From: "YO4 (Yoshinao Muramatsu) via ruby-core" <ruby-core@...>
Date: 2025-11-16T14:07:14+00:00
Subject: [ruby-core:123812]  [Ruby Bug#21687] IO＃pos goes wrong after EOF character(ctrl-z) met.

Issue #21687 has been updated by YO4 (Yoshinao Muramatsu).


Here is additional issue.
When an intermediate EOF character met, reading operation reports EOF but sometimes IO#eof? does not.
```ruby
require 'tempfile'

Tempfile.open do |f|
  str = "0123456789\x1A"
  f.write(str + "x"*(1024_0 - str.bytesize))
  f.rewind
  p f.readline # => "0123456789"
  p f.getbyte  # => nil (EOF met)
  p f.eof?     # => false (not EOF met, wrong)
end
```
In contrast, when the end of the file fits within the read buffer, IO#eof? returns the correct result. This suggests that it recognizes the end of the file rather than an EOF character.
```ruby
Tempfile.open do |f|
  str = "0123456789\x1A"
  f.write(str + "x"*(1024 - str.bytesize))
  f.rewind
  p f.readline # => "0123456789"
  p f.getbyte  # => nil (EOF met)
  p f.eof?     # => true (EOF met, pseudo good probably)
end
```
This seems to related to #21634-2

----------------------------------------
Bug #21687: IO���pos goes wrong after EOF character(ctrl-z) met.
https://bugs.ruby-lang.org/issues/21687#change-115214

* Author: YO4 (Yoshinao Muramatsu)
* Status: Open
* ruby -v: ruby 3.4.7 (2025-10-08 revision 7a5688e2a2) +PRISM [x64-mingw-ucrt]
* Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN
----------------------------------------
In Windows environment, when opening a file with the "r", encountering an EOF character (Ctrl-Z, "\x1A") during reading causes the IO to report END-OF-FILE.

```ruby
require 'tempfile'

Tempfile.open do |f|
  str = "0123456789\x1A"
  f.write(str + "x"*(1024_0 - str.bytesize))
  f.rewind
  p f.readline.size # => 10
  p f.pos           # => 8191, shuld be 10 but equals rbuf size
end
```
When the file reaches its end, the file position moves to the position after the last character.
Therefore, when encountering the EOF character, the file position is expected to move to the position of the EOF character.

This is somewhat unrelated, but I understand that self.seek(0, File::SEEK_END) not positioning at the EOF character is a known limitation.



-- 
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