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Bug #21260
closed
duping stringio objects shares cursors
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]
Description
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 
Updated by jeremyevans0 (Jeremy Evans) 7 months ago
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 
Updated by nobu (Nobuyoshi Nakada) 7 months ago
- Status changed from Open to Feedback
Updated by chucke (Tiago Cardoso) 7 months ago
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.
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