('filename.rb')
f2 = f.dup
f2.pos # => 0
f.read 4
f2.pos # => 4

[nobu (Nobuyoshi Nakada)]

• Status changed from Open to Feedback

[chucke (Tiago Cardoso)]

I see. Alignment with File makes sense. At least my expectation was that, by dup'ing the IO object, I'd receive a separate independent object with which to traverse the file. But perhaps that does not make sense, as at least in the case of File, that'd mean a second OS file descriptor, and that's heavy? Also not sure whether this expectation is general.